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Animating SVG topic maps with Inkscape, Emacs, FFmpeg, and Reveal.js

| emacs, drawing, org, ffmpeg, video

tldr (2167 words): I can make animating presentation maps easier by writing my own functions for the Emacs text editor. In this post, I show how I can animate an SVG element by element. I can also add IDs to the path and use CSS to build up an SVG with temporary highlighting in a Reveal.js presentation.

Text from the sketch
  • PNG: Inkscape: trace
  • Supernote (e-ink)
  • iPad: Adobe Fresco

Convert PDF to SVG with Inkscape (Cairo option) or pdftocairo)

  • PNG / Supernote PDF: Combined shapes. Process
    1. Break apart, fracture overlaps
    2. Recombine
    3. Set IDs
    4. Sort paths -> Animation style 1
  • Adobe Fresco: individual elements in order; landscape feels natural

Animation styles

  • Animation style 1: Display elements one after another
  • Animation style 2: Display elements one after another, and also show/hide highlights
    • Table: slide ID, IDs to add, temporary highlights -> Reveal.js: CSS with transitions

Ideas for next steps:

  • Explore graphviz & other diagramming tools
  • Frame-by-frame SVGs
    • on include
    • write to files
  • FFmpeg crossfade
  • Recording Reveal.js presentations
  • Use OCR results?

I often have a hard time organizing my thoughts into a linear sequence. Sketches are nice because they let me jump around and still show the connections between ideas. For presentations, I'd like to walk people through these sketches by highlighting different areas. For example, I might highlight the current topic or show the previous topics that are connected to the current one. Of course, this is something Emacs can help with. Before we dive into it, here are quick previews of the kinds of animation I'm talking about:

Figure 1: Animation style 1: based on drawing order

Animation style 2: building up a map with temporary highlights

Getting the sketches: PDFs are not all the same

Let's start with getting the sketches. I usually export my sketches as PNGs from my Supernote A5X. But if I know that I'm going to animate a sketch, I can export it as a PDF. I've recently been experimenting with Adobe Fresco on the iPad, which can also export to PDF. The PDF I get from Fresco is easier to animate, but I prefer to draw on the Supernote because it's an e-ink device (and because the kiddo usually uses the iPad).

If I start with a PNG, I could use Inkscape to trace the PNG and turn it into an SVG. I think Inkscape uses autotrace behind the scenes. I don't usually put my highlights on a separate layer, so autotrace will make odd shapes.

It's a lot easier if you start off with vector graphics in the first place. I can export a vector PDF from the SuperNote A5X and either import it into Inkscape using the Cairo option or use the command-line pdftocairo tool.

I've been looking into using Adobe Fresco, which is a free app available for the iPad. Fresco's PDF export can be converted to an SVG using Inkscape or PDF to Cairo. What I like about the output of this app is that it gives me individual elements as their own paths and they're listed in order of drawing. This makes it really easy to animate by just going through the paths in order.

Animation style 1: displaying paths in order

Here's a sample SVG file that pdfcairo creates from an Adobe Fresco PDF export:

pdftocairo -svg ~/Downloads/subed-audio.pdf ~/Downloads/subed-audio.svg
Sample SVG

Adobe Fresco also includes built-in time-lapse, but since I often like to move things around or tidy things up, it's easier to just work with the final image, export it as a PDF, and convert it to an SVG.

I can make a very simple animation by setting the opacity of all the paths to 0, then looping through the elements to set the opacity back to 1 and write that version of the SVG to a separate file. From how-can-i-generate-png-frames-that-step-through-the-highlights:

my-animate-svg-paths: Add one path at a time. Save the resulting SVGs to OUTPUT-DIR.
(defun my-animate-svg-paths (filename output-dir)
  "Add one path at a time. Save the resulting SVGs to OUTPUT-DIR."
  (unless (file-directory-p output-dir)
    (make-directory output-dir t))
  (let* ((dom (xml-parse-file filename))
         (paths (seq-filter (lambda (e) (dom-attr e 'style))
                            (dom-by-tag dom 'path)))
         (total (length paths))
         (frame-num (length paths))
    (dolist (elem paths)
      (dom-set-attribute elem 'style
                          (dom-attr elem 'style)
    (with-temp-file (expand-file-name (format "frame-%03d.svg" (1+ frame-num)) output-dir)
      (xml-print dom))
    (dolist (elem paths)
      (dom-set-attribute elem 'style
                          (dom-attr elem 'style)
    (dolist (elem paths)
      (with-temp-file (expand-file-name
                       (format "frame-%03d.svg"
                               (- total frame-num))
        (message "%03d" frame-num)
        (dom-set-attribute elem 'style
                           (concat (dom-attr elem 'style)
        (push (list (format "frame-%03d.svg"
                            (1+ (- total frame-num)))
                    (dom-attr elem 'id))
        (setq frame-num (1- frame-num))
        (xml-print dom)))
    (reverse result)))

Here's how I call it:

(my-animate-svg-paths "~/Downloads/subed-audio.svg" "/tmp/subed-audio/frames" t)

Then I can use FFmpeg to combine all of those frames into a video:

ffmpeg -i frame-%03d.svg -vf palettegen -y palette.png
ffmpeg -framerate 30 -i frame-%03d.svg -i palette.png -lavfi "paletteuse" -loop 0 -y animation-loop.gif
Figure 2: Animating SVG paths based on drawing order

Neither Supernote nor Adobe Fresco give me the original stroke information. These are filled shapes, so I can't animate something drawing it. But having different elements appear in sequence is fine for my purposes. If you happen to know how to get stroke information out of Supernote .note files or of an iPad app that exports nice single-line SVGs that have stroke direction, I would love to hear about it.

Identifying paths from Supernote sketches

When I export a PDF from Supernote and convert it to an SVG, each color is a combined shape with all the elements. If I want to animate parts of the image, I have to break it up and recombine selected elements (Inkscape's Ctrl-k shortcut) so that the holes in shapes are properly handled. This is a bit of a tedious process and it usually ends up with elements in a pretty random order. Since I have to reorder elements by hand, I don't really want to animate the sketch letter-by-letter. Instead, I combine them into larger chunks like topics or paragraphs.

The following code takes the PDF, converts it to an SVG, recolours highlights, and then breaks up paths into elements:

my-sketch-convert-pdf-and-break-up-paths: Convert PDF to SVG and break up paths.
(defun my-sketch-convert-pdf-and-break-up-paths (pdf-file &optional rotate)
  "Convert PDF to SVG and break up paths."
  (interactive (list (read-file-name
                      (format "PDF (%s): "
                              (my-latest-file "~/Dropbox/Supernote/EXPORT/" "pdf"))
                      (my-latest-file "~/Dropbox/Supernote/EXPORT/" "pdf")
                      (lambda (s) (string-match "pdf" s)))))
  (unless (file-exists-p (concat (file-name-sans-extension pdf-file) ".svg"))
    (call-process "pdftocairo" nil nil nil "-svg" (expand-file-name pdf-file)
                  (expand-file-name (concat (file-name-sans-extension pdf-file) ".svg"))))
  (let ((dom (xml-parse-file (expand-file-name (concat (file-name-sans-extension pdf-file) ".svg"))))
    (setq highlights (dom-node 'g '((id . "highlights"))))
    (dom-append-child dom highlights)
    (dolist (path (dom-by-tag dom 'path))
      ;;  recolor and move
      (unless (string-match (regexp-quote "rgb(0%,0%,0%)") (or (dom-attr path 'style) ""))
        (dom-remove-node dom path)
        (dom-append-child highlights path)
         path 'style
          (regexp-quote "rgb(78.822327%,78.822327%,78.822327%)")
          (or (dom-attr path 'style) ""))))
      (let ((parent (dom-parent dom path)))
        ;; break apart
        (when (dom-attr path 'd)
          (dolist (part (split-string (dom-attr path 'd) "M " t " +"))
             (dom-node 'path `((style . ,(dom-attr path 'style))
                               (d . ,(concat "M " part))))))
          (dom-remove-node dom path))))
    ;; remove the use
    (dolist (use (dom-by-tag dom 'use))
      (dom-remove-node dom use))
    (dolist (use (dom-by-tag dom 'image))
      (dom-remove-node dom use))
    ;; move the first g down
    (let ((g (car (dom-by-id dom "surface1"))))
      (setf (cddar dom)
            (seq-remove (lambda (o)
                          (and (listp o) (string= (dom-attr o 'id) "surface1")))
                        (dom-children dom)))
      (dom-append-child dom g)
      (when rotate
        (let* ((old-width (dom-attr dom 'width))
               (old-height (dom-attr dom 'height))
               (view-box (mapcar 'string-to-number (split-string (dom-attr dom 'viewBox))))
               (rotate (format "rotate(90) translate(0 %s)" (- (elt view-box 3)))))
          (dom-set-attribute dom 'width old-height)
          (dom-set-attribute dom 'height old-width)
          (dom-set-attribute dom 'viewBox (format "0 0 %d %d" (elt view-box 3) (elt view-box 2)))
          (dom-set-attribute highlights 'transform rotate)
          (dom-set-attribute g 'transform rotate))))
    (with-temp-file (expand-file-name (concat (file-name-sans-extension pdf-file) "-split.svg"))
      (svg-print (car dom)))))

Figure 3: Image after splitting up into elements

You can see how the spaces inside letters like "o" end up being black. Selecting and combining those paths fixes that.

Combining paths in Inkscape

If there were shapes that were touching, then I need to draw lines and fracture the shapes in order to break them apart.

Fracturing shapes and checking the highlights

The end result should be an SVG with the different chunks that I might want to animate, but I need to identify the paths first. You can assign object IDs in Inkscape, but this is a bit of an annoying process since I haven't figured out a keyboard-friendly way to set object IDs. I usually find it easier to just set up an Autokey shortcut (or AutoHotkey in Windows) to click on the ID text box so that I can type something in.

Autokey script for clicking
import time
x, y = mouse.get_location()
# Use the coordinates of the ID text field on your screen; xev can help
mouse.click_absolute(3152, 639, 1)
mouse.move_cursor(x, y)

Then I can select each element, press the shortcut key, and type an ID into the textbox. I might use "t-…" to indicate the text for a map section, "h-…" to indicate a highlight, and arrows by specifying their start and end.

Setting IDs in Inkscape

To simplify things, I wrote a function in Emacs that will go through the different groups that I've made, show each path in a different color and with a reasonable guess at a bounding box, and prompt me for an ID. This way, I can quickly assign IDs to all of the paths. The completion is mostly there to make sure I don't accidentally reuse an ID, although it can try to combine paths if I specify the ID. It saves the paths after each change so that I can start and stop as needed. Identifying paths in Emacs is usually much nicer than identifying them in Inkscape.

Identifying paths inside Emacs

my-svg-identify-paths: Prompt for IDs for each path in FILENAME.
(defun my-svg-identify-paths (filename)
  "Prompt for IDs for each path in FILENAME."
  (interactive (list (read-file-name "SVG: " nil nil
                                     (lambda (f) (string-match "\\.svg$" f)))))
  (let* ((dom (car (xml-parse-file filename)))
         (paths (dom-by-tag dom 'path))
         (vertico-count 3)
         (ids (seq-keep (lambda (path)
                          (unless (string-match "path[0-9]+" (or (dom-attr path 'id) "path0"))
                            (dom-attr path 'id)))
         (edges (window-inside-pixel-edges (get-buffer-window)))
    (my-svg-display "*image*" dom nil t)
    (dolist (path paths)
      (when (string-match "path[0-9]+" (or (dom-attr path 'id) "path0"))
        ;; display the image with an outline
              (my-svg-display "*image*" dom (dom-attr path 'id) t)
              (setq id (completing-read
                        (format "ID (%s): " (dom-attr path 'id))
              ;; already exists, merge with existing element
              (if-let ((old (dom-by-id dom id)))
                     (concat (dom-attr (dom-by-id dom id) 'd)
                             " "
                             ;; change relative to absolute
                             (replace-regexp-in-string "^m" "M"
                                                       (dom-attr path 'd))))
                    (dom-remove-node dom path)
                    (setq id nil))
                (dom-set-attribute path 'id id)
                (add-to-list 'ids id))))
        ;; save the image just in case we get interrupted halfway through
        (with-temp-file filename
          (svg-print dom))))))

Sorting and animating the paths by IDs

Then I can animate SVGs by specifying the IDs. I can reorder the paths in the SVG itself so that I can animate it group by group, like the way that the Adobe Fresco SVGs were animated element by element.

Reordering paths
(my-svg-reorder-paths "~/proj/2023-12-audio-workflow/map.svg"
                      '("t-start" "h-audio" "h-capture" "t-but" "t-mic" "h-mic"
                        "t-reviewing" "h-reviewing"
                        "t-words" "h-words" "t-workflow" "h-workflow"
                        "t-lapel" "h-lapel"
                        "mic-recorder" "t-recorder" "h-recorder"
                        "t-syncthing" "h-sync"
                        "t-keywords" "h-keywords" "t-keyword-types"
                        "t-lines" "h-lines"
                        "t-align" "h-align"
                        "t-org" "h-org" "t-todo" "h-todo" "h-linked"
                        "t-jump" "h-jump"
                        "t-waveform" "h-waveform"
                        "t-speech-recognition" "h-speech-recognition"
                        "t-ai" "h-ai"
(my-animate-svg-paths "~/proj/2023-12-audio-workflow/map-output.svg"
Table of filenames after reordering paths and animating the image
frame-001.svg t-start
frame-002.svg h-audio
frame-003.svg h-capture
frame-004.svg t-but
frame-005.svg t-mic
frame-006.svg h-mic
frame-007.svg t-reviewing
frame-008.svg h-reviewing
frame-009.svg t-words
frame-010.svg h-words
frame-011.svg t-workflow
frame-012.svg h-workflow
frame-013.svg t-lapel
frame-014.svg h-lapel
frame-015.svg mic-recorder
frame-016.svg t-recorder
frame-017.svg h-recorder
frame-018.svg t-syncthing
frame-019.svg h-sync
frame-020.svg t-keywords
frame-021.svg h-keywords
frame-022.svg t-keyword-types
frame-023.svg t-lines
frame-024.svg h-lines
frame-025.svg t-align
frame-026.svg h-align
frame-027.svg arrow
frame-028.svg t-org
frame-029.svg h-org
frame-030.svg t-todo
frame-031.svg h-todo
frame-032.svg h-linked
frame-033.svg t-jump
frame-034.svg h-jump
frame-035.svg t-waveform
frame-036.svg h-waveform
frame-037.svg t-someday
frame-038.svg h-sections
frame-039.svg t-speech-recognition
frame-040.svg h-speech-recognition
frame-041.svg t-ai
frame-042.svg h-ai
frame-043.svg t-summary
frame-044.svg extra

The table of filenames makes it easy to use specific frames as part of a presentation or video.

Here is the result as a video:

(let ((compile-media-output-video-width 1280)
      (compile-media-output-video-height 720))
   (directory-files "~/proj/2023-12-audio-workflow/frames/" t "\\.svg$")
   (expand-file-name "~/proj/2023-12-audio-workflow/frames/animation.webm")

Animation of SVG by paths

The way it works is that the my-svg-reorder-paths function removes and readds elements following the list of IDs specified, so everything's ready to go for step-by-step animation. Here's the code:

my-svg-reorder-paths: Sort paths in FILENAME.
(defun my-svg-reorder-paths (filename &optional ids output-filename)
  "Sort paths in FILENAME."
  (interactive (list (read-file-name "SVG: " nil nil (lambda (f) (string-match "\\.svg$" f)))
                     nil (read-file-name "Output: ")))
  (let* ((dom (car (xml-parse-file filename)))
         (paths (dom-by-tag dom 'path))
         (parent (dom-parent dom (car paths)))
          (nreverse (seq-keep (lambda (path)
                                (unless (string-match "path[0-9]+" (or (dom-attr path 'id) "path0"))
                                  (dom-attr path 'id)))
    (when (called-interactively-p)
      (while ids-left
        (my-svg-display "*image*" dom (car ids-left))
        (let ((current (completing-read
                        (format "ID (%s): "
                                (car ids-left))
                        ids-left nil nil nil nil (car ids-left)))
          (add-to-list 'ids current)
          (setq ids-left (seq-remove (lambda (o) (string= o current)) ids-left)))))
    (if ids ;; reorganize under the first path's parent
          (dolist (id ids)
            (if-let ((node (car (dom-by-id dom id))))
                  (dom-remove-node dom node)
                  (dom-append-child parent node))
              (message "Could not find %s" id)))
          (with-temp-file (or output-filename filename)
            (svg-print dom))))
    (nreverse (seq-keep (lambda (path)
                          (unless (string-match "path[0-9]+" (or (dom-attr path 'id) "path0"))
                            (dom-attr path 'id)))
                        (dom-by-tag dom 'path)))))

Animation style 2: Building up a map with temporary highlights

I can also use CSS rules to transition between opacity values for more complex animations. For my EmacsConf 2023 presentation, I wanted to make a self-paced, narrated presentation so that people could follow hyperlinks, read the source code, and explore. I wanted to include a map so that I could try to make sense of everything. For this map, I wanted to highlight the previous sections that were connected to the topic for the current section.

I used a custom Org link to include the full contents of the SVG instead of just including it with an img tag.

#+ATTR_HTML: :class r-stretch
my-include:~/proj/emacsconf-2023-emacsconf/map.svg?wrap=export html

my-include-export: Export PATH to FORMAT using the specified wrap parameter.
(defun my-include-export (path _ format _)
  "Export PATH to FORMAT using the specified wrap parameter."
  (let (params body start end)
    (when (string-match "^\\(.*+?\\)\\(?:::\\|\\?\\)\\(.*+\\)" path)
      (setq params (save-match-data (org-protocol-convert-query-to-plist (match-string 2 path)))
            path (match-string 1 path)))
      (insert-file-contents-literally path)
      (when (string-match "\\.org$" path)
      (if (plist-get params :name)
          (when (org-babel-find-named-block (plist-get params :name))
            (goto-char (org-babel-find-named-block (plist-get params :name)))
            (let ((block (org-element-context)))
              (setq start (org-element-begin block)
                    end (org-element-end block))))
        (goto-char (point-min))
        (when (plist-get params :from-regexp)
          (re-search-forward (url-unhex-string (plist-get params :from-regexp)))
          (goto-char (match-beginning 0)))
        (setq start (point))
        (setq end (point-max))
        (when (plist-get params :to-regexp)
          (re-search-forward (url-unhex-string (plist-get params :to-regexp)))
          (setq end (match-beginning 0))))
      (setq body (buffer-substring start end)))
      (when (plist-get params :wrap)
        (let* ((wrap (plist-get params :wrap))
               block args)
          (when (string-match "\\<\\(\\S-+\\)\\( +.*\\)?" wrap)
            (setq block (match-string 1 wrap))
            (setq args (match-string 2 wrap))
            (setq body (format "#+BEGIN_%s%s\n%s\n#+END_%s\n"
                               block (or args "")
      (when (plist-get params :summary)
        (setq body (format "#+begin_my_details %s\n%s\n#+end_my_details\n"
                           (plist-get params :summary)
      (insert body)
      (org-export-as format nil nil t))))

I wanted to be able to specify the entire sequence using a table in the Org Mode source for my presentation. Each row had the slide ID, a list of highlights in the form prev1,prev2;current, and a comma-separated list of elements to add to the full-opacity view.

Slide Highlight Additional elements
props-map h-email;h-properties t-email,email-properties,t-properties
file-prefixes h-properties;h-filename t-filename,properties-filename
renaming h-filename;h-renaming t-renaming,filename-renaming
shell-scripts h-renaming;h-shell-scripts renaming-shell-scripts,t-shell-scripts
availability h-properties;h-timezone t-timezone,properties-timezone
schedule h-timezone;h-schedule t-schedule,timezone-schedule
emailing-speakers h-timezone,h-mail-merge;h-emailing-speakers schedule-emailing-speakers,t-emailing-speakers
template h-properties;h-template t-template,properties-template
wiki h-template;h-wiki t-wiki,template-wiki,schedule-wiki
pad h-template;h-pad template-pad,t-pad
mail-merge h-template;h-mail-merge t-mail-merge,template-mail-merge,schedule-mail-merge,emailing-speakers-mail-merge
bbb h-bbb t-bbb
checkin h-mail-merge;h-checkin t-checkin,bbb-checkin
redirect h-bbb;h-redirect t-redirect,bbb-redirect
shortcuts h-email;h-shortcuts t-shortcuts,email-shortcuts
logbook h-shortcuts;h-logbook shortcuts-logbook,t-logbook
captions h-captions t-captions,captions-wiki
tramp h-captions;h-tramp t-tramp,captions-tramp
crontab h-tramp;h-crontab tramp-crontab,bbb-crontab,t-crontab
transitions h-crontab;h-transitions shell-scripts-transitions,t-transitions,shortcuts-transitions,transitions-crontab
irc h-transitions;h-irc t-irc,transitions-irc

Reveal.js adds a "current" class to the slide, so I can use that as a trigger for the transition. I have a bit of Emacs Lisp code that generates some very messy CSS, in which I specify the ID of the slide, followed by all of the elements that need their opacity set to 1, and also specifying the highlights that will be shown in an animated way.

my-reveal-svg-progression-css: Make the CSS.
(defun my-reveal-svg-progression-css (map-progression &optional highlight-duration)
  "Make the CSS.
map-progression should be a list of lists with the following format:
((\"slide-id\" \"prev1,prev2;cur1\" \"id-to-add1,id-to-add2\") ...)."
  (setq highlight-duration (or highlight-duration 2))
  (let (full)
      (lambda (slide)
        (setq full (append (split-string (elt slide 2) ",") full))
        (format "#slide-%s.present path { opacity: 0.2 }
%s { opacity: 1 !important }
                (car slide)
                (mapconcat (lambda (id) (format "#slide-%s.present #%s" (car slide) id))
                           ", ")
                (my-reveal-svg-highlight-different-colors slide)))
I call it from my Org file like this:

#+NAME: progression-css
#+begin_src emacs-lisp :exports code :var map-progression=progression :var highlight-duration=2 :results silent
(my-reveal-svg-progression-css map-progression highlight-duration)

Here's an excerpt showing the kind of code it makes:

<style>#slide-props-map.present path { opacity: 0.2 }
#slide-props-map.present #t-email, #slide-props-map.present #email-properties, #slide-props-map.present #t-properties { opacity: 1 !important }
#slide-props-map.present #h-email { fill: #c6c6c6; opacity: 1 !important; transition: fill 0.5s; transition-delay: 0.0s }#slide-props-map.present #h-properties { fill: #f6f396; opacity: 1 !important; transition: fill 0.5s; transition-delay: 0.5s }
#slide-file-prefixes.present path { opacity: 0.2 }
#slide-file-prefixes.present #t-filename, #slide-file-prefixes.present #properties-filename, #slide-file-prefixes.present #t-email, #slide-file-prefixes.present #email-properties, #slide-file-prefixes.present #t-properties { opacity: 1 !important }
#slide-file-prefixes.present #h-properties { fill: #c6c6c6; opacity: 1 !important; transition: fill 0.5s; transition-delay: 0.0s }#slide-file-prefixes.present #h-filename { fill: #f6f396; opacity: 1 !important; transition: fill 0.5s; transition-delay: 0.5s }

Since it's automatically generated, I don't have to worry about it once I've gotten it to work. It's all hidden in a results drawer. So this CSS highlights specific parts of the SVG with a transition, and the highlight changes over the course of a second or two. It highlights the previous names and then the current one. The topics I'd already discussed would be in black, and the topics that I had yet to discuss would be in very light gray. This could give people a sense of the progress through the presentation.

Code for making the CSS
(defun my-reveal-svg-animation (slide)
    (lambda (step-ids i)
      (format "%s { fill: #f6f396; transition: fill %ds; transition-delay: %ds }"
               (lambda (id) (format "#slide-%s.present #%s" (car slide) id))
               (split-string step-ids ",")
               ", ")
              (* i highlight-duration)))
    (split-string (elt slide 1) ";"))

(defun my-reveal-svg-highlight-different-colors (slide)
  (let* ((colors '("#f6f396" "#c6c6c6")) ; reverse
         (steps (split-string (elt slide 1) ";"))
         (step-length 0.5))
      (lambda (step-ids i)
        (format "%s { fill: %s; opacity: 1 !important; transition: fill %.1fs; transition-delay: %.1fs }"
                 (lambda (id) (format "#slide-%s.present #%s" (car slide) id))
                 (split-string step-ids ",")
                 ", ")
                (elt colors (- (length steps) i 1))
                (* i 0.5)))

(defun my-reveal-svg-progression-css (map-progression &optional highlight-duration)
  "Make the CSS.
map-progression should be a list of lists with the following format:
((\"slide-id\" \"prev1,prev2;cur1\" \"id-to-add1,id-to-add2\") ...)."
  (setq highlight-duration (or highlight-duration 2))
  (let (full)
      (lambda (slide)
        (setq full (append (split-string (elt slide 2) ",") full))
        (format "#slide-%s.present path { opacity: 0.2 }
%s { opacity: 1 !important }
                (car slide)
                (mapconcat (lambda (id) (format "#slide-%s.present #%s" (car slide) id))
                           ", ")
                (my-reveal-svg-highlight-different-colors slide)))

As a result, as I go through my presentation, the image appears to build up incrementally, which is the effect that I was going for. I can test this by exporting only my map slides:

  (goto-char (org-babel-find-named-block "progression-css"))
(let ((org-tags-exclude-from-inheritance "map")
      (org-export-select-tags '("map")))

Ideas for next steps

  • Graphviz, mermaid-js, and other diagramming tools can make SVGs. I should be able to adapt my code to animate those diagrams by adding other elements in addition to path. Then I'll be able to make diagrams even more easily.
  • Since SVGs can contain CSS, I could make an SVG equivalent of the CSS rules I used for the presentation, maybe calling a function with a Lisp expression that specifies the operations (ex: ("frame-001.svg" "h-foo" opacity 1)). Then I could write frames to SVGs.
  • FFmpeg has a crossfade filter. With a little bit of figuring out, I should be able to make the same kind of animation in a webm form that I can include in my regular videos instead of using Reveal.js and CSS transitions.
  • I've also been thinking about automating the recording of my Reveal.js presentations. For my EmacsConf talk, I opened my presentation, started the recording with the system audio and the screen, and then let it autoplay the presentation. I checked on it periodically to avoid the screensaver/energy saving things from kicking in and so that I could stop the recording when it's finished. If I want to make this take less work, one option is to use ffmpeg's "-t" argument to specify the expected duration of the presentation so that I don't have to manually stop it. I'm also thinking about using Puppeteer to open the presentation, check when it's fully loaded, and start the process to record it - maybe even polling to see whether it's finished. I haven't gotten around to it yet. Anyhow, those are some ideas to explore next time.
  • As for animation, I'm still curious about the possibility of finding a way to access the raw stroke information if it's even available from my Supernote A5X (difficult because it's a proprietary data format) or finding an app for the iPad that exports single line SVGs that use stroke information instead of fill. That would only be if I wanted to do those even fancier animations that look like the whole thing is being drawn for you. I was trying to figure out if I could green screen the Adobe Fresco timelapse videos so that even if I have a pre-sketch to figure out spacing and remind me what to draw, I can just export the finished elements. But there's too much anti-aliasing and I haven't figured out how to do it cleanly yet. Maybe some other day.
  • I use Google Cloud Vision's text detection engine to convert my handwriting to text. It can give me bounding polygons for words or paragraphs. I might be able to figure out which curves are entirely within a word's bounding polygon and combine those automatically.
  • It would be pretty cool if I could combine the words recognized by Google Cloud Vision with the word-level timestamps from speech recognition so that I could get word-synced sketchnote animations with maybe a little manual intervention.

Anyway, those are some workflows for animating sketches with Inkscape and Emacs. Yay Emacs!

View org source for this post

Quick notes on livestreaming to YouTube with FFmpeg on a Lenovo X230T

| video, youtube, streaming, ffmpeg, yay-emacs

[2024-01-05 Fri]: Updated scripts

Text from the sketch

Quick thoughts on livestreaming


  • work out loud
  • share tips
  • share more
  • spark conversations
  • (also get questions about things)

Doable with ffmpeg on my X230T:

  • streaming from my laptop
  • lapel mic + system audio,
  • second screen for monitoring

Ideas for next time:

  • Overall notes in Emacs with outline, org-timer timestamped notes; capture to this file
  • Elisp to start/stop the stream → find old code
  • Use the Yeti? Better sound
  • tee to a local recording
  • grab screenshot from SuperNote mirror?

Live streaming info density:

  • High: Emacs News review, package/workflow demo
  • Narrating a blog post to make it a video
  • Categorizing Emacs News, exploring packages
  • Low: Figuring things out

YouTube can do closed captions for livestreams, although accuracy is low. Videos take a while to be ready to download.

Experimenting with working out loud

I wanted to write a report on EmacsConf 2023 so that we could share it with speakers, volunteers, participants, donors, related organizations like the Free Software Foundation, and other communities. I experimented with livestreaming via YouTube while I worked on the conference highlights.

It's a little over an hour long and probably very boring, but it was nice of people to drop by and say hello.

The main parts are:

  • 0:00: reading through other conference reports for inspiration
  • 6:54: writing an overview of the talks
  • 13:10: adding quotes for specific talks
  • 25:00: writing about the overall conference
  • 32:00: squeezing in more highlights
  • 49:00: fiddling with the formatting and the export

It mostly worked out, aside from a brief moment of "uhhh, I'm looking at our private file on stream". Fortunately, the e-mail addresses that were showed were the public ones.

Technical details


  • I set up environment variables and screen resolution:

      # From pacmd list-sources | egrep '^\s+name'
      LAPEL=alsa_input.usb-Jieli_Technology_USB_Composite_Device_433035383239312E-00.mono-fallback #
      # MIC=$LAPEL
      # AUDIO_WEIGHTS="1 1"
      AUDIO_WEIGHTS="0.5 0.5"
      SCREEN=LVDS-1  # from xrandr
      xrandr --output $SCREEN --mode 1280x720
  • I switch to a larger size and a light theme. I also turn consult previews off to minimize the risk of leaking data through buffer previews.
    my-emacsconf-prepare-for-screenshots: Set the resolution, change to a light theme, and make the text bigger.
    (defun my-emacsconf-prepare-for-screenshots ()
      (shell-command "xrandr --output LVDS-1 --mode 1280x720")
      (modus-themes-load-theme 'modus-operandi)
      (set-face-attribute 'default nil :height 170)


ffmpeg -f x11grab -video_size $SIZE -i :0.0$OFFSET -y /tmp/test.png; display /tmp/test.png
ffmpeg -f pulse -i $MIC -f pulse -i $SYSTEM -filter_complex amix=inputs=2:weights=$AUDIO_WEIGHTS:duration=longest:normalize=0 -y /tmp/test.mp3; mpv /tmp/test.mp3
DATE=$(date "+%Y-%m-%d-%H-%M-%S")
ffmpeg -f x11grab -framerate 30 -video_size $SIZE -i :0.0$OFFSET -f pulse -i $MIC -f pulse -i $SYSTEM -filter_complex "amix=inputs=2:weights=$AUDIO_WEIGHTS:duration=longest:normalize=0" -c:v libx264 -preset fast -maxrate 690k -bufsize 2000k -g 60 -vf format=yuv420p -c:a aac -b:a 96k -y -flags +global_header "/home/sacha/recordings/$DATE.flv" -f flv


DATE=$(date "+%Y-%m-%d-%H-%M-%S")
ffmpeg -f x11grab -framerate 30 -video_size $SIZE -i :0.0$OFFSET -f pulse -i $MIC -f pulse -i $SYSTEM -filter_complex "amix=inputs=2:weights=$AUDIO_WEIGHTS:duration=longest:normalize=0[audio]" -c:v libx264 -preset fast -maxrate 690k -bufsize 2000k -g 60 -vf format=yuv420p -c:a aac -b:a 96k -y -f tee -map 0:v -map '[audio]' -flags +global_header  "/home/sacha/recordings/$DATE.flv|[f=flv]rtmp://$YOUTUBE_KEY"

To restore my previous setup:

my-emacsconf-back-to-normal: Go back to a more regular setup.
(defun my-emacsconf-back-to-normal ()
  (shell-command "xrandr --output LVDS-1 --mode 1366x768")
  (modus-themes-load-theme 'modus-vivendi)
  (set-face-attribute 'default nil :height 115)
  (keycast-mode -1))

Ideas for next steps

I can think of a few workflow tweaks that might be fun:

  • a stream notes buffer on the right side of the screen for context information, timestamped notes to make editing/review easier (maybe using org-timer), etc. I experimented with some streaming-related code in my config, so I can dust that off and see what that's like. I also want to have an org-capture template for it so that I can add notes from anywhere.
  • a quick way to add a screenshot from my Supernote to my Org files

I think I'll try going through an informal presentation or Emacs News as my next livestream experiment, since that's probably higher information density.

View org source for this post

Figuring out how to use ffmpeg to mask a chroma-keyed video based on the differences between images

| linux, geek, ffmpeg, video

A- is really into Santa and Christmas because of the books she's read. Last year, she wanted to set up the GoPro to capture footage during Christmas Eve. I helped her set it up for a timelapse video. After she went to bed, we gradually positioned the presents. I extracted the frames from the video, removed the ones that caught us moving around, and then used Krita's new animation features to animate sparkles so that the presents magically appeared. She mentioned the sparkles a number of times during her deliberations about whether Santa exists or not.

This year, I want to see if I can use green-screen videos like this reversed-spin sparkle or this other sparkle video. I'm going to take a series of images, with each image adding one more gift. Then I'm going to make a mask in Krita with white covering the gift and a transparent background for the rest of the image. Then I'll use chroma-key to drop out the green screen of the sparkle video and mask it in so that the sparkles only happen within the boundaries of the gift that was added. I also want to fade one image into the other, and I want the sparkles to fade out as the gift appears.

Figuring things out

I didn't know how to do any of that yet with ffmpeg, so here's how I started figuring things out. First, I wanted to see how to fade test.jpg into test2.jpg over 4 seconds.

ffmpeg -y -loop 1 -i test.jpg -loop 1 -i test2.jpg -filter_complex "[1:v]fade=t=in:d=4:alpha=1[fadein];[0:v][fadein]overlay[out]" -map "[out]" -r 1 -t 4 -shortest test.webm

Here's another way using the blend filter:

ffmpeg -y -loop 1 -i test.jpg -loop 1 -i test2.jpg -filter_complex "[1:v][0:v]blend=all_expr='A*(if(gte(T,4),1,T/4))+B*(1-(if(gte(T,4),1,T/4)))" -t 4 -r 1 test.webm

Then I looked into chromakeying in the other video. I used balloons instead of sparkles just in case she happened to look at my screen.

ffmpeg -y -i test.webm -i balloons.mp4 -filter_complex "[1:v]chromakey=0x00ff00:0.1:0.2[ckout];[0:v][ckout]overlay[out]" -map "[out]" -shortest -r 1 overlaid.webm

I experimented with the alphamerge filter.

ffmpeg -y -i test.jpg -i test2.jpg -i mask.png -filter_complex "[1:v][2:v]alphamerge[a];[0:v][a]overlay[out]" -map "[out]" masked.jpg

Okay! That overlaid test.jpg with a masked part of test2.jpg. How about alphamerging in a video? First, I need a mask video…

ffmpeg -y -loop 1 -i mask.png  -r 1 -t 4  mask.webm

Then I can combine that:

ffmpeg -loglevel 32 -y -i test.webm -i balloons.mp4 -i mask.webm -filter_complex "[1:v][2:v]alphamerge[masked];[0:v][masked]overlay[out]" -map "[out]" -r 1 -t 4 alphamerged.webm

Great, let's figure out how to combine chroma-key and alphamerge video. The naive approach doesn't work, probably because they're both messing with the alpha layer.

ffmpeg -loglevel 32 -y -i test.webm -i balloons.mp4 -i mask.webm -filter_complex "[1:v]chromakey=0x00ff00:0.1:0.2[ckout];[ckout][2:v]alphamerge[masked];[0:v][masked]overlay[out]" -map "[out]" -r 1 -t 4 masked.webm

So I probably need to blend the chromakey and the mask. Let's see if I can extract the chromakey alpha.

ffmpeg -loglevel 32 -y -i test.webm -i balloons.mp4 -i mask.webm -filter_complex "[1:v]chromakey=0x00ff00:0.1:0.2,format=rgba,alphaextract[out]" -map "[out]" -r 1 -t 4

Now let's blend it with the mask.webm.

ffmpeg -loglevel 32 -y -i test.webm -i balloons.mp4 -i mask.webm -filter_complex "[1:v]chromakey=0x00ff00:0.1:0.2,format=rgba,alphaextract[ckalpha];[ckalpha][2:v]blend=all_mode=and[out]" -map "[out]" -r 1 -t 4 masked-alpha.webm

Then let's use it as the alpha:

ffmpeg -loglevel 32 -y -i test.webm -i balloons.mp4 -i masked-alpha.webm -filter_complex "[2:v]format=rgba[mask];[1:v][mask]alphamerge[masked];[0:v][masked]overlay[out]" -map "[out]" -r 1 -t 4 alphamerged.webm

Okay, that worked! Now how do I combine everything into one command? Hmm…

ffmpeg -loglevel 32 -y -loop 1 -i test.jpg -t 4 -loop 1 -i test2.jpg -t 4 -i balloons.mp4 -loop 1 -i mask.png -t 4 -filter_complex "[1:v][0:v]blend=all_expr='A*(if(gte(T,4),1,T/4))+B*(1-(if(gte(T,4),1,T/4)))'[fade];[2:v]chromakey=0x00ff00:0.1:0.2,format=rgba,alphaextract[ckalpha];[ckalpha][3:v]blend=all_mode=and,format=rgba[maskedalpha];[2:v][maskedalpha]alphamerge[masked];[fade][masked]overlay[out]" -map "[out]" -r 5 -t 4 alphamerged.webm

Then I wanted to fade the masked video out by the end.

ffmpeg -loglevel 32 -y -loop 1 -i test.jpg -t 4 -loop 1 -i test2.jpg -t 4 -i balloons.mp4 -loop 1 -i mask.png -t 4 -filter_complex "[1:v][0:v]blend=all_expr='A*(if(gte(T,4),1,T/4))+B*(1-(if(gte(T,4),1,T/4)))'[fade];[2:v]chromakey=0x00ff00:0.1:0.2,format=rgba,alphaextract[ckalpha];[ckalpha][3:v]blend=all_mode=and,format=rgba[maskedalpha];[2:v][maskedalpha]alphamerge[masked];[masked]fade=type=out:st=2:d=1:alpha=1[maskedfade];[fade][maskedfade]overlay[out]" -map "[out]" -r 10 -t 4 alphamerged.webm

Making the video

When A- finally went to bed, we arranged the presents, using the GoPro to take a picture at each step of the way. I cropped and resized the images, using Krita to figure out the cropping rectangle and offset.

for FILE in *.JPG; do convert $FILE -crop 1558x876+473+842 -resize 1280x720 cropped/$FILE; done

I used ImageMagick to calculate the masks automatically.

while [ "$j" -lt $len ]; do
  compare -fuzz 15% cropped/${files[$i]} cropped/${files[$j]} -compose Src -highlight-color White -lowlight-color Black masks/${files[$j]}
  convert -morphology Open Disk -morphology Close Disk -blur 20x5 masks/${files[$j]} processed-masks/${files[$j]}

Then I faded the images together to make a video.

import ffmpeg
import glob
files = glob.glob("images/cropped/*.JPG")
fps = 15
crf = 32
out = ffmpeg.input(files[0], loop=1, r=fps)
duration = 3
for i in range(1, len(files)):
    out = ffmpeg.filter([out, ffmpeg.input(files[i], loop=1, r=fps).filter('fade', t='in', d=duration, st=i*duration, alpha=1)], 'overlay')
args = out.output('images.webm', t=len(files) * duration, r=fps, y=None, crf=crf).compile()
print(' '.join(f'"{item}"' for item in args))

"ffmpeg" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2317.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2318.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2319.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2320.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2321.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2322.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2323.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2324.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2325.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2326.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2327.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2328.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2329.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2330.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2331.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2332.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2333.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2334.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2335.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2336.JPG" "-loop" "1" "-r" "15" "-i" "images/cropped/GOPR2337.JPG" "-filter_complex" "[1]fade=alpha=1:d=3:st=3:t=in[s0];[0][s0]overlay[s1];[2]fade=alpha=1:d=3:st=6:t=in[s2];[s1][s2]overlay[s3];[3]fade=alpha=1:d=3:st=9:t=in[s4];[s3][s4]overlay[s5];[4]fade=alpha=1:d=3:st=12:t=in[s6];[s5][s6]overlay[s7];[5]fade=alpha=1:d=3:st=15:t=in[s8];[s7][s8]overlay[s9];[6]fade=alpha=1:d=3:st=18:t=in[s10];[s9][s10]overlay[s11];[7]fade=alpha=1:d=3:st=21:t=in[s12];[s11][s12]overlay[s13];[8]fade=alpha=1:d=3:st=24:t=in[s14];[s13][s14]overlay[s15];[9]fade=alpha=1:d=3:st=27:t=in[s16];[s15][s16]overlay[s17];[10]fade=alpha=1:d=3:st=30:t=in[s18];[s17][s18]overlay[s19];[11]fade=alpha=1:d=3:st=33:t=in[s20];[s19][s20]overlay[s21];[12]fade=alpha=1:d=3:st=36:t=in[s22];[s21][s22]overlay[s23];[13]fade=alpha=1:d=3:st=39:t=in[s24];[s23][s24]overlay[s25];[14]fade=alpha=1:d=3:st=42:t=in[s26];[s25][s26]overlay[s27];[15]fade=alpha=1:d=3:st=45:t=in[s28];[s27][s28]overlay[s29];[16]fade=alpha=1:d=3:st=48:t=in[s30];[s29][s30]overlay[s31];[17]fade=alpha=1:d=3:st=51:t=in[s32];[s31][s32]overlay[s33];[18]fade=alpha=1:d=3:st=54:t=in[s34];[s33][s34]overlay[s35];[19]fade=alpha=1:d=3:st=57:t=in[s36];[s35][s36]overlay[s37];[20]fade=alpha=1:d=3:st=60:t=in[s38];[s37][s38]overlay[s39]" "-map" "[s39]" "-crf" "32" "-r" "15" "-t" "63" "-y" "images.webm"

Next, I faded the masks together. These ones faded in and out so that only one mask was active at a time.

import ffmpeg
import glob
files = glob.glob("images/processed-masks/*.JPG")
files = files[:-2]  # Omit the last two, where I'm just turning off the lights
fps = 15
crf = 32
out = ffmpeg.input('color=black:s=1280x720', f='lavfi', r=fps)
duration = 3
for i in range(0, len(files)):
    out = ffmpeg.filter([out, ffmpeg.input(files[i], loop=1, r=fps).filter('fade', t='in', d=1, st=(i + 1)*duration, alpha=1).filter('fade', t='out', st=(i + 2)*duration - 1)], 'overlay')
args = out.output('processed-masks.webm', t=len(files) * duration, r=fps, y=None, crf=crf).compile()
print(' '.join(f'"{item}"' for item in args))

"ffmpeg" "-f" "lavfi" "-r" "15" "-i" "color=s=1280x720" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2318.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2319.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2320.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2321.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2322.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2323.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2324.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2325.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2326.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2327.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2328.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2329.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2330.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2331.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2332.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2333.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2334.JPG" "-loop" "1" "-r" "15" "-i" "images/processed-masks/GOPR2335.JPG" "-filter_complex" "[1]fade=alpha=1:d=1:st=3:t=in[s0];[s0]fade=st=5:t=out[s1];[0][s1]overlay[s2];[2]fade=alpha=1:d=1:st=6:t=in[s3];[s3]fade=st=8:t=out[s4];[s2][s4]overlay[s5];[3]fade=alpha=1:d=1:st=9:t=in[s6];[s6]fade=st=11:t=out[s7];[s5][s7]overlay[s8];[4]fade=alpha=1:d=1:st=12:t=in[s9];[s9]fade=st=14:t=out[s10];[s8][s10]overlay[s11];[5]fade=alpha=1:d=1:st=15:t=in[s12];[s12]fade=st=17:t=out[s13];[s11][s13]overlay[s14];[6]fade=alpha=1:d=1:st=18:t=in[s15];[s15]fade=st=20:t=out[s16];[s14][s16]overlay[s17];[7]fade=alpha=1:d=1:st=21:t=in[s18];[s18]fade=st=23:t=out[s19];[s17][s19]overlay[s20];[8]fade=alpha=1:d=1:st=24:t=in[s21];[s21]fade=st=26:t=out[s22];[s20][s22]overlay[s23];[9]fade=alpha=1:d=1:st=27:t=in[s24];[s24]fade=st=29:t=out[s25];[s23][s25]overlay[s26];[10]fade=alpha=1:d=1:st=30:t=in[s27];[s27]fade=st=32:t=out[s28];[s26][s28]overlay[s29];[11]fade=alpha=1:d=1:st=33:t=in[s30];[s30]fade=st=35:t=out[s31];[s29][s31]overlay[s32];[12]fade=alpha=1:d=1:st=36:t=in[s33];[s33]fade=st=38:t=out[s34];[s32][s34]overlay[s35];[13]fade=alpha=1:d=1:st=39:t=in[s36];[s36]fade=st=41:t=out[s37];[s35][s37]overlay[s38];[14]fade=alpha=1:d=1:st=42:t=in[s39];[s39]fade=st=44:t=out[s40];[s38][s40]overlay[s41];[15]fade=alpha=1:d=1:st=45:t=in[s42];[s42]fade=st=47:t=out[s43];[s41][s43]overlay[s44];[16]fade=alpha=1:d=1:st=48:t=in[s45];[s45]fade=st=50:t=out[s46];[s44][s46]overlay[s47];[17]fade=alpha=1:d=1:st=51:t=in[s48];[s48]fade=st=53:t=out[s49];[s47][s49]overlay[s50];[18]fade=alpha=1:d=1:st=54:t=in[s51];[s51]fade=st=56:t=out[s52];[s50][s52]overlay[s53]" "-map" "[s53]" "-crf" "32" "-r" "15" "-t" "54" "-y" "processed-masks.webm"

I ended up using this particle glitter video because the gifts were small, so I wanted a video that was dense with sparkly things. I also wanted the sparkles to be more concentrated on the area where the gifts were, so I resized it and positioned it.

ffmpeg -loglevel 32 -y -f lavfi -i color=black:s=1280x720 -i sparkles4.webm -ss 13 -filter_complex "[1:v]scale=700:392[sparkles];[0:v][sparkles]overlay=x=582:y=194,setpts=(PTS-STARTPTS)*1.05[out]" -map "[out]" -r 15 -t 53 -shortest sparkles-trimmed.webm
ffmpeg -y -stream_loop 2 -i sparkles-trimmed.webm -t 57 sparkles-looped.webm              

Lastly, I combined the videos with the sparkles.

ffmpeg -loglevel 32 -y -i images.webm -i sparkles-looped.webm -i processed-masks.webm -filter_complex "[1:v]chromakey=0x0a9d06:0.1:0.2,format=rgba,alphaextract[ckalpha];[ckalpha][2:v]blend=all_mode=and,format=rgba[maskedalpha];[1:v][maskedalpha]alphamerge[masked];[masked]fade=t=out:st=57:d=1:alpha=1[maskedfaded];[0:v][maskedfaded]overlay[combined];[combined]tpad=start_mode=clone:start_duration=4:stop_mode=clone:stop_duration=4[out]" -map "[out]" -r 15 -crf 32 output.webm

After many iterations and a very late night, I got (roughly) the video I wanted, which I'm not posting here because of reasons. But it worked, yay! Now I don't have to manually place stars frame-by-frame in Krita, and I can just have all that magic happen semi-automatically.

Converting our VTT files to TTML

| emacsconf, geek, ffmpeg

I wanted to convert our VTT files to TTML files so that we might be able to use them for training lachesis for transcript segmentation. I downloaded the VTT files from EmacsConf 2021 to a directory and copied the edited captions from the EmacsConf 2022 backstage area (using head -1 ${FILE} | grep -q "captioned" to distinguish them from the automatic ones). I installed the ttconv python package. Then I used the following command to convert the TTML files:

for FILE in *.vtt; do
    BASE=$(basename -s .vtt "$FILE");
    ffmpeg -y -i $FILE $; tt convert -i $ -o $BASE.ttml

I haven't gotten around to installing whanever I need in order to get lachesis to work under Python 2.7, since it hasn't been updated for Python 3. It'll probably be a low-priority project anyway, as EmacsConf is fast approaching. Anyway, I thought I'd stash this in my blog somewhere in case I need to make TTML files again!

Re-encoding the EmacsConf videos with FFmpeg and GNU Parallel

| geek, linux, emacsconf, ffmpeg, video

It turns out that using -crf 56 compressed the EmacsConf a little too aggressively, losing too much information in the video. We wanted to reencode everything, maybe going back to the default value of -crf 32. My laptop would have taken a long time to do all of those videos. Fortunately, one of the other volunteers shared a VM on a machine with 12 cores, and I had access to a few other systems. It was a good opportunity to learn how to use GNU Parallel to send jobs to different machines and retrieve the results.

First, I updated the compression script,

ffmpeg -y -i "$FILE"  -pixel_format yuv420p -vf $VIDEO_FILTER -colorspace 1 -color_primaries 1 -color_trc 1 -c:v libvpx-vp9 -b:v 0 -crf $Q -aq-mode 2 -tile-columns 0 -tile-rows 0 -frame-parallel 0 -cpu-used 8 -auto-alt-ref 1 -lag-in-frames 25 -g 240 -pass 1 -f webm -an -threads 8 /dev/null &&
if [[ $FILE =~ "webm" ]]; then
    ffmpeg -y -i "$FILE" $*  -pixel_format yuv420p -vf $VIDEO_FILTER -colorspace 1 -color_primaries 1 -color_trc 1 -c:v libvpx-vp9 -b:v 0 -crf $Q -tile-columns 2 -tile-rows 2 -frame-parallel 0 -cpu-used -5 -auto-alt-ref 1 -lag-in-frames 25 -pass 2 -g 240 -ac 2 -threads 8 -c:a copy "${FILE%.*}--compressed$SUFFIX.webm"
    ffmpeg -y -i "$FILE" $*  -pixel_format yuv420p -vf $VIDEO_FILTER -colorspace 1 -color_primaries 1 -color_trc 1 -c:v libvpx-vp9 -b:v 0 -crf $Q -tile-columns 2 -tile-rows 2 -frame-parallel 0 -cpu-used -5 -auto-alt-ref 1 -lag-in-frames 25 -pass 2 -g 240 -ac 2 -threads 8 -c:a libvorbis "${FILE%.*}--compressed$SUFFIX.webm"

I made an originals.txt file with all the original filenames. It looked like this:


I set up a ~/.parallel/emacsconf profile with something like this so that I could use three computers and my laptop, sending one job each and displaying progress:

--sshlogin computer1 --sshlogin computer2 --sshlogin computer3 --sshlogin : -j 1 --progress --verbose --joblog parallel.log

I already had SSH key-based authentication set up so that I could connect to the three remote computers.

Then I spread the jobs over four computers with the following command:

cat originals.txt | parallel -J emacsconf \
                             --transferfile {} \
                             --return '{=$_ =~ s/\..*?$/--compressed32.webm/=}' \
                             --cleanup \
                             --basefile \
                             bash 32 {}

It copied each file over to the computer it was assigned to, processed the file, and then copied the file back.

It was also helpful to occasionally do echo 'killall -9 ffmpeg' | parallel -J emacsconf -j 1 --onall if I cancelled a run.

It still took a long time, but less than it would have if any one computer had to crunch through everything on its own.

This was much better than my previous way of doing things, which involved copying the files over, running ffmpeg commands, copying the files back, and getting somewhat confused about which directory I was in and which file I assigned where and what to do about incompletely-encoded files.

I sometimes ran into problems with incompletely-encoded files because I'd cancelled the FFmpeg process. Even though ffprobe said the files were long, they were missing a large chunk of video at the end. I added a compile-media-verify-video-frames function to compile-media.el so that I could get the last few seconds of frames, compare them against the duration, and report an error if there was a big gap.

Then I changed emacsconf-publish.el to use the new filenames, and I regenerated all the pages. For EmacsConf 2020, I used some Emacs Lisp to update the files. I'm not particularly fond of wrangling video files (lots of waiting, high chance of error), but I'm glad I got the computers to work together.

Update on Emacs Conf 2015 videos; Org Mode tables and time calculations

| emacs, geek, org, ffmpeg

I spent the day cutting up the rest of the videos from the Emacs Conference 2015 stream into individual talks. I’d already cut the set of talks before lunch, but there were quite a few more after. As it turned out, keeping the video data in .ts format instead of converting it to .mp4 is actually better for Youtube processing.

Since Camtasia Studio and Movie Maker were both having problems with the large videos, I used VLC to play the video and find the timestamps at which I needed to cut the segments. I made an Org Mode table with the start and end times, and then I used the ;T flag in a table function to get the duration. A little bit of Emacs Lisp code later, and I had my ffmpeg commands. Here’s the source from my Org file:

#+NAME: emacsconf-c.ts
| Notes                                            |      Start |        End | Duration |
| Emacs configuration                              | 4:02:25.37 | 4:27:09.30 | 00:24:44 |
| Hearing from Emacs Beginners                     |    4:27:27 |    5:01:00 | 00:33:33 |
| Lightning talk: Emacs Club                       | 5:03:19.30 | 5:19:37.83 | 00:16:18 |
| Starting an Emacs Meetup - Harry Schwartz part 1 | 5:31:52.03 |    6:01:20 | 00:29:28 |
#+TBLFM: $4=$3-$2;T

#+NAME: emacsconf-a.ts
| Notes                                                    |   Start |     End | Duration |
| Starting an Emacs Meetup - Harry Schwartz part 2         |  0:0:00 | 0:20:04 | 00:20:04 |
| Literate Devops - Howard Abrams                          | 1:28:20 | 2:08:15 | 00:39:55 |
| Lightning talk: Wanderlust and other mail clients        | 2:15:04 | 2:26:55 | 00:11:51 |
| Making Emacs a Better Tool for Scholars - Erik Hetzner   | 2:27:00 | 2:57:38 | 00:30:38 |
| Wrapping up and going forward                            | 2:58:09 | 2:59:44 | 00:01:35 |
| Lightning talk: Collaborative coding with tmux and tmate | 3:00:20 | 3:05:53 | 00:05:33 |
| Lightning talk: Cask and Pellet                          | 3:05:56 | 3:09:04 | 00:03:08 |
| Lightning talk: File sharing with Git and save hooks     | 3:09:34 | 3:17:50 | 00:08:16 |
| Lightning talk: Calc                                     | 3:18:42 | 3:33:20 | 00:14:38 |
| Lightning talk: Magit                                    | 3:35:15 | 3:49:42 | 00:14:27 |
| Lightning talk: gist.el                                  | 3:53:50 | 4:01:58 | 00:08:08 |
| Lightning talk: Go                                       | 4:02:45 | 4:16:37 | 00:13:52 |
| Question: Emacs Lisp backtraces                          | 4:16:50 | 4:20:09 | 00:03:19 |
#+TBLFM: $4=$3-$2;T

#+begin_src emacs-lisp :var data=emacsconf-a.ts :var data2=emacsconf-c.ts :colnames t :results output
(let ((format-str "ffmpeg -i %s -ss %s -t %s -c:v copy -c:a copy \"EmacsConf 2015 - %s.ts\"\n"))
  (mapc (lambda (file)
    (mapc (lambda (row) 
      (princ (format format-str (car file) (elt row 1) (elt row 3) (my/convert-sketch-title-to-filename (elt row 0))))) 
     (cdr file)))
    `(("emacsconf-c.ts" . ,data2)
      ("emacsconf-a.ts" . ,data))))

and the output:

ffmpeg -i emacsconf-c.ts -ss 4:02:25.37 -t 00:24:44 -c:v copy -c:a copy "EmacsConf 2015 - Emacs configuration.ts"
ffmpeg -i emacsconf-c.ts -ss 4:27:27 -t 00:33:33 -c:v copy -c:a copy "EmacsConf 2015 - Hearing from Emacs Beginners.ts"
ffmpeg -i emacsconf-c.ts -ss 5:03:19.30 -t 00:16:18 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Emacs Club.ts"
ffmpeg -i emacsconf-c.ts -ss 5:31:52.03 -t 00:29:28 -c:v copy -c:a copy "EmacsConf 2015 - Starting an Emacs Meetup - Harry Schwartz part 1.ts"
ffmpeg -i emacsconf-a.ts -ss 0:0:00 -t 00:20:04 -c:v copy -c:a copy "EmacsConf 2015 - Starting an Emacs Meetup - Harry Schwartz part 2.ts"
ffmpeg -i emacsconf-a.ts -ss 1:28:20 -t 00:39:55 -c:v copy -c:a copy "EmacsConf 2015 - Literate Devops - Howard Abrams.ts"
ffmpeg -i emacsconf-a.ts -ss 2:15:04 -t 00:11:51 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Wanderlust and other mail clients.ts"
ffmpeg -i emacsconf-a.ts -ss 2:27:00 -t 00:30:38 -c:v copy -c:a copy "EmacsConf 2015 - Making Emacs a Better Tool for Scholars - Erik Hetzner.ts"
ffmpeg -i emacsconf-a.ts -ss 2:58:09 -t 00:01:35 -c:v copy -c:a copy "EmacsConf 2015 - Wrapping up and going forward.ts"
ffmpeg -i emacsconf-a.ts -ss 3:00:20 -t 00:05:33 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Collaborative coding with tmux and tmate.ts"
ffmpeg -i emacsconf-a.ts -ss 3:05:56 -t 00:03:08 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Cask and Pellet.ts"
ffmpeg -i emacsconf-a.ts -ss 3:09:34 -t 00:08:16 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - File sharing with Git and save hooks.ts"
ffmpeg -i emacsconf-a.ts -ss 3:18:42 -t 00:14:38 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Calc.ts"
ffmpeg -i emacsconf-a.ts -ss 3:35:15 -t 00:14:27 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Magit.ts"
ffmpeg -i emacsconf-a.ts -ss 3:53:50 -t 00:08:08 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - gist.el.ts"
ffmpeg -i emacsconf-a.ts -ss 4:02:45 -t 00:13:52 -c:v copy -c:a copy "EmacsConf 2015 - Lightning talk - Go.ts"
ffmpeg -i emacsconf-a.ts -ss 4:16:50 -t 00:03:19 -c:v copy -c:a copy "EmacsConf 2015 - Question - Emacs Lisp backtraces.ts"

You can watch the Emacs Conference 2015 playlist on YouTube. At some point, each talk will probably have individual wiki pages and IRC logs at . =) Enjoy!

Related tech notes: Emacs Conf video tech notes:,, livestreamer, ffmpeg

Emacs Conf video tech notes:,, livestreamer, ffmpeg

Posted: - Modified: | emacs, geek, ffmpeg, streaming

Last week’s Emacs Conf was fantastic. There were lots of people at the in-person event in San Francisco, and people could also watch the stream through and ask questions through IRC. There were remote speakers and in-person speakers, and that mix even worked for the impromptu lightning talks sprinkled throughout the day.

This is how the tech worked:

  • Before the conference started, the organizers set up a laptop for streaming on This was hooked up to the main display (a large television with speakers). They also configured the account to record and archive videos. In the free account, recorded videos are available for 14 days.
  • Remote speakers were brought in using the Jitsi open source video conferencing system, using the public servers at This was on the same computer that did the streaming, so people watching the stream could see whatever was shared through Jitsi. Organizers read out questions from the in-person audience and from the IRC channel. The audio from Jitsi wasn’t directly available through, though. Instead, the audio came in as a recording from the laptop’s microphone.
  • Local speakers either used the streaming laptop to go to a specific webpage they wanted to talk about, or joined the Jitsi web conference using Google Chrome or Chromium so that they could share their screen. The organizers muted the second Jitsi client to avoid audio feedback loops.

That worked out really well. There were more than a hundred remote viewers. As one of them, I can definitely rate the experience as surprisingly smooth.

All that’s left now is to figure out how to make a more lasting archive of the Emacs Conf videos. As it turns out, or online tools don’t make it easy to download stream recordings that are longer than three hours. Fortunately, livestreamer can handle the job. Here’s what I did to download the timestream data from one of the recordings of EmacsConf:

livestreamer -o emacsconf-1.ts --hls-segment-threads 4 best
ffmpeg -i emacsconf-1.ts -acodec copy -absf aac_adtstoasc -vcodec copy emacsconf-1.mp4

I normally use Camtasia Studio to edit videos, but for some reason, it kept flaking out on me today. After the umpteenth crash, I decided to keep things simple by using ffmpeg to extract the relevant part of the video. To extract a segment, you can use -ss to specify the start time and t to specify the duration. Here’s a sample command:

ffmpeg -i emacsconf-1.mp4 -ss 1:18:06.11 -t 0:03:32.29 -c:v copy -c:a copy emacsconf-engine-mode.mp4

Your version of ffmpeg might have a -to option, which would let you specify the end time instead of using -t to specify duration.

I’m coordinating with the other organizers to see if there’s a better way to process the videos, so that’s why we haven’t released them publicly yet. (Soon!) It would be nice to improve the audio, especially for some of the talks, and maybe it would be good to add overlays or zoom in as well. The on-site organizers captured backup videos and screen recordings, too, so we might want to edit some of those clips into the streamed recording. One of the organizers has access to better video editing tools, so we’ll try that out.

Anyway, those were the commands that helped me get started with command-line conversion and editing of recorded videos. Hope they come in handy for other people too.

For more info about EmacsConf 2015, check out There’ll probably be an announcement there once the videos are up. =)

Hat tip to Reddit and for tips.