Loop Cover skill

Looping cover GIF

A cover that moves gets noticed in a feed full of stills. It only works if the move is small, the loop never jumps and the first frame still makes sense on its own.

Before you start: read brand.md and MOTION.md if they exist. If not, run brand-intake first, or ask for hex codes, a font and a logo file. Never call a result on-brand without them.

Step 1: Ask for three things, then wait

1. The cover. A finished still (PNG) or a cover built in HTML. 896 x 640, or a larger image with the same 7:5 shape.
2. The one element that moves, named plainly. "The lighthouse beam." "The orange line drawing itself." "The lines between the cells."
3. How it moves. A pulse, a draw-on, a slow drift or a glow. One move, not three.

Step 2: Build it

If the cover is HTML: animate only the named element as a pure function of time, add window.seek(seconds), and capture 180 frames at 20fps (9 seconds).

If the cover is an image: animate the still in code. Make a mask of the named element by colour (for example every pixel where blue is much stronger than red), soften its edge a few pixels, and change only those pixels, frame by frame. Nothing outside the mask moves.

For both, use one clean sine cycle or one draw-on, hold and fade across the 9 seconds, so the last frame flows into the first.

Rotate the loop so frame 1 is the finished picture. Inboxes and link previews show frame 1 as a still. Frame i draws the moment ((i + offset) mod 180) / 20 seconds, where offset puts the fully drawn state first.

Encode with no dithering, because dither adds flicker to every frame of a flat image:

ffmpeg -y -framerate 20 -i f%03d.png -filter_complex "split[x][y];[x]palettegen=max_colors=128:stats_mode=full[p];[y][p]paletteuse=dither=none" -loop 0 cover.gif

Step 3: Measure it

ffmpeg -v error -y -i cover.gif -vf scale=216:154,format=gray -f rawvideo frames.raw
python3 -c "
n = 216 * 154
d = open('frames.raw', 'rb').read()
f = [d[i:i + n] for i in range(0, len(d), n)]
diff = lambda a, b: sum(abs(x - y) for x, y in zip(a, b)) / n
print('frames', len(f))
print('seam', round(diff(f[-1], f[0]), 2))
print('excursion', round(max(diff(x, f[0]) for x in f), 2))
"

Targets from covers I shipped: 180 frames, a seam of 0.0 to 0.3, an excursion of about 0.7 to 2.8, and a file between 0.6 and 2.3 MB. An excursion near zero means nothing visible moves.

What I learned the hard way

• "You changed the image entirely." My first GIF swept a band across the whole frame. Animate only the thing that was named.
• A video model adds a camera move. Told "camera holds completely still", it still pushed in. Animating the still in code is the only way to lock the camera.
• Never measure motion at the halfway frame. A sine is back at zero there, so a working animation reports no motion. Measure the biggest change against frame 0.
• In ffmpeg's eq filter, add eval=frame, or the expression runs once and nothing moves.
• Shrink it to 240px wide and look again. That is the size in an inbox. If the moving element disappears, it is too small.
• Upload the GIF itself. Some upload tools convert to PNG and the animation is silently gone. Check the uploaded link still ends in .gif.