Entry 241

A shot run as an animated WebP

2026-09-27tooling

Filescrates/piney-game/src/webp.rscrates/piney-game/src/main.rscrates/piney-game/Cargo.toml

Asked for: a way to share what the port draws, such as STRT.BIN's scenes (worklog 240), as animated WebP.

--shot takes new options:

• --webp OUT.webp writes the run's frames as one animated WebP, looping.
• --webp-from F skips the frames before F.
• --webp-every N keeps every Nth frame.
• --webp-quality Q sets the lossy quality (default 80).

Each kept frame lasts the vertical blanks the game spent up to the next one (the mode's frame rate times N) at 59.94 a second. So the file runs at the game's own speed. webp::Recorder encodes with webp-animation (libwebp's animation encoder), reading each frame back from the GS after it is drawn.

The first encode had blocky checkers over the Mutation demo title's logo. ImageMagick's decoder, which goes through libwebp, showed them too, so they were in the file. With no key frames after the first, libwebp encodes each frame as a blended difference. It marks the pixels it takes for unchanged transparent, and over lossy frames the error builds up into blocks. With every frame a key frame (kmax 1) the frames match the PNG shots. One loop of STRT stream 2 (453 frames, 15.1 s) is 8.2 MB, where the diffed file was 5.8 MB.

The file's frame times were read back from its ANMF chunks: 15,115 ms in all, 453 game frames at rate 2. libwebp merges identical consecutive frames into longer ones. The test frames_timed_on_the_game_clock checks three frames' ANMF durations against the game clock.

Tried from the command line as --webp out.webp --shot, it wrote nothing. A --shot with no file after it started no headless run, so the window came up and ignored --webp. Now:

• --webp alone, or a bare --shot, runs headless;
• --shot OUT.webp records the WebP instead of writing PNG bytes under that name;
• with no --frames, a mode that ends by itself (Mode::ends, the loose streams) runs until it has played through once (Mode::finished), rather than 60 frames. The frame that would start it over is not recorded.

The written files are printed with their full paths.