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bflatasticandClaude Opus 5.5
ac77b52ad3
document.export {format: "png8" | "gif"} reduces the art to a palette of at most `colors`
(2–256) entries: median cut over a 15-bit histogram refined by k-means (perceptual,
selective or adaptive weighting), or the web-safe, black-and-white or grey palettes; art
with that many colours or fewer keeps them exactly. Dithering is Floyd–Steinberg diffusion,
an 8×8 ordered pattern or noise, with an amount. Pixels under half opacity become one
transparent entry; partly transparent edges blend over the matte. PNG-8 is an indexed PNG
(PLTE, tRNS, 1–8 bits per pixel, Adam7); GIF is a GIF89a (optionally interlaced). Both are in
document.formats, Export As (PNG-8 / GIF Options dialogs) and Export for Screens.
document.formats' writable list and the MCP export tool's format enum gain gif and png8
(their tests list them).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
34 lines
1.7 KiB
Rust
34 lines
1.7 KiB
Rust
//! GIF export: a palette image ([`super::quantize`]) as a one-frame GIF89a with its transparent
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//! entry, optionally interlaced (rows in four passes, so the image builds up while it loads).
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use std::borrow::Cow;
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use super::quantize::Indexed;
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/// Encode `img` as a GIF file.
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pub fn encode(img: &Indexed, interlaced: bool) -> Result<Vec<u8>, String> {
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let fail = |e: gif::EncodingError| format!("GIF encoding failed: {e}");
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let (Ok(w), Ok(h)) = (u16::try_from(img.width), u16::try_from(img.height)) else {
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return Err(format!("{} × {} pixels is too large for GIF (at most 65535 pixels a side): lower the resolution", img.width, img.height));
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};
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if w == 0 || h == 0 || img.indices.len() != w as usize * h as usize || img.palette.is_empty() || img.palette.len() > 256 {
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return Err("GIF encoding failed: the palette image is malformed".into());
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}
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let buffer = if interlaced { Cow::Owned(interlace(&img.indices, w as usize)) } else { Cow::Borrowed(img.indices.as_slice()) };
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let mut enc = gif::Encoder::new(Vec::new(), w, h, &img.palette.concat()).map_err(fail)?;
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let frame = gif::Frame { width: w, height: h, buffer, transparent: img.transparent, interlaced, ..gif::Frame::default() };
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enc.write_frame(&frame).map_err(fail)?;
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enc.into_inner().map_err(fail)
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}
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/// The rows of a `width`-wide image in GIF's interlaced order: every 8th row from 0, every 8th
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/// from 4, every 4th from 2, then every 2nd from 1.
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fn interlace(px: &[u8], width: usize) -> Vec<u8> {
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let rows: Vec<&[u8]> = px.chunks(width.max(1)).collect();
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[(0, 8), (4, 8), (2, 4), (1, 2)]
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.into_iter()
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.flat_map(|(start, step)| rows.iter().skip(start).step_by(step))
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.flat_map(|r| r.iter().copied())
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.collect()
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}
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