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dvdsub: studio-range YCbCr palette conversion, warn on truncated SPU

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This commit is contained in:
Matthew Jackson
2026-10-01 14:36:14 -07:00
parent b62efb79a8
commit 82217737e1
1 changed files with 40 additions and 31 deletions
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src/mux/codec/dvdsub.rs
+40 -31
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@@ -45,7 +45,15 @@ impl DvdSubParser {
// `complete` is only true when `self.pending` was `Some` above, but
// borrow through `?` rather than a prod `unwrap()` on a fact the
// compiler can't see across the two lines.
let (facts, _, data) = self.pending.take()?;
let (facts, size, data) = self.pending.take()?;
if data.len() < size {
tracing::warn!(
target: "mux",
have = data.len(),
declared = size,
"dvdsub: emitting a truncated subpicture unit"
);
}
// Guaranteed Some: `pending` is only ever created from a PES with
// `pts.is_some()` (L054 dropped the no-PTS orphan path), so this
// never takes the fallback; kept as unwrap_or, never a prod unwrap.
@@ -165,7 +173,6 @@ impl CodecParser for DvdSubParser {
// ── YCbCr → RGB conversion and palette formatting ─────────────────────────
// Byte-2-as-Cb is invisible on achromatic (Cb=Cr=128) palette entries.
/// Convert a single YCbCr color to RGB, clamping to [0, 255].
///
/// Input: `[padding, Y, Cr, Cb]` (as stored in DVD IFO PGC data). Note the
@@ -174,15 +181,17 @@ impl CodecParser for DvdSubParser {
///
/// Returns `[R, G, B]`.
///
/// Uses full-range (JFIF) BT.601 coefficients, matching the VobSub `.idx` on-disk convention.
/// The DVD CLUT is studio-range (Y 16..=235, chroma 16..=240), so this applies
/// the limited-range BT.601 expansion: Y=16 maps to black, Y=235 to white. The
/// result is the full-range RGB a VobSub `.idx` palette carries.
pub fn ycbcr_to_rgb(color: &[u8; 4]) -> [u8; 3] {
let y = color[1] as f64;
let cr = color[2] as f64;
let cb = color[3] as f64;
let y = 1.164 * (color[1] as f64 - 16.0);
let cr = color[2] as f64 - 128.0;
let cb = color[3] as f64 - 128.0;
let r = y + 1.402 * (cr - 128.0);
let g = y - 0.344 * (cb - 128.0) - 0.714 * (cr - 128.0);
let b = y + 1.772 * (cb - 128.0);
let r = y + 1.596 * cr;
let g = y - 0.392 * cb - 0.813 * cr;
let b = y + 2.017 * cb;
[clamp_u8(r), clamp_u8(g), clamp_u8(b)]
}
@@ -424,22 +433,16 @@ mod tests {
#[test]
fn ycbcr_to_rgb_white() {
// White in YCbCr: Y=235, Cb=128, Cr=128 → R=235, G=235, B=235
// Studio-range white: Y=235 at neutral chroma expands to full white.
let color = [0x00, 235, 128, 128];
let [r, g, b] = ycbcr_to_rgb(&color);
assert_eq!(r, 235);
assert_eq!(g, 235);
assert_eq!(b, 235);
assert_eq!(ycbcr_to_rgb(&color), [255, 255, 255]);
}
#[test]
fn ycbcr_to_rgb_black() {
// Black: Y=16, Cb=128, Cr=128 → R=16, G=16, B=16
// Studio-range black: Y=16 at neutral chroma is true black.
let color = [0x00, 16, 128, 128];
let [r, g, b] = ycbcr_to_rgb(&color);
assert_eq!(r, 16);
assert_eq!(g, 16);
assert_eq!(b, 16);
assert_eq!(ycbcr_to_rgb(&color), [0, 0, 0]);
}
#[test]
@@ -465,9 +468,6 @@ mod tests {
// Approximate red: Y=82, Cr=240, Cb=90 — on disc as [pad, Y, Cr, Cb].
let color = [0x00, 82, 240, 90];
let [r, g, b] = ycbcr_to_rgb(&color);
// R = 82 + 1.402*(240-128) = 82 + 156.9 ≈ 239
// G = 82 - 0.344*(90-128) - 0.714*(240-128) = 82 + 13.1 - 79.97 ≈ 15
// B = 82 + 1.772*(90-128) = 82 - 67.3 ≈ 15
assert!(r > 200, "R should be high for red, got {}", r);
assert!(g < 30, "G should be low for red, got {}", g);
assert!(b < 30, "B should be low for red, got {}", b);
@@ -486,7 +486,7 @@ mod tests {
"on-disc red [0,Y=76,Cr=255,Cb=85] must render red-dominant, \
got R={r} G={g} B={b} (R and B swapped => byte 2/3 are transposed)"
);
assert_eq!([r, g, b], [254, 0, 0], "exact full-range BT.601 red");
assert_eq!([r, g, b], [255, 0, 0], "exact studio-range BT.601 red");
// And the converse: a saturated BLUE on-disc entry (Y=29, Cr=107, Cb=255)
// must not come out red.
@@ -545,8 +545,8 @@ mod tests {
#[test]
fn format_palette_hex_format() {
// Y=128, Cb=128, Cr=128 → R=128, G=128, B=128 → "808080"
let palette = vec![[0x00, 128, 128, 128]];
// Y=126 at neutral chroma → R=G=B=128 → "808080"
let palette = vec![[0x00, 126, 128, 128]];
let result = format_palette(&palette, 0, 0);
let text = String::from_utf8(result).unwrap();
assert_eq!(text, "palette: 808080\n");
@@ -556,7 +556,7 @@ mod tests {
fn format_palette_emits_size_line_before_palette() {
// With non-zero dimensions the `.idx` `size:` line is prepended ahead of
// the palette so players place/scale the VobSub bitmap (PAL 720x576).
let palette = vec![[0x00, 128, 128, 128]];
let palette = vec![[0x00, 126, 128, 128]];
let result = format_palette(&palette, 720, 576);
let text = String::from_utf8(result).unwrap();
assert_eq!(
@@ -569,7 +569,7 @@ mod tests {
fn format_palette_omits_size_line_when_dimensions_unknown() {
// 0 width/height (unknown resolution) omits the size line rather than
// emitting a 0x0 frame; the palette line is still present.
let palette = vec![[0x00, 128, 128, 128]];
let palette = vec![[0x00, 126, 128, 128]];
let result = format_palette(&palette, 0, 576);
let text = String::from_utf8(result).unwrap();
assert_eq!(text, "palette: 808080\n", "no size line when a dim is 0");
@@ -717,12 +717,21 @@ mod tests {
#[test]
fn ycbcr_blue_channel_clamps_high() {
// B = Y + 1.772*(Cb-128). Y=128, Cb=255 → 128 + 1.772*127 ≈ 353 → clamp 255.
// B = 1.164*(Y-16) + 2.017*(Cb-128) ≈ 130 + 256 → clamp 255.
// Cb is byte 3 in the on-disc [pad, Y, Cr, Cb] layout.
let [_r, _g, b] = ycbcr_to_rgb(&[0x00, 128, 128, 255]);
assert_eq!(b, 255, "blue clamps at 255");
}
#[test]
fn ycbcr_to_rgb_green_uses_both_chroma_terms() {
// Y=128, Cr=100, Cb=90: G = 1.164*112 - 0.392*(-38) - 0.813*(-28) = 167.9.
assert_eq!(ycbcr_to_rgb(&[0x00, 128, 100, 90])[1], 168);
// Each chroma term alone: Cr moves G by -0.813/step, Cb by -0.392/step.
assert_eq!(ycbcr_to_rgb(&[0x00, 128, 100, 128])[1], 153);
assert_eq!(ycbcr_to_rgb(&[0x00, 128, 128, 90])[1], 145);
}
#[test]
fn format_palette_empty_is_just_prefix() {
// An empty palette yields "palette: \n" (prefix + newline, no entries).
@@ -733,9 +742,9 @@ mod tests {
#[test]
fn format_palette_pads_each_channel_to_two_hex_digits() {
// Each RGB channel is formatted as exactly 2 hex digits (zero-padded).
// Y=16,neutral → 0x10 → "101010" (each channel two digits).
let result = format_palette(&[[0x00, 16, 128, 128]], 0, 0);
assert_eq!(String::from_utf8(result).unwrap(), "palette: 101010\n");
// Y=20,neutral → 5 → "050505" (each channel two digits).
let result = format_palette(&[[0x00, 20, 128, 128]], 0, 0);
assert_eq!(String::from_utf8(result).unwrap(), "palette: 050505\n");
}
// Text guard in codec/mod.rs can't see `source: facts.source` with no