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MCC export: end each ancillary packet with its checksum byte so FFmpeg reads the captions (#544)

This commit is contained in:
MarekSpodBiedry 2026-10-10 09:14:06 +02:00
parent 7bd7621262
commit 544f561587
1 changed files with 10 additions and 2 deletions
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12
crates/captions/src/mcc.rs
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@ -17,7 +17,8 @@
//! - **CDP (SMPTE ST 334-2):** identifier `96 69`, length, frame-rate code, flags, sequence
//! counter, a `cc_data` section (`72`, `E0 | cc_count`, cc_count × (marker/valid/type, two
//! bytes)), optional time code / service info sections, footer `74`, the counter again and a
//! checksum that makes the packet's byte sum 0 mod 256.
//! checksum that makes the packet's byte sum 0 mod 256. The ancillary packet then ends with one ST 291 checksum byte
//! (low 8 bits of DID + SDID + data count + CDP bytes).
//! - **Writing** (29.97 fps, `30DF` or `30` timecode labels): every frame from the first caption
//! data to one frame after the last carries a CDP with 20 cc_data triplets: field 1 CEA-608
//! pop-on data (the same schedule as [SCC](crate::scc)), a null field-2 pair, CEA-708
@ -161,6 +162,9 @@ pub fn build_cdp(seq: u16, data: &FrameData) -> Vec<u8> {
cdp.push(((256 - sum % 256) % 256) as u8);
let mut out = vec![0x61, 0x01, cdp.len() as u8];
out.extend(cdp);
// SMPTE ST 291 ancillary checksum: low 8 bits of the sum of DID, SDID, data count and the user data words.
let anc_sum: u32 = out.iter().map(|&b| b as u32).sum();
out.push((anc_sum & 0xff) as u8);
out
}
@ -661,7 +665,11 @@ mod tests {
let d = FrameData { f1: Some((0x14, 0x20)), dtvcc: vec![(true, 0x02, 0x21), (false, 0x89, 0x01)] };
let b = build_cdp(7, &d);
assert_eq!(&b[..3], &[0x61, 0x01, 73]);
let cdp = &b[3..];
// prefix (3) + CDP (73) + ancillary checksum byte (1)
assert_eq!(b.len(), 77);
let sum: u32 = b[..76].iter().map(|&x| x as u32).sum();
assert_eq!(b[76], (sum & 0xff) as u8, "ancillary checksum");
let cdp = &b[3..76];
assert_eq!(cdp.len(), 73);
assert_eq!(&cdp[..9], &[0x96, 0x69, 73, 0x4f, 0x43, 0, 7, 0x72, 0xf4]);
assert_eq!(cdp.iter().map(|&x| x as u32).sum::<u32>() % 256, 0, "checksum");