FEATURE: Add V_MPEG2 track support in MKV demuxer for CC extraction

[Varadraj75]

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Summary

CCExtractor's MKV demuxer only recognized V_MPEG4/ISO/AVC and V_MPEGH/ISO/HEVC
tracks, silently skipping V_MPEG2 tracks. MKV files with MPEG-2 video — common
in DVD-sourced content — produced no output at all.

Reported in #2149 by a user trying to extract CC3 captions from a Fairly OddParents DVD.

Changes

• Add mpeg2_codec_id = "V_MPEG2" to matroska.h
• Add mpeg2_track_number field to matroska_ctx struct
• Detect V_MPEG2 track during track entry parsing alongside AVC/HEVC
• Add process_mpeg2_frame_mkv() reusing the existing process_m2v()
  infrastructure (same path used by mp4.c and general_loop.c)
• Dispatch MPEG2 frames in parse_simple_block()
• Initialize, track and report mpeg2_track_number alongside AVC/HEVC

Testing

Tested with the sample from #2149 (cc3.mkv, V_MPEG2, captions on CC3/Field 2):

# Before: no output
ccextractor cc3.mkv --output-field 2   →  "Found no AVC/HEVC track. No captions found."

# After: captions extracted correctly
ccextractor cc3.mkv --output-field 2   →  "Found MPEG2 track." + full SRT output

Closes #2149

[Copilot]

Pull request overview

Adds Matroska (MKV) demuxer support for V_MPEG2 video tracks so CCExtractor can extract EIA-608/708 captions from MPEG-2-in-MKV content (e.g., DVD-sourced rips), addressing the “no output” scenario reported in #2149.

Changes:

• Add "V_MPEG2" codec-id recognition and track-number tracking in the Matroska parser.
• Dispatch MPEG-2 SimpleBlock frames through a new process_mpeg2_frame_mkv() that reuses the existing MPEG-2 elementary stream processing path (process_m2v()).
• Update Matroska loop reporting to include MPEG-2 track detection.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/lib_ccx/matroska.h	Adds MPEG-2 codec id constant, a new mpeg2_track_number field, and a new frame-processing function prototype.
src/lib_ccx/matroska.c	Detects MPEG-2 tracks, dispatches MPEG-2 frames for processing, initializes/report MPEG-2 track presence, and adjusts return behavior.

Comments suppressed due to low confidence (2)

src/lib_ccx/matroska.c:699

• In the non-video-track fast-path, skip_bytes(file, len - 1) assumes the TrackNumber VINT is always 1 byte. Track numbers in Matroska are variable-length VINTs, so this can seek to the wrong position and desynchronize parsing. Compute the remaining bytes to skip based on the block start (pos) and current file position instead of subtracting 1.

	int is_mpeg2 = (track == mkv_ctx->mpeg2_track_number);
	if (!is_avc && !is_hevc && !is_mpeg2)
	{
		// Skip everything except AVC/HEVC tracks
		skip_bytes(file, len - 1); // 1 byte for track
		return;

src/lib_ccx/matroska.c:699

• The comment says "Skip everything except AVC/HEVC tracks" but this block now also allows MPEG-2. Please update the comment to match the behavior so future changes don’t reintroduce accidental skips.

	{
		// Skip everything except AVC/HEVC tracks
		skip_bytes(file, len - 1); // 1 byte for track
		return;

---

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[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit 733ed89...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., Last passed: Never
• ccextractor --out=spupng c83f765c66..., Last passed: Never
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

All tests passed completely.

Check the result page for more info.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on windows. Below is a summary of the test results, when compared to test for commit 733ed89...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., Last passed: Never
• ccextractor --out=spupng c83f765c66..., Last passed: Never
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

All tests passed completely.

Check the result page for more info.