feat(mp4): Add VOBSUB subtitle extraction with OCR for MP4 files

[cfsmp3]

Summary

• Adds support for extracting VOBSUB (bitmap) subtitles from MP4 files with OCR conversion to text formats
• Creates shared vobsub_decoder module for SPU parsing and OCR integration
• Detects subp:MPEG tracks in MP4 container and processes them through OCR pipeline

Changes

New Files

• src/lib_ccx/vobsub_decoder.c - VOBSUB decoder with SPU parsing and OCR
• src/lib_ccx/vobsub_decoder.h - Public API header

Modified Files

• src/lib_ccx/mp4.c - Add VOBSUB track detection and processing
• src/lib_ccx/matroska.c - Integrate shared VOBSUB decoder for MKV OCR support

Features

The VOBSUB decoder module provides:

• SPU control sequence parsing (timing, colors, coordinates)
• RLE-encoded bitmap decoding (interlaced format)
• Palette parsing from idx header format
• Integration with Tesseract OCR via ocr_rect()

Test Results

Tested with sample from issue #1349:

Track 3, type=subp subtype=MPEG
MP4: found 4 tracks: 1 avc, 0 hevc, 1 cc, 1 vobsub
Processing VOBSUB track (128 samples)
VOBSUB processing complete

Successfully extracted 61 subtitles with accurate OCR output:

1
00:22:31,000 --> 00:22:32,874
I have a message from the Shield of Light

2
00:22:33,333 --> 00:22:35,040
Remember the old ways

Test plan

• Compiles with OCR support (-DWITH_OCR=ON)
• Compiles without OCR support
• Extracts VOBSUB from MP4 with accurate OCR text
• Proper error message when OCR not available

Fixes #1349

🤖 Generated with Claude Code

[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 5beb438...:

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=ttxt --latin1 1974a299f0..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 132d7df7e9..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 99e5eaafdc..., Last passed: Never
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., 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 ec30a79...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	6/7
DVD	3/3
DVR-MS	2/2
General	25/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	80/86
Teletext	21/21
WTV	13/13
XDS	34/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65...
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b...
• ccextractor --out=spupng c83f765c66...
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.