System Information

OpenCV version: 4.6.0
Operating System / Platform: Windows 11
Compiler & compiler version: Visual Studio 2022

Detailed description

VideoCapture with the FFmpeg back end returns the incorrect codec fourcc when the codec_tag represents the underlying stream type.

The existing logic is to return the codec_tag if it exists and igored the codec_id

This is fine if the codec_tag is valid but, if it is invalid and the codec_id is valid this logic fails.

For example this MPEG-TS video has a valid
codec_id== AV_CODEC_ID_HEVC (173)
and invalid
codec_tag ==36
the MPEG-TS stream type.

Additonaly because the codec_tag is

/**
 * Additional information about the codec (corresponds to the AVI FOURCC).
 */
uint32_t         codec_tag;

the fourcc returned for the three test video's below is mp4v

highgui/video/big_buck_bunny.mp4
highgui/video/sample_322x242_15frames.yuv420p.mpeg2video.mp4
highgui/video/sample_322x242_15frames.yuv420p.mjpeg.mp4

when they use MPEG-4, MPEG-2 and MJPG respectively.

I can think of easy fixes:

1. Only use the codec_tag if the codec_id is unavaliable. This may pass all the test cases but break existing user code.
2. Add an additional property, i.e. CAP_PROP_CODEC_ID_FOURCC or similar, which a user can fall back on if the fourcc returned from CAP_PROP_FOURCC is garbage. This is clunky but it is guaranteed not to break any existing user code.

Steps to reproduce

Video from https://4kmedia.org/lg-new-york-hdr-uhd-4k-demo/

VideoCapture cap("LG New York HDR UHD 4K Demo.ts",CAP_FFMPEG);
const double fourcc = cap.get(CAP_PROP_FOURCC);

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)