System information (version)

• OpenCV => 4.4
• Operating System / Platform => MacOS Big Sur
• Compiler => Clang 12

Detailed description

The JPEG2000 standard defines both a "codestream" format and a file format for JP2K data. A codestream should ideally always be wrapped in the file format container, but in practice raw codestreams are sometimes found in the wild.

The OpenCV integration of OpenJPEG can read the JP2K file format, but not raw codestreams (also checked today's master). OpenJPEG itself has no problem reading codestreams (tested with the opj_decompress tool).

A codestream can be identified by its 4-byte header FF 4F FF 51.

OpenCV's JP2K file signature check should test for both the file header and the codestream header.

Steps to reproduce

Call cv::imread on the attached file.
balloon.j2c.zip