JPEG Bitstream Reconstruction (`JxlEncoderAddJPEGFrame`) Failing for Some Images about libjxl HOT 3 OPEN

These JPEGs contain two quantization tables but only use one. There is no way in jxl to store a quantization table that is not used, so it is impossible to reconstruct the jpeg bitstreams in this case. That's why JXL_ENC_ERR_JBRD is returned.

This is a limitation of the jpeg bitstream reconstruction specification: jpeg quantization tables are restored based on the corresponding quantization table in the jxl codestream, but in jpeg you can specify a quantization table and then never use it (which is of course just a waste of bytes, and I wonder why any jpeg encoder would ever want to do that), while in jxl you can only specify quantization tables that are actually used.

The jpeg image data itself can be represented just fine when JXL_ENC_ERR_JBRD is returned, it's just impossible to create the jpeg bitstream reconstruction data that will allow you to reconstruct the exact same jpeg file.

One option is to fall back in this case to encoding with JxlEncoderStoreJPEGMetadata set to false. Another option is to 'sanitize' the jpeg with a tool like jpegtran, which will remove the unused quantization table and create a (smaller) jpeg file that can actually be reconstructed.

From the libjxl point of view, I don't think there's much we can do here...

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Thank you for getting back to me. I hadn't realised that there was an unused table. I'm sure I can figure out a way to strip that out. I'll give jpegtran a try and see how that goes.

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jpegtran does appear to strip out these unused tables just fine, allowing the image to be repackaged using libjxl. Thank you for the help. As libjpeg and jpegtran didn't complain about these files, and they open as usual in an image viewer/web browser, it was hard to identify the issue.

It may be helpful to be a little more verbose/granular in the libjxl errors, if possible, to help identify files such as this. I found JxlEncoderError JXL_ENC_ERR_JBRD a bit too broad to be helpful. Also, I was not sure what is meant by "too much tail data" and a quick web search did not help me here. I think this means extra data appended to the JPEG file after an EOI marker but it would be nice to have this documented somewhere as I had to guess what this means.

EDIT: Maybe just a list and a brief explanation of some common causes of JXL_ENC_ERR_JBRD in the docs would be enough to be very helpful with debugging.

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