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Add djxl option to disable decoding to pixels #4498

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Merged
merged 3 commits into from
Nov 3, 2025
Merged

Add djxl option to disable decoding to pixels #4498

merged 3 commits into from
Nov 3, 2025

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@alistair7
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@alistair7 alistair7 commented Oct 30, 2025

Description

I noticed there's no easy way to tell djxl not to fall back on decoding to pixels if JPEG reconstruction fails, so I added a --reconstruct_jpeg flag that means "losslessly reconstruct a JPEG or do nothing". Mutually exclusive with --pixels_to_jpeg and --jpeg_quality.

I was going to extend --pixels_to_jpeg to take a 1 or 0 argument, with 0 having this effect, but decided that was more confusing (and less backwards compatible).

$ ./djxl -v -h
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
...
 -J, --reconstruct_jpeg
    Losslessly reconstruct a JPEG if possible, and fail otherwise.
...

New Behaviour

$ ./djxl good_jpeg.jxl reconstructed.jpg; echo $?
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
Reconstructed to JPEG.
4094 x 2894, 17.721 MP/s, 5.186 MB/s, 8 threads. 
0

$ ./djxl bad_jpeg.jxl reconstructed.jpg; echo $?
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
./lib/jxl/jpeg/jpeg_data.cc:121: JXL_FAILURE: Unknown app marker type 4
./lib/jxl/fields.cc:394: JXL_RETURN_IF_ERROR code=1: visitor.Visit(fields)
./lib/jxl/jpeg/dec_jpeg_data.cc:31: JXL_RETURN_IF_ERROR code=1: Bundle::Read(&br, jpeg_data)
Failed to decode image
Warning: could not decode losslessly to JPEG. Retrying with --pixels_to_jpeg...
Decoded to pixels.
4094 x 2894, 17.608 MP/s, 8 threads.
0

$ ./djxl --reconstruct_jpeg bad_jpeg.jxl reconstructed.jpg; echo $?
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
./lib/jxl/jpeg/jpeg_data.cc:121: JXL_FAILURE: Unknown app marker type 4
./lib/jxl/fields.cc:394: JXL_RETURN_IF_ERROR code=1: visitor.Visit(fields)
./lib/jxl/jpeg/dec_jpeg_data.cc:31: JXL_RETURN_IF_ERROR code=1: Bundle::Read(&br, jpeg_data)
Failed to decode image
Error: could not decode losslessly to JPEG.
1

$ ./djxl good_jpeg.jxl pixels.png; echo $?
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
Decoded to pixels.
4094 x 2894, 17.653 MP/s, 8 threads.
0

$ ./djxl --reconstruct_jpeg good_jpeg.jxl pixels.png; echo $?
JPEG XL decoder v0.12.0 bcb9a074 [_AVX2_,SSE4,SSE2] {GNU 13.3.0}
Error: --reconstruct_jpeg requires output format to be JPEG.
1

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@eustas
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eustas commented Oct 31, 2025

Hello. Thank you for your PR.

@eustas eustas added this pull request to the merge queue Nov 3, 2025
Merged via the queue into libjxl:main with commit 261599f Nov 3, 2025
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@alistair7 @eustas