singe/thirdparty/SDL3_image/external/libjxl/tools

High bit depth JPEG encoder

cjpeg_hdr is an (experimental) JPEG encoder that can preserve a higher bit depth than a traditional JPEG encoder. In particular, it may be used to produce HDR JPEGs that do not show obvious signs of banding.

Note that at this point in time cjpeg_hdr does not attempt to actually compress the image - it behaves in the same way as a "quality 100" JPEG encoder would normally do, i.e. no quantization, to achieve the maximum possible visual quality. Moreover, no Huffman optimization is performed.

Generating HBD JPEGs

Note: this and the following sections assume that libjxl has been built in the build/ directory, either by using CMake or by running ./ci.sh opt.

It should be sufficient to run build/tools/cjpeg_hdr input_image output.jpg. Various input formats are supported, including NetBPM and (8- or 16-bit) PNG.

If the PNG image includes a colour profile, it will be copied in the resulting JPEG image. If this colour profile approximates the PQ or HLG transfer curves, some applications will consider the resulting image to be HDR.

To attach a PQ profile to an image without a colour profile (or with a different colour profile), the following command can be used:

 build/tools/decode_and_encode input RGB_D65_202_Rel_PeQ output_with_pq.png 16

Similarly, to attach an HLG profile, the following command can be used

 build/tools/decode_and_encode input RGB_D65_202_Rel_HLG output_with_pq.png 16

Decoding HBD JPEGs

HBD JPEGs are fully retrocompatible with libjpeg, and any JPEG viewer ought to be able to visualize them. Nonetheless, to achieve the best visual quality, a high bit depth decoder should be used.

Such a decoder does not exist today. As a workaround, it is possible to do a lossless conversion to JPEG XL and then view the resulting image:

  build/tools/cjxl --jpeg_transcode_disable_cfl hbd.jpeg hbd.jxl

The resulting JPEG XL file can be visualized, for example, in a browser, assuming that the corresponding flag is enabled in the settings.

In particular, if the HBD JPEG has a PQ or HLG profile attached and the current display is an HDR display, Chrome ought to visualize the image as HDR content.

It is also possible to convert the JPEG XL file back to a 16-bit PNG:

  build/tools/djxl hbd.jxl --bits_per_sample=16 output.png

Note however that as of today (2 Nov 2021) Chrome does not interpret such a PNG as an HDR image, even if a PQ or HLG profile is attached. Thus, to display the HDR image correctly it is recommended to either display the JPEG XL image directly or to convert the PNG to a format that Chrome interprets as HDR, such as AVIF. This can be done with the following command for a PQ image:

  avifenc -l -y 444 --depth 10 --cicp 9/16/9 image.png output.avif

and the following one for an HLG image:

  avifenc -l -y 444 --depth 10 --cicp 9/18/9 image.png output.avif