NAME

       cjpeg_hdr - cjpeg_hdr

SYNOPSIS

       cjpeg_hdr input output.jpg

DESCRIPTION

       #  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 ```

AUTHOR

       This manual page was written  by  Mathieu  Malaterre  <malat@debian.org>  for  the  Debian
       GNU/Linux system (but may be used by others).