[FFmpeg-trac] #10407(avcodec:new): Incorrect decode of 32 bit DWA encoded EXR

[FFmpeg]

#10407: Incorrect decode of 32 bit DWA encoded EXR
-------------------------------------+-------------------------------------
             Reporter:               |                     Type:  defect
  slipperyseal                       |
               Status:  new          |                 Priority:  normal
            Component:  avcodec      |                  Version:  git-
             Keywords:  exr dwaa     |  master
  dwab                               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 I created 16 and 32 bit DWA encoded EXR images from the same source image
 (from Blender).

 16 bit DWA files decode fine in ffmpeg while there seems to be an issue
 with 32 bit DWA.

 Looking at the code I can see when decoding DWA EXR files, ffmpeg runs 16
 bit or 32 bit decoders but always targets a 32bit space. There is common
 code which then transforms that intermediate output to linear space.


 {{{
 libavcodec/exr.c
 }}}

 On line 1122 is a scaling factor for the linear space conversion.

 {{{
 const float scale = s->pixel_type == EXR_FLOAT ? 2.f : 1.f;
 }}}

 When pixels are EXR_FLOAT (32 bit) the scaling factor is set to 2.f which
 produces underexposed images.
 If I set the scaling factor to always be 1.0, both file conversions appear
 correct and "identical".

 {{{
 const float scale = 1.f;
 }}}

 With this change applied, I logged pixel values (prior to apply_trc) and
 saw the results almost identical (probably because of float size
 precision).

 Perhaps this scaling factor was originally required at one time but now is
 compensated for in the 32 bit decoder?

 I havn't been able to locate any DWA files other than those I generate
 with Blender, but I can confirm that when importing both types into
 DaVinci Resolve, they appear identical. Therefor I don't think the 32 bit
 file is at fault.

 Note that a RGB colour space conversion also needs to be applied when
 converting EXRs.

 {{{
 # Convert dwaa_16_0001.exr to png

 ffmpeg -y -start_number 1 -f image2 -apply_trc iec61966_2_1 -i
 dwaa_16_%04d.exr dwaa_16_%04d.png

 # Convert dwaa_32_0001.exr to png

 ffmpeg -y -start_number 1 -f image2 -apply_trc iec61966_2_1 -i
 dwaa_32_%04d.exr dwaa_32_%04d.png
 }}}
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Ticket URL: <https://trac.ffmpeg.org/ticket/10407>
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