[cairo] Image Format / Pre-Multiplied Alpha Related Feature Request

[William Kappler]

Hello,

I am currently working on a 3D graphics/GUI project utilizing Cairo and
Pango, primarily for text rendering. However, I have run into an issue with
how Cairo handles its image data, and wanted to make a feature request
regarding that.

Currently, Cairo uses pre-multiplied alpha for CAIRO_FORMAT_ARGB32, which
is the only format currently supported combining color and alpha data. I
require the capability to have a full RGBA texel, but alpha
pre-multiplication presents several major issues for my use, or indeed any
use, of Cairo in 3D contexts:

   - Most libraries, shaders, and users/programmers expect textures in 3D
   contexts to be non-pre-multiplied. My system is no different – for various
   reasons, I need to use standard independent alpha for textures not
   generated by Cairo. I cannot easily include special handling of Cairo
   textures to sidestep that as it would make texture handling needlessly
   complex. I don't want to have to know which library the texture came from
   in a GLSL shader.


   - Reversing pre-multiplication is fairly simple, but still requires
   special conversion functionality in my library.


   - Conversion of texels introduces unnecessary texture write latency,
   especially for large images.


   - Most importantly, the dynamic range of the color is effectively
   reduced to 256*(1-alpha%) values, even on back-converted texels. For nearly
   transparent textures, this is especially problematic.
      - A texel with independent alpha has 256 discrete values (counting
      zero) per channel no matter its alpha
      - A pre-multiplied alpha texel with 50% opacity only supports 128
      discrete values per channel rounding issues make that even worse.
      - Going down to 12.5% opacity, only 32 values are possible per
      channel.
      - All this makes grayscale images that might have low opacity, which
      I expect I will use Pango-Cairo for fairly often, extremely poor quality.

As such, I would like to make a feature request that Cairo implement one of
the following:

   - The option to turn off pre-multiplied alpha in the existing
   CAIRO_FORMAT_ARGB32 format, without alpha reducing the range of possible
   values in the format.


   - A floating point image format, which would ease back-conversion of the
   pre-multiplication and eliminate most of the dynamic range issues.


   - An extended size integer format, such as 32 bits per channel ARGB /
   128 total, which would eliminate the dynamic range issues.

I’d prefer the options in that order, with the simple ability to turn off
premult the best from where I stand, but any would solve my problem and I
would be extremely appreciative if any could be implemented.

Thank you in advance for any assistance you might offer. If desired I can
help implement this change, but I have no prior experience with the
internals of Cairo, only experience with my usage of it thus far.

:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-
William E. Kappler II
LinkedIn <https://www.linkedin.com/in/williamkappler> · Blog
<http://williamkappler.blogspot.com/> · Project Website
<https://github.com/WilliamKappler/onathacar/wiki>
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