Ref T84819
Build System
============
This is an API breaking new version, and the updated code only builds with
OpenColorIO 2.0 and later. Adding backwards compatibility was too complicated.
* Tinyxml was replaced with Expat, adding a new dependency.
* Yaml-cpp is now built as a dependency on Unix, as was already done on Windows.
* Removed currently unused LCMS code.
* Pystring remains built as part of OCIO itself, since it has no good build system.
* Linux and macOS check for the OpenColorIO verison, and disable it if too old.
Ref D10270
Processors and Transforms
=========================
CPU processors now need to be created to do CPU processing. These are cached
internally, but the cache lookup is not fast enough to execute per pixel or
texture sample, so for performance these are now also exposed in the C API.
The C API for transforms will no longer be needed afer all changes, so remove
it to simplify the API and fallback implementation.
Ref D10271
Display Transforms
==================
Needs a bit more manual work constructing the transform. LegacyViewingPipeline
could also have been used, but isn't really any simpler and since it's legacy
we better not rely on it.
We moved more logic into the opencolorio module, to simplify the API. There is
no need to wrap a dozen functions just to be able to do this in C rather than C++.
It's also tightly coupled to the GPU shader logic, and so should be in the same
module.
Ref D10271
GPU Display Shader
==================
To avoid baking exposure and gamma into the GLSL shader and requiring slow
recompiles when tweaking, we manually apply them in the shader. This leads
to some logic duplicaton between the CPU and GPU display processor, but it
seems unavoidable.
Caching was also changed. Previously this was done both on the imbuf and
opencolorio module levels. Now it's all done in the opencolorio module by
simply matching color space names. We no longer use cacheIDs from OpenColorIO
since computing them is expensive, and they are unlikely to match now that
more is baked into the shader code.
Shaders can now use multiple 2D textures, 3D textures and uniforms, rather
than a single 3D texture. So allocating and binding those adds some code.
Color space conversions for blending with overlays is now hardcoded in the
shader. This was using harcoded numbers anyway, if this every becomes a
general OpenColorIO transform it can be changed, but for now there is no
point to add code complexity.
Ref D10273
CIE XYZ
=======
We need standard CIE XYZ values for rendering effects like blackbody emission.
The relation to the scene linear role is based on OpenColorIO configuration.
In OpenColorIO 2.0 configs roles can no longer have the same name as color
spaces, which means our XYZ role and colorspace in the configuration give an
error.
Instead use the new standard aces_interchange role, which relates scene linear
to a known scene referred color space. Compatibility with the old XYZ role is
preserved, if the configuration file has no conflicting names.
Also includes a non-functional change to the configuraton file to use an
XYZ-to-ACES matrix instead of REC709-to-ACES, makes debugging a little easier
since the matrix is the same one we have in the code now and that is also
found easily in the ACES specs.
Ref D10274
Cycles now uses the color space on the image datablock, and uses OpenColorIO
to convert to scene linear as needed. Byte images do not take extra memory,
they are compressed in scene linear + sRGB transfer function which in common
cases is a no-op.
Eevee and workbench were changed to work similar. Float images are stored as
scene linear. Byte images are compressed as scene linear + sRGB and stored in
a GL_SRGB8_ALPHA8 texture. From the GLSL shader side this means they are read
as scene linear, simplifying the code and taking advantage of hardware support.
Further, OpenGL image textures are now all stored with premultiplied alpha.
Eevee texture sampling looks a little different now because interpolation
happens premultiplied and in scene linear space.
Overlays and grease pencil work in sRGB space so those now have an extra
conversion to sRGB after reading from image textures. This is not particularly
elegant but as long as engines use different conventions, one or the other
needs to do conversion.
This change breaks compatibility for cases where multiple image texture nodes
were using the same image with different color space node settings. However it
gives more predictable behavior for baking and texture painting if save, load
and image editing operations have a single color space to handle.
Differential Revision: https://developer.blender.org/D4807
This is the groundwork for supporting loading image textures with arbitrary
color spaces through OpenColorIO.
Initial patch by Lukas, completed by Brecht.
Differential Revision: https://developer.blender.org/D3491
