Based on http://jcgt.org/published/0008/01/03/
This is a simple trick that does *not* have a huge performance impact but
does work pretty well. It just modifies the Fresnel term to account for
the multibounce energy loss (coloration).
However this makes the shader variations count double. To avoid this we
use a uniform and pass the multiscatter use flag inside the sign of f90.
This is a bit hacky but avoids many code duplication.
This uses the simplification proposed by McAuley in
A Journey Through Implementing Multiscattering BRDFs and Area Lights
This does not handle area light differently than the IBL case but that's
already an issue in current implementation.
This is related to T68460.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D8912
This patch continue the efforts to split the `gpu_shader_material` file
started in D5569.
Dependency resolution is now recursive. Each shading node gets its own
file. Additionally, some utility files are added to be shared between
files, like `math_util`, `color_util`, and `hash`. Some files are always
included because they may be used in the execution function, like
`world_normals`.
Some glsl functions appeared to be unused, so they were removed, like
`output_node`, `bits_to_01`, and `exp_blender`. Other functions have
been renamed to be more general and get used as utils, like `texco_norm`
which became `vector_normalize`.
A lot of the opengl tests fails, but those same tests also fail in
master, so this is probably unrelated to this patch.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5616
