This significantly improve the depth-buffer resolution utilization
through the near-infinity range on Metal and Vulkan.
On OpenGL, this benefit is only seen when glClipControl is available.
The user-facing clip plane is unchanged [-1,1], the conversion
happens in filament's vertex shaders.
The bulk of this change consists in:
- invert the clip space's z in the vertex shader
- clear the depth buffer with 0
- invert all depth function comparisons
- fix all screen-space effects, e.g. ssao, DoF
- fix shadows and shadow biases
- use floating-point depth
- add a driver API to query which clip-space is used
- add glClipControl support to the gl backend
This is more similar to the Vulkan shader pipeline and less magical.
There are 3 places in our shader code where we perform fixups like this:
- main.vs
- depth_main.vs
- post_process_getters.vs
That last one needs no change since it does not involve Z.
This works by aliasing CUSTOM0 - CUSTOM7 to morphing attributes, and by
extending our existing skinning variant.
This PR was tested against some upcoming changes to gltfio.
Issue #1149, #1417
Recall that Vulkan has a right-handed NDC system. Currently, our Vulkan
backend is not handling VERTEX_DOMAIN_DEVICE correctly, but we didn't
notice because the culling mode is not honored yet (a separate PR is on
the way for that).
To fix this, we considered adding a shader-based fixup only for the
device domain and keeping our Vulkanish projection matrix as-is.
However, this would cause the skybox shader to compute an incorrect
wrong eye vector due to in the inconsistent definition of clip space.
After discussion with Mathias and Ben, we decided that the most elegant
fix is for Filament to have only one canonical clip space, which for
now is the clip space that OpenGL requires.
Ben pointed out that spirv-cross has a flag for injecting shader-based
fixups. However we don't invoke spirv-cross for the Vulkan target, and
it's easy just to do this on our own.
