GLES uses a sample count of 0 to indicate non-multisampling, and a
sample count of 1 or greater to indicate multisampling. While the
Vulkan specification seems to support multisampling with a sample count
of 1, the following spec language effectively disallows it (even when
the underlying Vulkan driver supports it):
If the image was created with VkImageCreateInfo::samples equal to
VK_SAMPLE_COUNT_1_BIT, the instruction must have MS = 0.
The following GLES 3.1 dEQP tests use shaders that look for the sample
position, with a sample count of 1:
dEQP-GLES31.functional.texture.multisample.samples_1.*
For Vulkan drivers that fully support multisampling with a sample count
of 1, these tests pass, but get Vulkan validation errors because of the
above spec language.
This was discussed in the Khronos-private issue tracker (see:
https://gitlab.khronos.org/vulkan/vulkan/issues/1925). The Vulkan
working group wondered if ANGLE could live with not supporting a sample
count of 1 (but upgrading such requests to the next-largest supported
sample count, per the GLES specification). This change implements that
approach.
Bug: angleproject:4197
Bug: angleproject:4212
Change-Id: I009ba47b48efe5af464011c7c15022fc01f45e76
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1974842Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>
