GSplat: Initial rendering support for Cycles

Based on the "Stochastic ray tracing of transparent 3D Gaussians" paper
by Xin Sun et. al. The basic idea: perform stochastic intersection with
the Gaussian splat based on its transparency.

Gaussian splats are implemented as a dedicated primitive type, but it
shares the same layout for position and radius as points, so a lot of
existing functions (positions, attributes, etc) work for both points
and splats.

For the Embree and hardware intersection it is implemented as a custom
primitive type.

The choice of using bounding spheres mainly comes from a balance between
performance and memory usage. More ideal would be to use OBB, but it is
not supported for custom primitive types in Embree and GPU HW-RT on all
backends.

There is a known limitation that comes from the fact that the datasets
are trained in sRGB space and Cycles work in Linear space: areas with
low opacity and high radiance render noticeably differently from the
ground-truth implementation.

Ref #159470

Pull Request: https://projects.blender.org/blender/blender/pulls/163103

tests/python/CMakeLists.txt

@@ -925,6 +925,7 @@ if((WITH_CYCLES OR WITH_GPU_RENDER_TESTS) AND TEST_SRC_DIR_EXISTS)
     camera
     colorspace
     bsdf
+    gsplat
     hair
     image_colorspace
     image_data_types

tests/python/cycles_render_tests.py

@@ -351,6 +351,11 @@ def main():
     # OSL blackbody output is a little different.
     if (test_dir_name in {'colorspace'}):
         report.set_fail_threshold(0.05)
+    # Keeping Gaussian splats rendered exactly the same on CPU and GPU is
+    # tricky due to stochastic nature of the algorithm.
+    if (test_dir_name in {'gsplat'}):
+        report.set_fail_threshold(0.05)
+        report.set_fail_percent(4)
 
     ok = report.run(args.testdir, args.blender, get_arguments, batch=args.batch)
 

tests/python/eevee_render_tests.py

@@ -82,6 +82,7 @@ BLOCKLIST = [
     "image_mipmap_world_sun.blend",
 
     ### Cycles only tests go here ###
+    "gsplat.*.blend",
 ]
 
 BLOCKLIST_METAL = [

tests/python/storm_render_tests.py

@@ -50,6 +50,8 @@ BLOCKLIST_HYDRA = [
     "many_lights.blend",
     # The result differs between platforms
     "principled_bsdf_dispersion.blend",
+    # No Gaussian splat rendering in hydra.
+    "gsplat_.*.blend",
 ]
 
 BLOCKLIST_USD = [
@@ -70,6 +72,8 @@ BLOCKLIST_USD = [
     "osl_camera_.*.blend",
     # The result in incorrect
     "principled_bsdf_dispersion.blend",
+    # No Gaussian splat rendering in hydra.
+    "gsplat_.*.blend",
 ]
 
 # Metal support in Storm is no as good as OpenGL, though this needs to be