Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 3060 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.89
Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash: rB0f45cab862b8
Worked: (newest version of Blender that worked as expected)
Short description of error
When rendering using OptiX on a RTX3060TI graphics card, placing 64 or more fully transparent (Alpha = 0) layers results in a shadow. This is irrespective of the transparent bounces setting in the render menu. When the renderer is set to OptiX CPU or to CUDA, this issue does not occur.
It is also noted that, under the visibility setting in the object menu, setting and unsetting the 'shadow' ray visibility option does not reliably enable or disable this option. Sometimes, when unsetting the shadow ray visibility option, the shadow is still rendered.
I have searched the forums and the bug tracker for this issue but have been unable to find it being reported. Since the issue occurs when 64 or more transparent layers are used, I strongly suspect a bug, or perhaps intentional limitation, in the OptiX renderer or driver. Or perhaps this is a little known limitation in the RTX ray tracing cores?
Exact steps for others to reproduce the error
Create 64 planes with full transparent material (either Principled BSDF Alpha=0 or Transparent BSDF) and place any light source directly above it. Ensure Alpha Blend and Shadow modes are set accordingly (Hashed or Blend). Ensure that transparency bounces are set to more than 64. Render using OptiX in GPU mode on an RTX 3060TI (Uncertain whether this issue is hardware related.). Do the same experiment with only 63 layers, and the shadow disappears. See the attached .blend file.