Cycles: Divide by zero with sky texture #83185

New Issue

Robert Guetzkow · 2020-11-29T14:37:29+01:00

Robert Guetzkow commented

2020-11-29 14:37:29 +01:00

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30

Blender Version
Broken: version: 2.92.0 Alpha, branch: master, commit date: 2020-11-29 08:04, hash: 13c820d87b
Worked: -

Short description of error
Using a Sky Texture node for the world background results in a divide by zero in background_cdf (light.cpp ), because cdf_total is zero.

Exact steps for others to reproduce the error

Start Blender with a debugger attached. Set a breakpoint on the line float cdf_total_inv = 1.0f / cdf_total; with the condition set to cdf_total == 0.0f.
Set the render engine to Cycles.
In the shader editor for the world, add a Sky Texture node and connect it to the Background node.
Set viewport shading to Rendered.

or

Start Blender with a debugger attached. Set a breakpoint on the line float cdf_total_inv = 1.0f / cdf_total; with the condition set to cdf_total == 0.0f.
Open attached file.

Set viewport shading to Rendered.

2020-11-29-sky-texture-div-by-zero.blend

**System Information** Operating system: Windows-10-10.0.19041-SP0 64 Bits Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30 **Blender Version** Broken: version: 2.92.0 Alpha, branch: master, commit date: 2020-11-29 08:04, hash: `13c820d87b` Worked: - **Short description of error** Using a *Sky Texture* node for the world background results in a divide by zero in `background_cdf` ([light.cpp ](https://developer.blender.org/diffusion/B/browse/master/intern/cycles/render/light.cpp)), because `cdf_total` is zero. **Exact steps for others to reproduce the error** - Start Blender with a debugger attached. Set a breakpoint on the line `float cdf_total_inv = 1.0f / cdf_total;` with the condition set to `cdf_total == 0.0f`. - Set the render engine to Cycles. - In the shader editor for the world, add a *Sky Texture* node and connect it to the *Background* node. - Set viewport shading to *Rendered*. or - Start Blender with a debugger attached. Set a breakpoint on the line `float cdf_total_inv = 1.0f / cdf_total;` with the condition set to `cdf_total == 0.0f`. - Open attached file. # Set viewport shading to *Rendered*. [2020-11-29-sky-texture-div-by-zero.blend](https://archive.blender.org/developer/F9407680/2020-11-29-sky-texture-div-by-zero.blend)

Robert Guetzkow commented

2020-11-29 14:37:29 +01:00

Added subscriber: @rjg

Robert Guetzkow commented

2020-11-29 14:39:22 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Robert Guetzkow commented

2020-11-29 14:39:22 +01:00

Added subscriber: @brecht

Robert Guetzkow commented

2020-11-29 14:39:22 +01:00

@brecht I'm setting this to high priority since this causes undefined behavior and could lead to a crash.

Marco commented

2020-11-29 16:29:55 +01:00

Added subscriber: @nacioss

Brecht Van Lommel commented

2020-11-29 17:25:01 +01:00

Float division by zero is not undefined behavior, it's well defined that the result is inf in this case. If it causes a crashes or other actual bug it might be high priority, but it's not clear to me from the report.

Float division by zero is not undefined behavior, it's well defined that the result is `inf` in this case. If it causes a crashes or other actual bug it might be high priority, but it's not clear to me from the report.

Brecht Van Lommel commented

2020-11-29 17:35:01 +01:00

The division by zero is harmless in this case, but the code can be tweaked a bit for clarity by moving it inside the if (cdf_total > 0.0f) test.

The division by zero is harmless in this case, but the code can be tweaked a bit for clarity by moving it inside the ` if (cdf_total > 0.0f)` test.

Robert Guetzkow commented

2020-11-29 18:14:41 +01:00

@brecht My bad, seems like the version of UBSan I'm using is being overly cautious as theoretically some compiler might not implement Annex F. GCC and CLANG have the expected IEEE-754 behavior.

See discussion in:
https://bugs.llvm.org/show_bug.cgi?id=19535
https://stackoverflow.com/a/51487703

@brecht My bad, seems like the version of UBSan I'm using is being overly cautious as theoretically some compiler might not implement Annex F. GCC and CLANG have the expected IEEE-754 behavior. See discussion in: https://bugs.llvm.org/show_bug.cgi?id=19535 https://stackoverflow.com/a/51487703

Robert Guetzkow commented

2020-11-29 19:07:39 +01:00

@brecht Does it make sense to add the check in this case then? Technically, it could cause undefined behavior for compilers that don't implement Annex F / IEEE-754, not sure if that is actually the case for any compilers that are used today.

Brecht Van Lommel commented

2020-11-30 13:42:20 +01:00

Changed status from 'Confirmed' to: 'Resolved'

Brecht Van Lommel closed this issue

2020-11-30 13:42:20 +01:00

Brecht Van Lommel self-assigned this 2020-11-30 13:42:20 +01:00

Brecht Van Lommel commented

2020-11-30 13:42:20 +01:00

Solved in c986e46.

Solved in c986e46.

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Cycles: Divide by zero with sky texture #83185

Set viewport shading to Rendered.