GLSL 'clamp' issue? #83494

New Issue

Stanislav Blinov · 2020-12-07T11:13:19+01:00

Stanislav Blinov commented

2020-12-07 11:13:19 +01:00

System Information
Operating system: Linux 5.9.10-arch1-1
Graphics card: NVIDIA GTX 980Ti 455.45.01

Blender Version
Broken: 2.91, 2.92 master
Worked: ???

Short description of error

According to GLSL spec, clamp(x, a, b) == min(max(x,a), b). So then, clamp(0, 1, 0) should yield 0. But that doesn't seem to be the case, at least on my system. With this node setup:

...the output should be black. However...

Eevee material preview:

Eevee render:

Cycles render:

Exact steps to reproduce error

Switch to the Shading workspace
Remove Principled BSDF
Add the Converter -> Clamp node
Set mode to 'Min Max', Value to 0, Min to 1 and Max to 0
Connect Clamp's output to the Material Output (Surface)

Observe difference between material/render with Eevee vs. Cycles. Every time Eevee is used (material preview and render), the ouput will be white, though it should be black. Cycles render produces correct result.

Sample blend file:
clamp_issue.blend

**System Information** Operating system: Linux 5.9.10-arch1-1 Graphics card: NVIDIA GTX 980Ti 455.45.01 **Blender Version** Broken: 2.91, 2.92 master Worked: ??? **Short description of error** According to GLSL spec, clamp(x, a, b) == min(max(x,a), b). So then, clamp(0, 1, 0) should yield 0. But that doesn't seem to be the case, at least on my system. With this node setup: ![image.png](https://archive.blender.org/developer/F9487762/image.png) ...the output should be black. However... Eevee material preview: ![image.png](https://archive.blender.org/developer/F9487778/image.png) Eevee render: ![image.png](https://archive.blender.org/developer/F9487782/image.png) Cycles render: ![image.png](https://archive.blender.org/developer/F9487786/image.png) **Exact steps to reproduce error** - Switch to the Shading workspace - Remove Principled BSDF - Add the Converter -> Clamp node - Set mode to 'Min Max', Value to 0, Min to 1 and Max to 0 - Connect Clamp's output to the Material Output (Surface) Observe difference between material/render with Eevee vs. Cycles. Every time Eevee is used (material preview and render), the ouput will be white, though it should be black. Cycles render produces correct result. Sample blend file: [clamp_issue.blend](https://archive.blender.org/developer/F9500498/clamp_issue.blend)

Stanislav Blinov commented

2020-12-07 11:13:20 +01:00

Added subscriber: @Stan_Pancakes

Germano Cavalcante commented

2020-12-10 03:38:37 +01:00

Added subscribers: @fclem, @brecht, @mano-wii

Germano Cavalcante commented

2020-12-10 03:38:37 +01:00

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

Germano Cavalcante closed this issue

2020-12-10 03:38:37 +01:00

Germano Cavalcante commented

2020-12-10 03:38:37 +01:00

Thanks for the report, but I would not say that this is a bug.
It is expected that users set the maximum value to something always greater than the minimum.
If they don't, the result is unpredictable and will depend on the engine.
Apparently in Eevee it was decided that the minimum value prevails while in Cycles it is the maximum value.

Although consistency is more appreciated, this is something to decide outside the bug tracker. (https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests).
So I'm closing the report for now.

(Cc @fclem, @brecht)

Thanks for the report, but I would not say that this is a bug. It is expected that users set the maximum value to something always greater than the minimum. If they don't, the result is unpredictable and will depend on the engine. Apparently in Eevee it was decided that the minimum value prevails while in Cycles it is the maximum value. Although consistency is more appreciated, this is something to decide outside the bug tracker. (https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests). So I'm closing the report for now. (Cc @fclem, @brecht)

Clément Foucault commented

2020-12-10 06:01:03 +01:00

I cannot reproduce. It feels like it's an issue with the Nvidia driver. Are you using proprietary driver? please attach your system_info.txt.

We could patch the GLSL code but it seems like this is a quite basic thing to expect from driver to properly implement a clamp!

I cannot reproduce. It feels like it's an issue with the Nvidia driver. Are you using proprietary driver? please attach your `system_info.txt`. We could patch the GLSL code but it seems like this is a quite basic thing to expect from driver to properly implement a clamp!

Stanislav Blinov commented

2020-12-10 11:27:06 +01:00

@mano-wii, please don't do that. The two clamp options aren't called "Range" and "Implementation-defined". They're called "Range" and "Min Max".

@fclem Of course. See also https://www.shadertoy.com/view/Ms33R4 . I see this:

system-info.txt

@mano-wii, please don't do that. The two clamp options aren't called "Range" and "Implementation-defined". They're called "Range" and "Min Max". @fclem Of course. See also https://www.shadertoy.com/view/Ms33R4 . I see this: ![image.png](https://archive.blender.org/developer/F9499892/image.png) [system-info.txt](https://archive.blender.org/developer/F9499867/system-info.txt)

Dalai Felinto commented

2020-12-10 18:09:08 +01:00

Added subscriber: @dfelinto

Dalai Felinto commented

2020-12-10 18:09:08 +01:00

I cannot reproduce it either on:

Operating system: Linux-5.4.0-56-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.80.02

As in, I don't get the Cycles black render either.

I cannot reproduce it either on: Operating system: Linux-5.4.0-56-generic-x86_64-with-debian-bullseye-sid 64 Bits Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.80.02 As in, I don't get the Cycles black render either.

Stanislav Blinov commented

2020-12-10 18:18:07 +01:00

@dfelinto I've attached an example file in the description, so that we're on the same page.

Philip Holzmann commented

2020-12-10 18:49:28 +01:00

Added subscriber: @Foaly

Philip Holzmann commented

2020-12-10 18:49:28 +01:00

I get the same result as @radcapricorn.

Quote from the GLSL 4.6 spec:

Returns min(max(x, minVal), maxVal). Results are undefined if minVal > maxVal.

So if EEVEE wants to implement this consistently across platforms, I'd argue the clamp function cannot be used.
Or we can make it undefined as well in the documentation. I assume the Min Max type exists only for performance reasons anyway?

I get the same result as @radcapricorn. Quote from the GLSL 4.6 spec: > Returns min(max(x, minVal), maxVal). **Results are undefined if minVal > maxVal.** So if EEVEE wants to implement this consistently across platforms, I'd argue the clamp function cannot be used. Or we can make it undefined as well in the documentation. I assume the *Min Max* type exists only for performance reasons anyway?

Brecht Van Lommel commented

2020-12-10 19:20:44 +01:00

Changed status from 'Archived' to: 'Confirmed'

Brecht Van Lommel reopened this issue

2020-12-10 19:20:44 +01:00

Brecht Van Lommel commented

2020-12-10 19:20:44 +01:00

We don't want to expose undefined behavior to users, so this is a bug then.