Holdout shader and Transparent shader don't use the background's alpha channel when used with Light Path node. #39749

New Issue

Anna Celarek · 2014-04-15T21:56:36+02:00

Anna Celarek commented

2014-04-15 21:56:36 +02:00

System Information
Windows 7 64bit

Blender Version
Broken: 2.70a, daily build 3cf39fc (2014-04-14), 2.65

Short description of error
Holdout shader and Transparent shader don't use the background's alpha channel when used with a Light Path node.
See the image:
Left - the transparent shader renders the background color ignoring alpha
Right- the holdout shader renders black ignoring both color and alpha
Middle - an emission shader is linked into the same node socket for comparison.

Both holdout and transparent BSDF should render with alpha 0.

Exact steps for others to reproduce the error
render the attached .blend

backgroundtransparency.blend

**System Information** Windows 7 64bit **Blender Version** Broken: 2.70a, daily build 3cf39fc (2014-04-14), 2.65 **Short description of error** Holdout shader and Transparent shader don't use the background's alpha channel when used with a Light Path node. See the image: Left - the transparent shader renders the background color ignoring alpha Right- the holdout shader renders black ignoring both color and alpha Middle - an emission shader is linked into the same node socket for comparison. Both holdout and transparent BSDF should render with alpha 0. **Exact steps for others to reproduce the error** render the attached .blend [backgroundtransparency.blend](https://archive.blender.org/developer/F84964/backgroundtransparency.blend) ![backgroundtransparency.jpg](https://archive.blender.org/developer/F84965/backgroundtransparency.jpg)

Anna Celarek commented

2014-04-15 21:56:36 +02:00

Changed status to: 'Open'

Anna Celarek commented

2014-04-15 21:56:36 +02:00

Added subscriber: @AnnaCelarek

Anna Celarek commented

2014-04-15 21:59:38 +02:00

There was a fix for a similar problem, but works only if the Holdout or Transparent shader is connected directly into Material Output, not with the mix node inbetween.
https://developer.blender.org/rB7ffa42075e29033a54a606a75b802d17f4a0f079

There was a fix for a similar problem, but works only if the Holdout or Transparent shader is connected directly into Material Output, not with the mix node inbetween. https://developer.blender.org/rB7ffa42075e29033a54a606a75b802d17f4a0f079

Willi commented

2014-04-16 00:23:28 +02:00

Added subscriber: @willi-2

Willi commented

2014-04-16 00:23:28 +02:00

Looks to me it has less to do with transparent and holdout shaders, but it's the nature of the refraction shader that alpha is not passed through. Just try a plane with a refraction shader only, and use the same world background (alpha=0), and you'll get alpha=1 in the render result, too. Same is true for the glass shader.

Brecht Van Lommel was assigned by Bastien Montagne

2014-04-16 09:45:37 +02:00

Bastien Montagne commented

2014-04-16 09:45:37 +02:00

Added subscriber: @mont29

Brecht Van Lommel commented

2014-04-16 12:09:06 +02:00

Changed status from 'Open' to: 'Archived'

Brecht Van Lommel closed this issue

2014-04-16 12:09:06 +02:00

Brecht Van Lommel commented

2014-04-16 12:09:06 +02:00

This is working as designed, refraction does not give any alpha. It can't work because the ray refracts in a different direction, only with a transparent BSDF can you get correct alpha to composite the render over another image.

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Holdout shader and Transparent shader don't use the background's alpha channel when used with Light Path node. #39749