Principled BSDF base color alpha value is not reflected in viewport with alpha blend #62794

System Information
Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 860M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 398.36

Blender Version
Broken: version: 2.80 (sub 51), branch: master, commit date: 2019-03-20 16:33, hash: b6d1946c2e
Worked: (optional)

Short description of error
When using the Principled BSDF shader with a Base Color Alpha value less than 1.0, in conjunction with setting the material's Blend Mode to Alpha Blend, the Alpha value is not reflected in the viewport. It seems to display at a fixed semi-transparent value, regardless of what is set.

The Alpha is set to 0.010, which is almost wholly transparent:

This is what shows in viewport:

Note that the cube shows at approximately 0.5 alpha (gauged by eye), regardless of set value, and that the plane is not at all visible through the cube.

As it is now, this is the workflow for exporting transparency to glTF, so I believe it to be considerably important to be able to preview the transparency based on its value before exporting.

Exact steps for others to reproduce the error

Add a cube or any mesh
Add default material using Principled BSDF shader
Set Base Color and set Alpha to less than 1.0
Set material's Blend Mode to Alpha Blend
Adjust Base Color Alpha value and see no adjustments in viewport

Blend file
AlphaBlendIssue.blend

Added subscriber: @DanSinni

#67327 was marked as duplicate of this issue

#58281 was marked as duplicate of this issue

Added subscribers: @Ajkad, @brecht

Color sockets in Cycles and Eevee do not actually have an alpha value, this is a leftover from old Blender Internal code. This needs to be cleaned up in the UI still. For transparency you should mix with the Transparent BSDF.

Appreciate the feedback, Brecht. However, I don't believe the glTF exporter currently supports the Transparent BSDF shader; it relies on Alpha from either the Base Color or from a texture linked through the Base Color node of the Principled BSDF, Background, and possibly Emission shaders.

Note that the Alpha value does work as expected when linked through something like the Background shader (which is currently used to export unlit materials in glTF) or the Emission shader:

Added subscriber: @donmccurdy

Thanks Brecht – you mentioned earlier that you'd like to add opacity to the Principled BSDF node. I realize opacity and alpha are not the same thing, so just to clarify – do you still think there should eventually be an opacity socket on the Principled node? Or is mixing with the Transparent BSDF likely to be the long-term answer?

The glTF exporter can be made to support either one, but we'd like to have a future-proof way of representing opacity that feels natural in Blender, so we don't train our users to do something that's going to break in the near future. I misunderstood your earlier comment about opacity as meaning that you wanted to support an alpha channel on the basecolor socket, which is why the exporter is requiring that now, and if it's just a UI glitch that is going away then we should change that in the exporter. In general mixed nodes are hard for us to export reliably, part of the reason for our excitement about Principled BSDF, but I suppose an exception could be made for Transparent BSDF here.

Added subscriber: @elm19087

Added subscriber: @christian

This comment was removed by @christian

Added subscriber: @GaiaClary

Added subscriber: @fclem

Changed status from 'Confirmed' to: 'Resolved'

Alpha handling is now consistent with cycles due to the blend mode refactor done for 2.81.

I don't know about the glTF addon but I would expect that since the time, they would have fixed it.

The glTF addon uses the new 'alpha' socket on the Principled BSDF node, which works correctly in the viewport too. So yes, I think this is resolved. Thanks!

Is the Alpha value of the Base color in the Principled BSDF Shader still used anywhere?

The alpha values of the base color (or any color socket in shading nodes) should not be used.

We still have to remove it from the user interface.

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Principled BSDF base color alpha value is not reflected in viewport with alpha blend #62794