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"Material preview" shows always eevee mat branch / never cycles
Confirmed, NormalPublicTO DO

Description

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 980/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 432.00

Blender Version
Broken: version: 2.82 (sub 6), branch: master, commit date: 2019-12-18 22:47, hash: rBda6929488a64
Worked: never, I think

Short description of error
"Material Preview" shows always the material output assigned to eevee, but I want the cycles branch. Changing renderengine or the active material output node doesn't change anything.

Exact steps for others to reproduce the error
Open the blendfile and try to preview the material with "cycles" image - without going to rendered viewport. The material preview seems to always use the node tree going to the Eevee material output node, even when the render enging is set to Cycles.

Event Timeline

I'm not sure if this should be considered a bug. I can certainly agree that this is a missing feature that should be implemented soon!

I wonder how you would expect it to work. Should the material preview show the material for the output node for the Render Engine that is selected, or should there be a separate option?

It would probably be best if you talk to the appropriate developer directly regarding this feature request.

This happens often these times, that it's not easy to say where a bug end's and a feature starts. In general I think the gap it too large. In my eyes, blender tries always to be straight. Tries to not make mistakes, which everyone does, has it's own way, is able to reinvent. Be consistent is a part of this straight way - in my opinion. Maybe that's also why, some (most?) of my bugreports are closed, because they are not exactly bugs for developers. But for me as user: inconsistency is a bug.

Ok, what had I expected?

My Material has two shader branches. One Material Output for Cycles, another for Eevee. When my viewport "says" that he shows a "Material Preview", it should also show the appropriate Material output node for the engine.

Same with the export (like in T72600): When I export, I can choose "selected objects" - which an obvious option. But blender introduced these two comparable engines, where some shaders can be used from both engines, but some not. So an export should also make this difference.

Jacques Lucke (JacquesLucke) changed the task status from Needs Triage to Confirmed.Jan 8 2020, 3:08 PM

I can definitely follow your argument for the material preview in Blender. I just looked into the code. While it is quite easy to let eevee always use the cycles version I'm not quite sure what's the best way to pass the selected render engine down to the point where it is required. @Clément Foucault (fclem), can you estimate the complexity of this change?

Jeroen Bakker (jbakker) changed the subtype of this task from "Report" to "To Do".Jan 13 2020, 4:53 PM

I don't think that the complexity is on the technical level. Perhaps we need to recompile the shaders. The issue is on the user side of the change. How does the user get control over what will be rendered. In the original design the split between EEVEE and Cycles was to get better resource management as the can get to complex and we gave the user an option to create a better optimized shader version. This report asks a possibility to still display cycles shader graph.

IMO this falls into a feature development as the current system works as designed and a new design is needed for this. It falls in my scope, but I am not 100% sure I will get to this in the next 6 month...
Sorry about that, but will set it on To Do and discuss it here.

My opinion:
Since you would have to click on the output you want to render at any time anyway, using the Node Wrangler Addon to Ctrl + Shift on a node is just as valid as a workflow.
As long as the eevee output is selected it should be just as fast to use.
Sounds like a workaround and it is ... at least until this Node Wrangler feature get's a fully official default implementation.

On the other hand if an output is set to only render in either Cycles or Eevee then that should be followed through.
You could also reverse the argument so that you can't render the Eevee output when rendering in Cycles in the viewport.
This behaviour seems right to me.

Before we make that change, we should consider that it would break a very common use-case:

Users may use Material mode as a preview of their Cycles scene, but use a simpler set of nodes for the preview. For an animated character, they may not want to have hundreds of complex nodes, but only the main textures with some basic shaders for fast playback, but then then full shader when you hit render. That is possible to do now, and would be broken if the suggested change is made.

I'm not saying no changes should be made, but I think the above use-case is quite valuable to support somehow. I would suggest we make a design doc for this.