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Sorry for the vague title. I'm not sure if I'm asking the right question.

This cube has two materials slots, the faces facing the camera are completely red, and the back faces are completely blue.

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When "Blend Mode" is Opaque on both materials, it looks red, just like expected:

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(To simplify the problem, I removed the lights from the scene so it's only lightened up by the ambient.)

When the red material's "Blend Mode" is set to Alpha Blend, nothing really changes:

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But when both materials' "Blend Mode" is set to Alpha Blend, the red faces become invisible!

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So my question is, why doesn't it blend into some kind of purple? And how to control the transparency of these Diffuse shaders so they can blend together, like 10% red 90% blue?

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You can keep the alpha blend setting on the red material. You just need to add a mix shader and a transparent shader set to pure white :

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The factor of the mix shader drives how much the red blends with the blue.

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  • $\begingroup$ Thank you very much! If you don't mind, could you briefly explain why the red material is invisible in my set up? If Transparent BSDF "add transparency"(as docs.blender.org/manual/en/latest/render/shader_nodes/shader/… says) to a material, shouldn't a single Diffuse BSDF without Transparent BSDF, like the one in my question, be completely opaque? $\endgroup$ – Lai Yu-Hsuan Apr 2 at 13:52
  • $\begingroup$ @LaiYu-Hsuan Honestly I don't really understand why, I guess it intverts the normals of the face because when you check "Backface culling" and "Show backface" the behaviour changes... $\endgroup$ – Gorgious Apr 2 at 16:43
  • $\begingroup$ @LaiYu-Hsuan These settings have already been changed once during the development of Eevee and I suspect it will change again at one time for something more easily understandable $\endgroup$ – Gorgious Apr 2 at 16:44
  • $\begingroup$ Ok, thanks. These blending options really baffle me... $\endgroup$ – Lai Yu-Hsuan Apr 2 at 16:57

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