The interaction between Diffuse BSDF and EEVEE's Alpha Blend mode

Support Materials and Textures
1

I have a cube whose front faces are red (material slot 1) and back faces are blue (material slot 2). The shader is very simple:

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Of course the cube just looks very red:

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But what I found interesting is, when I set both of the materials’ Blend Mode to Alpha Blend, the red faces become completely invisible.

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It baffles me in many ways:

  1. Why are the red faces invisible? Shouldn’t Diffuse BSDF be opaque, regardless Blend Mode?
  2. Why doesn’t this happen when I use Alpha Clip or Alpha Hashed?
  3. Why doesn’t this happen when I only set the red faces’ Blend Mode, but not the blue faces’?

Any insight on this?

2

A face is a face and can only have one material. Delete the number two slot, and try to use geometry/backfacing to change behavior within the same material. I don’t know if Eevee supports this.

3

A face is a face and can only have one material. Delete the number two slot, and try to use geometry/backfacing to change behavior within the same material. I don’t know if Eevee supports this.

Sorry I didn’t make it clear. Each face has only one material. The 3 faces facing the camera have a red material, the other 3 have a blue one.

4

Maybe I shouldn’t have answered. I really have no idea what goes on in Eevee. There should be others around that can probable answer this.

5

It looks like a sorting problem. According to the manual, we’re supposed to disable “Show Backface” (Backside in the manual) when alpha blending is used. Try it, for me it works.

Edit: If you want to have transparent areas in the red material, for example, you can try to play with the options for both materials and see which combination does what you want. I don’t understand it all, but maybe you can find a setting that works.

6

After reading the manual page you posted I believe it’s the problem. Thank for pointing it out!

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