I am mixing colors and roughness of a Glossy shader through Layer weight node.


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Mix Shader:

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Mix RGB:

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Why do the same properties used in two similar approaches give a very different renders? Colors more or less equal, but roughness difference is quite visible.


Mix shader use a very simple method to combine two shader output. It doesn't guarantee any real material based on physic light reproduce. You are actually using alpha compositing rather than reevaluate a BSDF shader. See the example below.

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On the left we use Mix Shader to mix the Roughness between 1.0 and 0.0. Right we use real 0.5 value for a single Glossy BSDF.

You can see that the Glossy 100% reflection just blend into the rough one without any transition. This is totally impossible in real world unless a clear-coat is presented.

So if a single BSDF shader can do the job, don't try to mix other shader, just let it do the calculation and connect it to the output directly.

  • $\begingroup$ Great explanation! I always prefered mixing RGB's rather than shaders due to hypothetical performance, but now I see real reason here. $\endgroup$ – Serge L Nov 21 '19 at 9:46
  • $\begingroup$ @SergeL There is the performance difference. Double any BSDF shader is actually almost double the calculation of a surface. They are expensive shader compare to mix shader and mix RGB node. $\endgroup$ – HikariTW Nov 21 '19 at 10:01
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    $\begingroup$ @SergeL if you're interested at taking a look at the implementation of Mix RGB and Mix Shader as GLSL shader. The closure_mix is implemented in bsdf_common_lib.glsl. $\endgroup$ – Robert Gützkow Nov 21 '19 at 10:54
  • $\begingroup$ The OSL shader implementation for Cycles can be found here. $\endgroup$ – Robert Gützkow Nov 21 '19 at 11:03
  • $\begingroup$ @RobertGützkow So the mix shader is a linear interpolation of two shader closure output? And the closure contain absorption scatter emission anisotropy properties, and there is no roughness property left for an accurate glossy BSDF? $\endgroup$ – HikariTW Nov 21 '19 at 11:14

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