Hi!
I am trying to make a material where the main texture is yellow (yellow.jpg) and uses a black and white image (scratches.jpg) to allow metal(chrome.jpg) to show through the scratches in the paint where the scratches.jpg is black.

ie, bare metal showing through scratches in yellow paint.

I started with a layer shader , but i want to use mental ray, so tried messing around with mib_color_mix shader. No luck.

can anyone please advise how to do this in mib_color_mix, or in fact, if I even need to use this shader at all?

Is there a way to do it in a standard (phong/blinn) shader without the mib_color_mix ?

thanks for any help I can get!!

bik.

The color mix shader has to be the “assigned material”, but you can’t do that like you would with a blinn or a phong material; the color mix has no shading group node.  You have to trick it.

For the sake of this discussion, let’s say that your underlying metal is a gray blinn material, and the layered shader is a yellow phong.

Your scratches will be a b/w transparency map created in photoshop.

Assign the gray blinn material to your object.  Open up the SG node and scroll down to the mental ray section.  MMB drag your color mix node into the “Material Shader” slot. 

Double click on the color mix node, and drag the gray blinn to the “color base” slot all the way at the bottom.  Drag the yellow phong onto the Color 0 slot. 

Up at the top, set the “mode 0” to “mix”, and the “Num” to 1.  When you render, you should see only the yellow phong.

Drag your b/w scratches map onto the transparency channel of the yellow phong, and the gray blinn will show through.

The way you are doing this - with a yellow.jpg and a chrome.jpg, you are not taking advantage of the power of shading groups - i.e., reflections and specular highlights.  Your main visual differences between the paint and the chrome will be in these areas, but by using jpeg maps to define them, you are not getting that kind of surface quality.