Use texture as a mask

Unity Engine
1

Is it possible for one texture to act as a mask for another texture?

For example, lets say I have two textures on a polygon, A, and B. Parts of texture A are completely transparent (alpha=0) and others are opaque (alpha=1). Is it possible to have the transparent parts of texture A also cause texture B to be transparent in the same place, essentially acting like a mask?

To put some perspective on things, I’m trying to implement a “fog of war” on a 2d landscape, I’m doing that by overlaying the entire landscape by a large plane that uses a very low resolution texture where each pixel in the texture corresponds to one “square” in the 2d landscape. If I want the fog to cover that square I simply make that pixel alpha=1, or alpha=0 if I want to remove the fog. This works very nicely for my purposes, especially with filtering that smooths out the pixel edges. However, the fog ends up being a solid color, I would like to texture it a little.

I imagine something like that would be pretty straightforward with pixel shaders. Is it possible to do it without pixel shaders?

2

Sure. For example this shader uses one texture for the color and another one for the mask.

Shader "Mask" {
Properties {
	_MainTex ("Color (RGB)", 2D) = "white" {}
	_Mask ("Mask (A)", 2D) = "gray" {}
}
SubShader {
	Tags { "Queue" = "Transparent" }
	Pass {
		Blend SrcAlpha OneMinusSrcAlpha
		ZWrite Off
		AlphaTest Greater 0.01
		SetTexture [_MainTex] { combine texture }
		SetTexture [_Mask] { combine previous, texture }
	}
} 
}

BTW, shader related questions probably fit better in Shaderlab forum section.

3

Ah, you’ll notice I said that I was looking for a non-shader solution. For some odd reason I had it in my head that I can’t write custom shaders in Indie. Silly me. This shader works wonders, thank you very much, it’s perfect!

4

The above is not a pixel shader, just good ole’ multitexture. You can write pixel shaders in Indie as well of course.

I assumed that by “pixel shader” you just meant that your target hardware is old and won’t have them.