Alpha channel masking in texture

Suppose I'm doing a specialized kind of screen-capture, where I've managed to get two rendertextures. One has an alpha channel exactly like I want, but the lighting is wrong. The other has the lighting I want, but no transparency.

So is there a way to copy the RGB channels from one and the Alpha channel from the other into one texture?

This is for screen-capture exporting to a PNG, not for rendering in the engine, so I don't particularly need a real-time display to the screen. Though I see there's a Graphics.Blit that can be passed a material, so maybe the way is create a material that applies one texture as a mask of the other?

 

You can create a material shader that takes two texture parameters and combines the channels however you like.

In a vert/frag shader, you could have a frag method like this:

Code (CSharp):

1. sampler2D _MainTex;
2. sampler2D _AlphaTex;
3. fixed4 frag(v2f IN) : SV_Target
4. {
5.     fixed4 c = tex2D(_MainTex, IN.texcoord);
6.     float a = tex2D(_AlphaTex, IN.texcoord).a;
7.     // one of these should work, not sure which (maybe both? can't hurt)
8.     c.rgb *= a;
9.     c.a = a;
10.     return c;
11. }

with a properties block like this:

Code (CSharp):

1. Properties
2. {
3.     [PerRendererData] _MainTex("Color Texture", 2D) = "white" {}
4.     [PerRendererData] _AlphaTex ("Alpha Mask", 2D) = "white" {}
5. }

Whatever that material is applied to will be drawn as a composite of the two textures.

I'm not an expert so there may be better ways. What you're talking about is absolutely possible tho.

 

So if I use the Graphics.Blit with a material that uses this shader, I should be able to get the result into a final texture?

 

Yes I believe the destination texture parameter using the Blit function will do just that.