Basic Shader using only one channel of a texture (texture splat)

Hi everyone,

I'm just beginning to learn how to write my own shaders, and here's what i came up with:

Code (csharp):

1. Shader "StrumpyShaders/Basic_AlphaMapSplatR - GLSL"
2. {
3. Properties
4. {
5. _Color("Color", Color) = (1,1,1)
6. _AlphaMap("AlphaMap", 2D) = "black"
7. }
8.    
9. SubShader
10. {      
11. Pass
12. {
13. Tags {"queue"="Transparent"}
14. Blend SrcAlpha OneMinusSrcAlpha
15. Zwrite Off
16.            
17. GLSLPROGRAM
18.            
19. varying mediump vec2 uv;
20.                
21. uniform lowp sampler2D _AlphaMap;
22. uniform lowp vec3 _Color;
23.    
24. #ifdef VERTEX
25. void main()
26. {
27. gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
28. uv = gl_MultiTexCoord0.xy;
29. }
30. #endif
31.    
32. #ifdef FRAGMENT
33. void main()
34. {
35. vec4 AlphaMap = texture2D(_AlphaMap, uv);
36. vec4 AlphaChannel = AlphaMap.r;
37. gl_FragColor = vec4(_Color, AlphaChannel);
38. }
39. #endif  
40. ENDGLSL
41. }
42. }
43. }

So this is a GLSL shader as you can see, and the idea is to use only one channel of an image (here it is the red channel) as alpha map so that i could store up to 4 greyscale alpha textures in only one image.
It worked quite well back on the computer where i wrote this shader but when i came back home to use it on my own one, it doesn't work anymore and i get the following error :
"No subshaders can run on this graphics card"
and
"GLSL Error in Fragment Shader: Fragment shader failed to compile with following errors: ERROR: 0:37: error(#160) Cannot convert from 'high float' to 'highp 4-component vector of float' ERROR: error(#273) 1 compilation errors. No code generated at line 0"

Thank you for your help

 

Color properties are float4s (or vec4s in GL, I guess).

Also, you're assigning your alpha channel to be a vec4 - it only needs to be one channel so you need it to be just a float. I think that's what your error's about.

I'm not really up on GLSL, but this might work better (bold bits are bits I've changed);

Code (csharp):

1.  
2. Shader "StrumpyShaders/Basic_AlphaMapSplatR - GLSL"
3. {
4.     Properties
5.     {
6.         [b]_Color("Color", Color) = (1,1,1,1)[/b]
7.         _AlphaMap("AlphaMap", 2D) = "black"
8.     }
9.        
10.     SubShader
11.     {      
12.         Pass
13.         {
14.             Tags {"queue"="Transparent"}
15.             Blend SrcAlpha OneMinusSrcAlpha
16.             Zwrite Off
17.                        
18.             GLSLPROGRAM
19.                        
20.                 varying mediump vec2 uv;
21.                                
22.                 uniform lowp sampler2D _AlphaMap;
23.                 [b]uniform lowp vec4 _Color;[/b]
24.                    
25.                 #ifdef VERTEX
26.                 void main()
27.                 {
28.                     gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
29.                     uv = gl_MultiTexCoord0.xy;
30.                 }
31.                 #endif
32.                    
33.                 #ifdef FRAGMENT
34.                 void main()
35.                 {
36.                     vec4 AlphaMap = texture2D(_AlphaMap, uv);
37.                     [b]float AlphaChannel = AlphaMap.r;
38.                     gl_FragColor = vec4(_Color.rgb, AlphaChannel);[/b]
39.                 }
40.                 #endif  
41.             ENDGLSL
42.         }
43.     }
44. }
45.  

 

Thank you very much farfarer for this quick reply.
It's working now

I didn't see this technique treated often so i'm wondering if it makes sense using it on mobile platforms such as iphone.
Do you think it's worth doing such optimization ? Wouldn't there be some drawbacks ?

 

Really not sure. I'd add a selector for which channel to use, otherwise you'll only increase your draw-calls by needing one shader per channel used.

But then I'm not sure whether adding in if statements would affect the performance of the shader much.

 

I guess i should run some test to see how it affects performance. At least i hope it can saves on the texture size part.