GLSL for loop acting weird

Question

I tried to implement something in glsl to do texture splatting, but the for loop is acting weird and gives different results for code that does exactly the same.

Code 1:

for(int i = 0; i < 5; ++i) {
 if(i == 1) {
  float fade = texture2D(alphaTextures[i], texCoord.st).r;
  vec4 texCol = texture2D(textures[i], texCoord.ba);
  texColor = mix(texColor, texCol, fade);
 }
}

Code 2:

for(int i = 0; i < 6; ++i) {
 if(i == 1) {
  float fade = texture2D(alphaTextures[i], texCoord.st).r;
  vec4 texCol = texture2D(textures[i], texCoord.ba);
  texColor = mix(texColor, texCol, fade);
 }
}

The if statement is just for testing purposes so that it should give the same result. The only difference is the loop condition. I really have no idea why only Code 1 gives the correct result. Here are two pictures:

Code1

Code2

The result should be like in picture 1.

"*The if statement is just for testing purposes so that it should give the same result.*" Why should it give the same result? — Nicol Bolas, Jul 29 '13 at 08:16
Originally i tested it without the if statement and 7 textues, but the seventh texture made the whole result look completely different. The if statement should only let element 2 of the array through regardless of the texture count. — user2195441, Jul 29 '13 at 09:48

score 1 · Accepted Answer · edited May 23 '17 at 12:20

1

According to this answer, you can't iterate over a sampler array. The index alphaTextures[i] is invalid, you can only use alphaTextures[1].

This changes in GLSL 4.00+ (OpenGL 4.0+), where you can have a variable index, but it cannot be from a shader input/derived value.

edited May 23 '17 at 12:20

Community

1
1

answered Jul 29 '13 at 20:03

chronospoon

510
6
14

score 0 · Answer 2 · answered Jul 29 '13 at 18:11

One reason could be that Graphic processors don't like branched texture fetches.

Try this instead:

 for(int i = 0; i < 6; ++i) {
    float fade = texture2D(alphaTextures[i], texCoord.st).r;
    vec4 texCol = texture2D(textures[i], texCoord.ba);

    if(i == 1) {
       texColor = mix(texColor, texCol, fade);
    }
 }

(disclaimer) i am only guessing and this error is really weird.

GLSL for loop acting weird

2 Answers2