Question

I have two textures, cloud and hill, each with 512 x 512 size, and i intend to create a gl_FragColor output which will obtain the pixel values from the previous textures. In this case, i want to obtain the 1st pixel in gl_FragColor from the 1st pixel in the 1st texture, the 2nd pixel in the gl_FragColor from the 2nd pixel in the 2nd texture, the 3rd pixel in gl_FragColor from the 3rd pixel in the 1st texture an so on. Here is my fragment shader code:

 uniform sampler2D tex0;
 uniform sampler2D tex1;

 void main() {
     vec4 cloud = texture2D(tex0, gl_TexCoord[0].st);
     vec4 hill = texture2D(tex1, gl_TexCoord[0].st);
     for (int i=0;i<512;i++) {
       for (int j=0;j<512;j++) {
            if ( j%2 == 0)
                gl_FragColor =  cloud;
            else
                gl_FragColor =  hill;
          }
     }
  }

Here’s the texture unit setup:

t1 = loadTexture("pY.raw", 512, 512);
t2 = loadTexture("pZ.raw", 512, 512);

glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, t2);
glEnable(GL_TEXTURE_2D);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, t1);
glEnable(GL_TEXTURE_2D);

And here is the uniform setup:
glUseProgram(program);

GLuint t1Location = glGetUniformLocation(program, "tex0");
GLuint t2Location = glGetUniformLocation(program, "tex1");

glUniform1i(t1Location, 0);
glUniform1i(t2Location, 1);

The problem is, the output for the program is only the hill texture, and i don’t know how to fix this. Any suggestion?

Answer 1

You don’t need to do any iterations in your shader. Pixel shader will be called once for every pixel in your object. Instead use gl_TexCoord[0] to get current texture coordinates. Your code should look something like that:

uniform sampler2D tex0;
uniform sampler2D tex1;

void main()
{
    vec4 cloud = texture2D(tex0, gl_TexCoord[0].st);
    vec4 hill = texture2D(tex1, gl_TexCoord[0].st);

    if ( int(gl_TexCoord[0].x*512)%2 == 0)
        gl_FragColor =  cloud;
    else
        gl_FragColor =  hill;

    }
}

This one should work, even with older opengl:

#ifdef GL_ES
precision highp float;
#endif

uniform sampler2D tex0;
uniform sampler2D tex1;

void main(void)
{

    if((gl_FragCoord/32.0- vec4(ivec4(gl_FragCoord/32.0))).x<0.5)
        gl_FragColor = texture2D(tex0, gl_FragCoord.xy/512.0);
    else
        gl_FragColor = texture2D(tex1, gl_FragCoord.xy/512.0);
}

You can try it out with WebGL at: http://www.iquilezles.org/apps/shadertoy/