how to pick pixel value from a specific index of a texture in OpenGL GLSL

Question

I am rendering a texture where I am stuck at a point where I need to pick values from some specific index to update the current index.

For EG :

float someColor = texture2D(u_image, vTexCoord).r; //assume u_image is 10*10 image
if (someColor.r > 0.5) {
    someColor = someColorPalette[(zr*zcols)+(zc-1)]; //someColorPalette is another texture
//where (zr*zcols)+(zc-1) is getting the pixel value from some index using the calculation 
}

`

In the above snippet I know that someColorPalette[(zr*zcols)+(zc-1)] is a CPU statement and won't work in opengl. Can anyone suggest for some alternate solution to read from index of a texture?

Answer 1

For GLSL ES 1.0 to pull out a specific texel from a texture use

 vec2 resolutionOfTexture = vec2(someWidth, someHeight)
 int pixelX = ?
 int pixelY = ?

 vec2 uv = (vec2(pixelX, pixelY) + .5) / resolutionOfTexture;
 vec4 color = texture(someTexture, uv);

You need to add .5 to target the center of texels, otherwise you're targeting edges and you might get the wrong pixel.

Of course resolutionOfTexture and pixelX and pixelY can all be uniforms or whatever

For GLSL ES 3.0 use

 vec4 color = texelFetch(someTexture, ivec2(pixelX, pixelY), 0);

If you want a linear index then in GLSL ES 1.0

 pixelX = mod(someIndex, resolutionOfTexture.x);
 pixelY = floor(someIndex / resolutionOfTexture.x);

in GLSL ES 3.0

 ivec2 texSize = textureSize(someTexture, 0);
 pixelX = someIndex % texSize.x;
 pixelY = someIndex / texSize.x;