How should I read neighboring Texels in the Fragment Shader?

Question

I'm just beginning to learn Opengl, and I'm building a "falling sand" simulation using Love2D as a framework. My naive approach has been to take the texture_coord argument passed from the effect() function and offsetting it before feeding it into texel(). If there is an internal solution that has already been engineered for this, could someone point me in the right direction?

extern vec2 dim //the width and height of the display area

vec4 effect( vec4 color, Image tex, vec2 tCoords, vec2 sCoords ){
    
  //find the color of this cell's neighbors
  
  vec4 top = Texel(tex, vec2(tCoords.x,tCoords.y - 1/dim.y));
  //other neighbors excised for brevity
  
  vec4 self = Texel(tex, tCoords); //this pixel

  if (top == Yellow && self == Black){ //sand above and I'm empty
    self = Yellow;
  } 
  return self;
}

It works. I'm just curious if there is a better way.

Answer 1

Your code looks perfectly acceptable. However, there are some optimizations and alternatives you could consider depending on your specific requirements and constraints.

If your simulation requires sampling from multiple textures (e.g., for different layers or states), you might consider using texture arrays or 3D textures instead of individual 2D textures. This can simplify your sampling logic and reduce the number of texture lookups. And by the way, I hope you are using nearest-neighbor texture filtering (GL_NEAREST) instead of linear filtering (GL_LINEAR). Nearest-neighbor filtering will give you the exact texel values without interpolating between neighboring texels. Read More on it on OpenGL website.

For large simulations, it may be more efficient to use a separate buffer to store the simulation state instead of reading from and writing to a texture in the same pass. This can reduce the number of texture lookups and simplify the shader logic.

Example for your code:

extern vec2 dim; //the width and height of the display area

vec4 effect(vec4 color, Image tex, Image buffer, vec2 tCoords, vec2 sCoords) {
    // find the color of this cell's neighbors
    
    // Read from the buffer texture instead of tex
    vec4 top = Texel(buffer, vec2(tCoords.x, tCoords.y - 1.0 / dim.y));
    // other neighbors excised for brevity
    
    vec4 self = Texel(buffer, tCoords); // read the current state from the buffer

    if (top == Yellow && self == Black) { // sand above and I'm empty
        // Write the updated state to the buffer
        return Yellow;
    }
    
    return self; // return the current state if no update is needed
}

Whether it's important to use a separate buffer depends on the specific requirements and constraints of your simulation. However I would recommend it.