WebGL - Looping through an array in a shader

Question

I wondering how I am supposed to loop through an array from a shader. I've read a lot of things but I'm still quite confused.

Here is the JavaScript that creates the array:

var particles = []; // I'm creating an array of x and y coordinates
for (var i = 0; i < 100; i++) {
    particles.push([
            Math.floor(Math.random()*1000) + .5, 
            Math.floor(Math.random()*1000) + .5
        ]);
}
gl.uniform2fv(gl.getUniformLocation(glProgram, 'particles'), particles);

And here is a piece of my fragment shader:

uniform vec2 particles;

void main( void ) {
    bool found = false;
    for (int i = 0; i < 100; i ++) {
        if (particles[i].x == 1.0) {
            found = true;
        }
    }
}

The error I'm getting is "field selection requires structure or vector on left hand side". What am I missing?

Don't know much about WebGL, but I would guess your declaration of particles in your shader is incorrect, it should probably be uniform vec2 particles[100];. — jcaron
– jcaron, Commented Jan 3, 2018 at 14:34
FYI: Of course do whatever you want but it's not common to use large arrays of uniforms given that the min max size is 128. Meaning if you declare uniform vec2 particles[129] where will be many GPUs that can't run the shader because they support a maximum array size of 128. (currently 1 of 4 will fail) Typically if you need random access to data you put that data in a texture. — user128511
– user128511, Commented Jan 4, 2018 at 3:36
Passing by this question, but without additional context your code gives me the sense that you might want to re-organize your data so you know which points have a .x of 1.0 before the shader steps. You could create separate buffers. That way you could avoid the linear search. — synchronizer
– synchronizer, Commented Feb 9, 2018 at 1:23

jcaron · Accepted Answer · 2018-01-03 14:43:58Z

1

The declaration of particles in the shader was incorrect, it defined a single vec2 rather than an array of vec2.

The right declaration is:

uniform vec2 particles[100];

answered Jan 3, 2018 at 14:43

jcaron

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