index expression must be constant using array

Question

I would like to apply multiples lights in my shader. To do that, I've make the choice to put lights data in multiple arrays instead of one light = one struct to avoid doing "setUniforms" a lot of time.

I've got something like that:

// Light's data.
struct LightData 
{
   vec3 ambient[4];
   vec3 diffuse[4];
   vec3 position[4];
   vec3 specular[4];
};
uniform LightData lights;


vec3 applyLight( int index, vec3 cameraPos )
{
    vec3 position = lights.position[index]; //<<< Error here.
    …
    return result;
}

int main()
{
    vec3 color;

    for( int i = 0; i < 4; i++)
        color += applyLight(i, camera.position);
}

The problem is in the GLSL code: "index expression must be constant []". How can I do to have this code to work? I could give position, diffuse, ambient, specular to the function but it will by dirty.

What about performances? It is better to have something like what I've done or like the code bellow?

struct Light 
{
   vec3 ambient;
   vec3 diffuse;
   vec3 position;
   vec3 specular;
};
uniform Light lights[4];

Have a nice day!

Possible duplicate of Index expression must be constant - WebGL/GLSL error — Suma
– Suma, Commented Oct 1, 2018 at 10:00

Dragan Okanovic · Accepted Answer · 2015-08-05 16:16:35Z

2

Unroll your loop:

color += applyLight(0, camera.position);
color += applyLight(1, camera.position);
color += applyLight(2, camera.position);
color += applyLight(3, camera.position);

(not sure if this exact code will work, but you get the idea)

lights.position[index]; // no
lights.position[0]; // yes, etc

Option below (for uniform layout) might be better, it may have a better locality.

answered Aug 5, 2015 at 16:16

Dragan Okanovic

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2 Comments

SwagBoy93 Over a year ago

Thanks but "4" is for the exemple, I would like to manage less or more lights with another integer (lights.lightCount)

Dragan Okanovic Over a year ago

if(7<lightCount) { color+=lambert(light[7], normal); } something like should do the trick.

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