What can I use as an array index in GLSL in WebGL ?

Question

Are there ONLY constants allowed or can I use for loop indices or any other dynamic values ?

Answer 1

In WebGL/GLES2: Yes, only constants are allowed. However if your code can be unrolled (either by yourself or by the compiler) then it counts as a constant and you have a workaround. For example,

The problem:

uniform int i;
...
int a[4];
a[2] = 42; // ✓ a constant index, no worries
a[i] = 42; // ✗ dynamic indexing is not allowed

The workaround:

if (i == 0) a[0] = 42;
if (i == 1) a[1] = 42;
if (i == 2) a[2] = 42;
if (i == 3) a[3] = 42;

Ok, that's pretty painful. Lets get the compiler to do that:

for (int k = 0; k < 4; ++k)
    if (i == k)
        a[k] = 42; // ✓ k is a constant because the loop is bounded by one

Sometimes nesting branches can make things faster (not always and not necessarily in this case, it's just an example):

if (i == 0)
    a[0] = 42;
else
{
    if (i == 1)
        a[1] = 42;
    else
    {
        if (i == 2)
            a[2] = 42;
        else
        {
            if (i == 3)
                a[3] = 42;
        }
    }
}

// or equivalently...
for (int k = 0; k <= i && k < 4; ++k)
    if (i == k)
        a[k] = 42;

With regard to desktop GL where you do have dynamic indexing this is still quite useful. Especially in the case of dynamically indexed arrays, unrolling loops and using constants allows values to be placed in registers and you can greatly improve speed. As another side note, if the int[4] array is indexed dynamically it's placed into local memory and each element is padded to 16 bytes (i.e. ivec4[4]), which loop unrolling also fixes.