I'm doing an exercise in Shader School in which I implement a function that returns the nth power of a matrix. Even when I pass constant parameter n into the following function:
mat2 matrixPower(highp mat2 m, const int n) {
if(n==0) {
return mat2(1.0);
} else {
mat2 result = m;
for(int i=1; i < n; ++i) {
result = result * m;
}
return result;
}
}
I get the following error:
Loop index cannot be compared with non-constant expression
How is this possible?
The keyword const indicates that n is not writable, but it is still a variable and not a constant!
n varies with the actual parameter to the formal parameter when the function was called.
Note in Appandix A of The OpenGL ES Shading Language 1.0 are listed possible limitations for OpenGL ES 2.0:
for loops are supported but with the following restrictions:
- condition has the form
loop_index relational_operator constant_expression
where relational_operator is one of:>>=<<===or!=
In Chapter 5.10 Constant Expressions of The OpenGL ES Shading Language 1.0 is clearly said, that a const function parameter is not a constant expression:
A constant expression is one of
- a literal value (e.g., 5 or true)
- a global or local variable qualified as const excluding function parameters
- an expression formed by an operator on operands that are constant expressions, including getting an element of a constant vector or a constant matrix, or a field of a constant structure
- a constructor whose arguments are all constant expressions
- a built-in function call whose arguments are all constant expressions, with the exception of the texture lookup functions.
A workaround would be to create a loop which is limited by a constant expression and to break out off the loop with a condition:
for (int i = 1; i < 10; ++i)
{
if ( i >= n )
break;
result = result * m;
}