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Is it possible to have a dynamic array in a GLSL shader? For instance, what if I have something like this in my GLSL Shader:

uniform int size;
uniform SceneLights lights[size];

void main()
{
    for(int i = 0; i < size; i++) {
        /* Do Some Calculation */
    }
}

And this would be my C++ file:

for (GLuint i = 0; i < pointLights.size(); i++) {
    glUniform3f(glGetUniformLocation(shader, ("pointLights[" + std::to_string(i) + "].position").c_str()), lights[i].someValue);
}

I would like to do this because I have a file with the positions of all the light sources in my scene (this may vary for each scene) so I would like to have a dynamic array which will allow me to send different numbers of lights to my shader.

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I don't think uniform arrays can be dynamically sized. In your case you should define the array as the maximum number of lights you will process and then use a uniform to control the number of iterations you do on this array. On the CPU side you can set a subset of the lights[] array according to the 'size' variable.

e.g.

#define MAX_LIGHTS 128

uniform int size;
uniform SceneLights lights[MAX_LIGHTS];

void main()
{
    for(int i = 0; i < size; i++) {
        /* Do Some Calculation */
    }
}

I think (speculate) the likely reason for this is that it would be impossible to determine the location of a uniform if there are variable sized arrays in your uniform list which depend on the value of another uniform.. e.g.

uniform int arraySize;               // offset 0
uniform int myArray[arraySize];      // offset 4
uniform int anotherVar;              // offset ???? 

GLSL would not know were to place anotherVar, because it would need arraySize to be set with a value before it can compute the offset of 'anotherVar'. I suppose the compiler could be clever and re-arrange the order of uniforms to get around this, but this will fail again if you have 2 or more variable sized arrays..

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