Passing POD arrays to shader?

Question

How do I pass to my vertex shader an array of N elements?

From what I see I can only pass float arrays of 3|4 elements, vectors 2|3d and matrices, but I want to pass a POD array and use it like this:

void ShaderFunction(float* array,int count)
{
    for (int i = 0;i < count;i++) 
    {
        DoJob(array[i]);
    }
}

So far all of my tests fail.

Answer 1

You can define an array in GLSL like so:

uniform float array[size];  

Then passing in depends on what language you're using. For Java (LWJGL):

FloatBuffer floatBuffer = BufferUtils.createFloatBuffer(size);
floatBuffer.put(x);
floatBuffer.put(y);
...
int location = GL20.glGetUniformLocation(shaderProgram, "array");
GL20.glUniform1f(location, floatBuffer);

Or in C++

GLfloat floatArray[size] = {...};
int location = glGetUniformLocation(program, "array");
glUniform1fv(location, size, floatArray);

Then you can use it:

void ShaderFunction(int count)
{
    for (int i = 0;i < count;i++) 
    {
        DoJob(array[i]);
    }
}

Answer 2

Short answer: You can't.

Long answer: Need more info what you are trying to do and why.

Answer 3

Another way you might do this is by using a floating point texture - you would create a one-channel FP texture of the required size, upload your data to it, and sample from that in your shader (remember that vertex shaders can also sample from textures).

Where this is going to cause you huge trouble - irrespective of your chosen solution - is in terms of instruction counts. GPUs are not really designed around being used for this kind of programming, and if your count value is sufficiently high and/or if it varies between calls your performance is at risk of falling off the edge of a cliff.

Since you say that you're looking to do this in a vertex shader, it may yet turn out to be more performance-efficient (and even advantageous from a code-cleanliness perspective) to leave it running on the CPU instead.