Compare two states into the shader

Question

How can I compare into the shader two states from different frames by means of global variable? I need compare states of mouse position between two frames, and if it didn't change, to do {bla bla bla}.

For example:

vec2 focusNew = vec2(0.0);
float x;
float y;

void main
{
    vec2 focus = vec2 ( x, y-1);
    if ((focusNew - focus) <= 0.00001)  // (focusNew == focus)
        {bla bla bla}
    focusNew = focus; 
}

But focusNew doesn't save current state.

Answer 1

You can't. Or at least not that way. Remember: shaders are run thousands of times per frame.

I would explain how you might actually do that, but it's abundantly clear that you don't really mean what you're saying you mean. The mouse state changes from frame to frame. But that's all stuff that happens on the CPU, and it happens once, not once per shader. Every shader therefore would compute the same value.

So there's no point in making the shader do it. Just do the condition on the CPU, then provide a uniform that tells the shader(s) whether or not to do the {bla bla bla}.