Fragment shader operations before vector transformations

Question

I feel like I'm misunderstanding how to work with vector/fragment shaders. My vector shader is as follows:

uniform mat4 uVMatrix;    // view (camera transformations)
uniform mat4 uMMatrix;    // model (object transformations)
uniform mat4 uPMatrix;    // projection
attribute vec4 aVertexPosition;     // passed in
attribute vec4 aVertexColor;
varying vec4 vColor;

void main() {
    gl_Position = uPMatrix * uVMatrix * uMMatrix * aVertexPosition;
    vColor = aVertexColor;    // pass the vertex's color to the fragment shader
}

Pretty simple. Right now I just have a simple square that I'm transforming in 3D space and drawing. I want to make a see through circle in the middle of the suqare as follows:

This square has 4 vertices that I transform in the vector shader. Here's my fragment shader:

precision mediump float;    // how precise to be with floats
varying vec4 vColor;        // interpolated from the vertices

void main() {
    gl_FragColor = vColor;
}

Now, I've seen I have access to gl_FragCoord, but that coordinate is after all the vector transformations, right? How can I manipulate the pixels in the square, before all the projection transformations, etc.? I don't think I can do it in the vector shader, as there's only 4 vectors...

Answer 1

A simple way to achieve this would be to assign UV to your quad mesh, and then you can utilize that UV in your fragment shader to draw the circle by doing something like this (pseudocode):

color = uv.magnitude > 0.5f ? invisible : visible

This will work because the UV's are interpolated over the polygon.