Generating a gradient with Processing and an OpenGL texture

texture_gradient.pde

import processing.opengl.*;

void setup(){
  size(500, 500, OPENGL);
  noStroke();
}

void draw(){
  background(0);
  
  fill(255,0,0);
  rect(0,0,500,500);

  float percentHidden = map(mouseY, 0, 500, 0, 1);  
  
  beginShape(QUADS);
    fill(0,255);
    vertex(0,0);
    vertex(500,0);
    fill(0,0);
    vertex(500, 500 * percentHidden);
    vertex(0, 500 * percentHidden);
  endShape();

}