GLSL Gradient Shader with Processing

gradient.glsl

uniform float millis;
uniform float width;
uniform float height;

void main()
{
	float rx = gl_FragCoord.x / width;
	float ry = gl_FragCoord.y / height;

	float rate = sin(millis)/2.0 + 0.5;
	float rate_inv = 1.0 - rate;
	gl_FragColor = vec4(rate * rx, rate_inv * ry, 0.0, 1.0);
}

shader_gradient.pde

PShader myShader;
    
void setup() {
  size(640, 360, P3D);   
  myShader = loadShader("gradient.glsl");
  fill(255);
  noStroke();
  myShader.set("width", (float)width);
  myShader.set("height", (float)height);
}

void draw() {
  // saveFrame("rec/####.png");
  myShader.set("millis", millis()/1000.0);
  shader(myShader);
  background(0);
  rect(0,0,width,height);
}