Basic shader demo with Processing

demo.pde

PShader sh;
PShader f;
PShape square;
PGraphics canvas;

void setup() {
  size(640, 480, P3D);

  canvas = createGraphics(width, height, P3D);
  canvas.noStroke();
  canvas.noLights();
  
  square = createShape(RECT, 0, 0, width, height);
  square.setFill(color(0, 0, 0));
  square.setStroke(false);

  ortho(0, width, 0, height);
  sh = loadShader("scene.fs", "scene.vs");
  sh.set("resolution", float(width), float(height));
  sh.set("origin", 0.0, 0.0);
  
  f = loadShader("filter.fs");
}

void draw() {
  sh.set("radius", map(cos(millis()/1000.0), -1.0, 1.0, 0.001, 0.1));
  shader(sh);
  
  // Rendering scene to offscreen buffer
  canvas.beginDraw();
  canvas.shape(square, 0, 0);
  canvas.endDraw();
  
  // Applying post process
  image(canvas, 0, 0, width, height);
  filter(f);
}

filter.fs

#version 120

uniform sampler2D texture;
uniform vec2 texOffset;

varying vec4 vertColor;
varying vec4 vertTexCoord;

void main(void)
{
  vec4 pixel = texture2D(texture, vertTexCoord.xy);
  gl_FragColor = vec4(vec3(1.0) - pixel.rgb, 1.0);
}

scene.fs

#version 120

uniform vec2 resolution;
uniform vec2 origin;
uniform float radius;

void main(void)
{
	float aspect = resolution.y/resolution.x;
	vec2 p = 2.0*gl_FragCoord.xy/resolution - 1.0;
	p.y *= aspect;

  vec3 c = radius/(length(p - origin))*vec3(1.0);
  gl_FragColor = vec4(c, 1.0);
}

scene.vs

#version 120

uniform mat4 transform;

attribute vec4 vertex;

void main(void)
{
  gl_Position = transform * vertex;
}