drawGrahic.glsl

uniform sampler2D srcImg;

varying vec4 vertTexCoord;

void main() {
  gl_FragColor = texture2D(srcImg, vertTexCoord.st).rgba;
}

shader_pgraphics_test.pde

PShader shader;
PGraphics output;

void setup() {
  size(500, 500, P2D);
  output = createGraphics(500, 500, P2D);
  updateCanvas();
  initShader();
}

void initShader() {
  shader = loadShader("drawGraphic.glsl");
  shader.set("srcImg", output);
  println("shader loaded");
}

void updateCanvas(){
  output.beginDraw();
  output.background(random(255), random(255), random(255));
  output.fill(255, 0, 0);
  output.ellipse(250, 250, 20, 20);
  output.endDraw();
}

void draw() {
  shader(shader);
  drawOutput(0,0, width, height);
}

void mouseMoved(){
  updateCanvas();
  shader.set("srcImg", output);
}

// borrowed from Andres Colubri's CustomBlend example
void drawOutput(float x, float y, float w, float h) {
  pushMatrix();
  translate(x, y);
  noStroke();
  beginShape(QUAD);
  // Although we are not associating a texture to 
  // this shape, the uv coordinates will be stored
  // anyways so they can be used in the fragment 
  // shader to access the destination and source
  // images.
  vertex(0, 0, 0, 0);
  vertex(w, 0, 1, 0);
  vertex(w, h, 1, 1);
  vertex(0, h, 0, 1);
  endShape();
  popMatrix();
}

void keyPressed() {
  if (key == 'r') {
    initShader();
  }
}