Fisheye Lens Shader

Sample output from the sketch below

For a long time I have been meaning to investigate shaders with the idea of creating my own. Yesterday I took the plunge and started messing about with the examples that came with Processing. I decided that a good start would be a fisheye lens shader and here is my effort for you to play with.

You can also download the sketch in zip format

Sketch code

PImage img;
PShader shade;
int offsetX, offsetY;

public void setup() {
  size(520, 344, P2D);
  noCursor();
  img = loadImage("london.jpg");
  // The offsets are needed if the image size is 
  // different from the display size
  offsetX = (width - img.width)/2;
  offsetY = (height - img.height)/2;
  initShaders(img);
}

public void mouseMoved() {
  shade.set("lensPos", mouseX - offsetX, mouseY - offsetY);
}

public void draw() {
  background(128);
  shader(shade);
  drawOutput(offsetX, offsetY, img.width, img.height);
}

public void initShaders(PImage img) {
  shade = loadShader("fisheye.glsl");
  shade.set("image", img);
  shade.set("size", img.width, img.height);
  shade.set("lensSize", 80.0f);
  shade.set("lensPos", 200, 100);
}

public void drawOutput(float x, float y, float w, float h) {
  pushMatrix();
  translate(x, y);
  beginShape(QUAD);
  vertex(0, 0, 0, 0);
  vertex(0, h, 0, 1);
  vertex(w, h, 1, 1);
  vertex(w, 0, 1, 0);
  endShape();
  popMatrix();
}

Shader file: 'fisheye.glsl'
Use a text editor to create this file and put it in the sketch folder.

uniform sampler2D image;

// Image size in pixels
uniform ivec2 size;

// Lens position
uniform ivec2 lensPos;

// Lens size (radius in pixels)
uniform float lensSize;

varying vec4 vertTexCoord;

void main() {
    // Get current uv position
    vec2 st = vertTexCoord.st;  

    // get current pixel position
    vec2 pxl = size * st;

    // Get distance from lens centre
    float pxl2lens = distance(pxl, lensPos);

    // If outside lens use pixel
    if(pxl2lens > lensSize){
        gl_FragColor = texture2D(image, st).rgba; 
        return;
    }
    // We are inside the lens radius so calculate 
    // new uv coordinate

    // Get direction vector to pixel
    vec2 direction = pxl - lensPos;

    float factor = 0.4 + 0.9 * pxl2lens / lensSize;
    vec2 npxl = lensPos + direction * factor;
    vec2 nst = npxl / size;

    gl_FragColor = texture2D(image, nst).rgba;
}