Adding the shader examples

java/examples/Topics/Shaders/ImageMask/ImageMask.pde

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+/**
+ * Image Mask
+ * 
+ * Move the mouse to reveal the image through the dynamic mask.
+ */
+ 
+PShader maskShader;
+PImage srcImage;
+PGraphics maskImage;
+
+void setup() {
+  size(640, 360, P2D);
+  srcImage = loadImage("leaves.jpg");
+  maskImage = createGraphics(srcImage.width, srcImage.height, P2D);
+  maskImage.noSmooth();
+  maskShader = loadShader("mask.glsl");
+  maskShader.set("maskSampler", maskImage);
+  background(0);
+}
+
+void draw() { 
+  maskImage.beginDraw();
+  maskImage.background(0);
+  if (mouseX != 0 && mouseY != 0) {  
+    maskImage.noStroke();
+    maskImage.fill(255, 0, 0);
+    maskImage.ellipse(mouseX, mouseY, 50, 50);
+  }
+  maskImage.endDraw();
+
+  shader(maskShader);    
+  image(srcImage, 0, 0, width, height);
+}
+

java/examples/Topics/Shaders/ImageMask/data/mask.glsl

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+uniform sampler2D textureSampler;
+uniform sampler2D maskSampler;
+
+uniform vec2 texcoordOffset;
+varying vec4 vertColor;
+varying vec4 vertTexcoord;
+
+void main() {
+  vec4 texColor = texture2D(textureSampler, vertTexcoord.st).rgba;
+  vec4 maskColor = texture2D(maskSampler, vec2(vertTexcoord.s, 1.0 - vertTexcoord.t)).rgba;
+  gl_FragColor = mix(texColor, vec4(0, 0, 0, 0), 1.0 - maskColor.r);  
+}