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+/*
+From http://jgt.akpeters.com/papers/McGuire08/
+
+Efficient, High-Quality Bayer Demosaic Filtering on GPUs
+
+Morgan McGuire
+
+This paper appears in issue Volume 13, Number 4.
+---------------------------------------------------------
+Copyright (c) 2008, Morgan McGuire. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+//Pixel Shader
+
+/** Monochrome RGBA or GL_LUMINANCE Bayer encoded texture.*/
+uniform sampler2D       source;
+varying vec4            center;
+varying vec4            yCoord;
+varying vec4            xCoord;
+
+void main(void) {
+    #define fetch(x, y) texture2D(source, vec2(x, y)).r
+
+    float C = texture2D(source, center.xy).r; // ( 0, 0)
+    const vec4 kC = vec4( 4.0,  6.0,  5.0,  5.0) / 8.0;
+
+    // Determine which of four types of pixels we are on.
+    vec2 alternate = mod(floor(center.zw), 2.0);
+
+    vec4 Dvec = vec4(
+        fetch(xCoord[1], yCoord[1]),  // (-1,-1)
+        fetch(xCoord[1], yCoord[2]),  // (-1, 1)
+        fetch(xCoord[2], yCoord[1]),  // ( 1,-1)
+        fetch(xCoord[2], yCoord[2])); // ( 1, 1)
+
+    vec4 PATTERN = (kC.xyz * C).xyzz;
+
+    // Can also be a dot product with (1,1,1,1) on hardware where that is
+    // specially optimized.
+    // Equivalent to: D = Dvec[0] + Dvec[1] + Dvec[2] + Dvec[3];
+    Dvec.xy += Dvec.zw;
+    Dvec.x  += Dvec.y;
+
+    vec4 value = vec4(
+        fetch(center.x, yCoord[0]),   // ( 0,-2)
+        fetch(center.x, yCoord[1]),   // ( 0,-1)
+        fetch(xCoord[0], center.y),   // (-1, 0)
+        fetch(xCoord[1], center.y));  // (-2, 0)
+
+    vec4 temp = vec4(
+        fetch(center.x, yCoord[3]),   // ( 0, 2)
+        fetch(center.x, yCoord[2]),   // ( 0, 1)
+        fetch(xCoord[3], center.y),   // ( 2, 0)
+        fetch(xCoord[2], center.y));  // ( 1, 0)
+
+    // Even the simplest compilers should be able to constant-fold these to
+    // avoid the division.
+    // Note that on scalar processors these constants force computation of some
+    // identical products twice.
+    const vec4 kA = vec4(-1.0, -1.5,  0.5, -1.0) / 8.0;
+    const vec4 kB = vec4( 2.0,  0.0,  0.0,  4.0) / 8.0;
+    const vec4 kD = vec4( 0.0,  2.0, -1.0, -1.0) / 8.0;
+
+    // Conserve constant registers and take advantage of free swizzle on load
+    #define kE (kA.xywz)
+    #define kF (kB.xywz)
+
+    value += temp;
+
+    // There are five filter patterns (identity, cross, checker,
+    // theta, phi).  Precompute the terms from all of them and then
+    // use swizzles to assign to color channels.
+    //
+    // Channel   Matches
+    //   x       cross   (e.g., EE G)
+    //   y       checker (e.g., EE B)
+    //   z       theta   (e.g., EO R)
+    //   w       phi     (e.g., EO R)
+    #define A (value[0])
+    #define B (value[1])
+    #define D (Dvec.x)
+    #define E (value[2])
+    #define F (value[3])
+
+    // Avoid zero elements. On a scalar processor this saves two MADDs
+    // and it has no effect on a vector processor.
+    PATTERN.yzw += (kD.yz * D).xyy;
+
+    PATTERN += (kA.xyz * A).xyzx + (kE.xyw * E).xyxz;
+    PATTERN.xw  += kB.xw * B;
+    PATTERN.xz  += kF.xz * F;
+
+    gl_FragColor.rgb = (alternate.y == 0.0) ?
+        ((alternate.x == 0.0) ?
+            vec3(C, PATTERN.xy) :
+            vec3(PATTERN.z, C, PATTERN.w)) :
+        ((alternate.x == 0.0) ?
+            vec3(PATTERN.w, C, PATTERN.z) :
+            vec3(PATTERN.yx, C));
+}