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+#version 100
+
+precision mediump float;
+
+/*************************************************************************************
+
+  The Sieve of Eratosthenes -- a simple shader by ProfJski
+  An early prime number sieve: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+
+  The screen is divided into a square grid of boxes, each representing an integer value.
+  Each integer is tested to see if it is a prime number.  Primes are colored white.
+  Non-primes are colored with a color that indicates the smallest factor which evenly divdes our integer.
+
+  You can change the scale variable to make a larger or smaller grid.  
+  Total number of integers displayed = scale squared, so scale = 100 tests the first 10,000 integers.
+
+  WARNING: If you make scale too large, your GPU may bog down!
+
+***************************************************************************************/
+
+// Input vertex attributes (from vertex shader)
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+
+// Make a nice spectrum of colors based on counter and maxSize
+vec4 Colorizer(float counter, float maxSize)
+{
+    float red = 0.0, green = 0.0, blue = 0.0;
+    float normsize = counter/maxSize;
+    
+    red = smoothstep(0.3, 0.7, normsize);
+    green = sin(3.14159*normsize);
+    blue = 1.0 - smoothstep(0.0, 0.4, normsize);
+    
+    return vec4(0.8*red, 0.8*green, 0.8*blue, 1.0);
+}
+
+void main()
+{
+    vec4 color = vec4(1.0);
+    float scale = 1000.0; // Makes 100x100 square grid. Change this variable to make a smaller or larger grid.
+    float value = scale*floor(fragTexCoord.y*scale) + floor(fragTexCoord.x*scale);  // Group pixels into boxes representing integer values
+    int valuei = int(value);
+    
+    //if ((valuei == 0) || (valuei == 1) || (valuei == 2)) gl_FragColor = vec4(1.0);
+    //else
+    {
+        //for (int i = 2; (i < int(max(2.0, sqrt(value) + 1.0))); i++)
+        // NOTE: On GLSL 100 for loops are restricted and loop condition must be a constant
+        // Tested on RPI, it seems loops are limited around 60 iteractions
+        for (int i = 2; i < 48; i++)
+        {
+            if ((value - float(i)*floor(value/float(i))) <= 0.0)
+            {
+                gl_FragColor = Colorizer(float(i), scale);
+                //break;    // Uncomment to color by the largest factor instead
+            }
+        }
+    }
+}