new example: shaders_eratosthenes

Contributed by ProfJski

examples/shaders/resources/shaders/glsl100/eratosthenes.fs

@@ -0,0 +1,58 @@
+#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.
+	int value = int(scale*floor(fragTexCoord.y*scale) + floor(fragTexCoord.x*scale));  // Group pixels into boxes representing integer values
+
+    if ((value == 0) || (value == 1) || (value == 2)) gl_FragColor = vec4(1.0);
+    else
+    {
+        for (int i = 2; (i < max(2, sqrt(value) + 1)); i++) 
+        {
+            if ((value - i*floor(value/i)) == 0) 
+            {
+                color = Colorizer(float(i), scale);
+                //break;    // Uncomment to color by the largest factor instead
+            }
+        }
+
+        gl_FragColor = color;
+    }
+}