3x3 Median
Morgan McGuire and Kyle Whitson

// Input texture
uniform sampler2D T;

#ifndef G3D
// vec2(1/width, 1/height) of the texture
    uniform vec2 Tinvsize;
#    define Tinvsize g3d_sampler2DInvSize(T))

// Change these 2 defines to change precision,
#define vec vec3
#define toVec(x) x.rgb

//#define vec vec4
//#define toVec(x) x.rgba

#define s2(a, b)		temp = a; a = min(a, b); b = max(temp, b);
#define mn3(a, b, c)		s2(a, b); s2(a, c);
#define mx3(a, b, c)		s2(b, c); s2(a, c);

#define mnmx3(a, b, c)		mx3(a, b, c); s2(a, b);                         // 3 exchanges
#define mnmx4(a, b, c, d)	s2(a, b); s2(c, d); s2(a, c); s2(b, d);         // 4 exchanges
#define mnmx5(a, b, c, d, e)	s2(a, b); s2(c, d); mn3(a, c, e); mx3(b, d, e); // 6 exchanges
#define mnmx6(a, b, c, d, e, f) s2(a, d); s2(b, e); s2(c, f); \
                                mn3(a, b, c); mx3(d, e, f);                // 7 exchanges

void main() {

  vec v[9];

  // Add the pixels which make up our window to the pixel array.
  for(int dX = -1; dX <= 1; ++dX) {
    for(int dY = -1; dY <= 1; ++dY) {		
      vec2 offset = vec2(float(dX), float(dY));
      // If a pixel in the window is located at (x+dX, y+dY), 
      // put it at index (dX + R)(2R + 1) + (dY + R) of the
      // pixel array. This will fill the pixel array, 
      // with the top left pixel of the window at pixel[0] and the
      // bottom right pixel of the window at pixel[N-1].
      v[(dX + 1) * 3 + (dY + 1)] = toVec(texture2D(T, gl_TexCoord[0].xy + offset * Tinvsize));

  vec temp;

  // Starting with a subset of size 6, remove the min and max each time
  mnmx6(v[0], v[1], v[2], v[3], v[4], v[5]);
  mnmx5(v[1], v[2], v[3], v[4], v[6]);
  mnmx4(v[2], v[3], v[4], v[7]);
  mnmx3(v[3], v[4], v[8]);
  toVec(gl_FragColor) = v[4];


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