C:Chroma Key Algorithm

// Software Chroma Keying in an Immersive Virtual Environment
// Adapted by A. Schiffler from: 
//  F. van den Bergh , V. Lalioti!  
//  Department of Computer Science, University of Pretoria, South Africa, fvdbergh@cs.up.ac.za
//  Department of Computer Science, University of Pretoria, South Africa, vlalioti@cs.up.ac.za
//  http://www.makebelieve.gr/vl/Publications/SAICSIT99.pdf
//
// alpha_map [0..511]:
// is linar drop from 255 to 0 over the range 100-255
unsigned char alpha_map[512];
//
// Call before using chromakey()
void init_alpha_map() {
 int i;
 for (i=0; i<512; i++) {
  if (i<128) {
   alpha_map[i] = 255;
  } else if (i<256) {
   alpha_map[i] = 255-2*(i-127);
  } else {
   alpha_map[i] = 0;
  }
 }
}
//
// s points to the interleaved RGBA image 
void chromakey(unsigned char *s, int imgsize) {
 unsigned char *rp=s,*gp=s+1, *bp=s+2,*ap=s+3;
 int i; 
 for (i = 0; i < imgsize; i++) { 
  if ( (*bp > *rp) && (*bp > *gp) ) {
   *ap = alpha_map[ ( ( (unsigned int)(*bp - *rp) + (unsigned int)(*bp + *gp) )  & 0x000001ff) ]; 
  } else {
   *ap = 255;
  }
  rp += 4; 
  bp += 4; 
  gp += 4; 
  ap += 4; 
 }
}