#include <stdio.h> #include <stdlib.h> #include <queue>  typedef unsigned char byte; typedef std::pair<int,int> coord; const int    N = 1024; const size_t C = 4; const size_t S = 28000000;  byte M[N][N]; const int dx[4] = {1,0,-1,0};  const int dy[4] = {0,1,0,-1};  int main() {     memset(M,0,N*N); 	     for(size_t steps=0;steps<S;++steps) {         std::queue<coord> q;         if( ++M[N/2][N/2]>=C ) q.push( coord(N/2,N/2) );         while( !q.empty() ) {             const int x = q.front().first;             const int y = q.front().second;             q.pop();             if(M[x][y]<C) continue;             M[x][y] -= 4;             for(int d=0;d<4;++d) {                 const int lx = x+dx[d];                 const int ly = y+dy[d];                 if(lx<0 || lx>=N) continue;                 if(ly<0 || ly>=N) continue; 		if(++M[lx][ly]>=C) q.push( coord(lx,ly) );             }         }     }      const byte co[4][3] = { {255,255,255}, {255,0,0}, {0,255,0}, {0,0,255} };     FILE * fo = fopen("backtang2.ppm","wb");     fprintf(fo,"P6\n%d %d\n255\n",N,N);     for(int x=0;x<N;++x) for(int y=0;y<N;++y)         fwrite(co[M[x][y]],1,3,fo);     fclose(fo);      return 0; }