Image filter code problem

SDL_Surface *applyfilter(double *filter,int w, int h, SDL_Surface *image)
{
 SDL_Surface *ret=SDL_CreateRGBSurface(SDL_SWSURFACE,image->w,image->h,24,0,0,0,0);
 SDL_LockSurface(ret);
 for(int y=0;y<image->h;y++)
 {
  for(int x=0;x<image->w;x++)
  {
   int used=0,c=0,d=0;
   double sumr=0,sumg=0,sumb=0;
   for(int b=y-((h-1)/2);;b++)
   {
    for(int a=x-((w-1)/2);;a++)
    {
     if((a>0 && a<image->w)&&(b>0 && b<image->h))
     {
      used++;
      Uint8 red,green,blue;
      SDL_GetRGB(GetPixel(image,a,b),image->format,&red,&green,&blue);
      sumr+=(int)(red)*filter[c+(d*w)];
      sumg+=(int)(green)*filter[c+(d*w)];
      sumb+=(int)(blue)*filter[c+(d*w)];
     }
     c++;
     if(c==w)
      break;
    }
    c=0;
    d++;
    if(d==h)
      break;
   }
   sumr/=used/(w*h); 
   sumg/=used/(w*h);
   sumb/=used/(w*h);
   SetPixel(ret,x,y,(int)(sumr+0.5),(int)(sumg+0.5),(int)(sumb+0.5));
  }
 }
 SDL_UnlockSurface(ret);
 return ret;
}