fugly!

unsigned char *data = main_ptr; // pointer the image data

for ( ii = 0; ii < _img.h; ii++)
{
    for ( b = 0; b < _img.w; b++)
    {
        *data = getGrayScale(*data);
        ++data;
    }

}


 int getGrayScale(int c) {


    int p[4];

    p[0] = (int) ((c & 0xff000000) >> 24); // Opacity level
    p[1] = (int) ((c & 0x00ff0000) >> 16); // Red level
    p[2] = (int) ((c & 0x0000ff00) >> 8); // Green level
    p[3] = (int) (c & 0x000000ff);  // Blue level


         // calculate the grayscale
         int bw=(p[1]+p[2]+p[3])/3;
         int result = (0x000000FF<<24)|(bw<<16)|(bw<<8)|bw;

         // create the new color values
         int new_r = (result >> 16) & 0xFF;
         int new_g = (result >> 8) & 0xFF;
         int new_b = result & 0xFF;

        int gc = (p[0] << 24 | new_r << 16 | new_g << 8| new_b);

     return gc;


 }