void
pgm_save(unsigned char *buf, int wrap, int xsize, int ysize, char *filename)
{
	FILE *f;
	int i;

	f = fopen(filename, "w");
	fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);
	for(i=0; i < ysize; i++)
		fwrite(buf + i * wrap, 1, xsize, f);
	fclose(f);
}