Image DownScale(const Image& img, int nx, int ny)
{
	ASSERT(nx > 0 && ny > 0);
	Size ssz = img.GetSize();
	Size tsz = Size((ssz.cx + nx - 1) / nx, (ssz.cy + ny - 1) / ny);
	int div = nx * ny;
	Buffer<RGBAV> b(tsz.cx);
	ImageBuffer ib(tsz);
	RGBA *it = ~ib;
	int scx0 = ssz.cx / nx * nx;
	for(int yy = 0; yy < ssz.cy; yy += ny) {
		for(int i = 0; i < tsz.cx; i++)
			b[i].Clear();
		for(int yi = 0; yi < ny; yi++) {
			int y = yy + yi;
			if(y < ssz.cy) {
				const RGBA *s = img[y];
				const RGBA *e = s + scx0;
				const RGBA *e2 = s + ssz.cx;
				RGBAV *t = ~b;
				while(s < e) {
					for(int n = nx; n--;)
						t->Put(*s++);
					t++;
				}
				while(s < e2)
					t->Put(*s++);
				ASSERT(s <= ~img + img.GetLength());
			}
		}
		const RGBAV *s = ~b;
		for(int x = 0; x < tsz.cx; x++)
			*it++ = (s++)->Get(div);
	}
	return ib;
}