code: func (p *ycc) SubImage(r image.Rectangle) image.Image {
  	r = r.Intersect(p.Rect)
  	if r.Empty() {
  		return &ycc{SubsampleRatio: p.SubsampleRatio}
  	}
  	i := p.PixOffset(r.Min.X, r.Min.Y)
  	return &ycc{
  		Pix:            p.Pix[i:],
  		Stride:         p.Stride,
  		Rect:           r,
  		SubsampleRatio: p.SubsampleRatio,
  	}
  }
docs: // SubImage returns an image representing the portion of the image p visible
  // through r. The returned value shares pixels with the original image.