func newProjector(dst image.Rectangle, src image.Rectangle) *projector {
	return &projector{
		rx: float64(dst.Dx()) / float64(src.Dx()),
		ry: float64(dst.Dy()) / float64(src.Dy()),
		dx: float64(dst.Min.X - src.Min.X),
		dy: float64(dst.Min.Y - src.Min.Y),
		r:  dst,
	}
}
newProjector creates a Projector with source src and target dst.
package detect
import (
	"image"
)
type projector struct {
	rx, ry float64
	dx, dy float64
	r image.Rectangle
}