rotateimage.go

func RotateAround(pt, origin image.Point, angle float64) (int, int) {
	x1, y1 := float64(pt.X-origin.X), float64(pt.Y-origin.Y)
	magnitude := math.Hypot(x1, y1)
	rotationAngle := math.Atan2(y1, x1) + angle
	return origin.X + (int(math.Cos(rotationAngle) * magnitude)), origin.Y + (int(math.Sin(rotationAngle) * magnitude))
}


func RotatedImage(i *ebiten.Image, rotation float64) *ebiten.Image {
	if i == nil {
		return i
	}

	w, h := i.Size()
	cx, cy := w/2, h/2
	maxDim := int(math.Hypot(float64(w), float64(h)))
	center := maxDim / 2
	newimage := ebiten.NewImage(maxDim, maxDim)
	rx, ry := RotateAround(
		image.Point{center - cx, center - cy},
		image.Point{center, center},
		rotation,
	)

	op := ebiten.DrawImageOptions{}
	op.GeoM.Rotate(rotation)
	op.GeoM.Translate(float64(rx), float64(ry))
	newimage.DrawImage(i, &op)

	return newimage
}