add ExtractBandToImage (davidbyttow#324)

* Update image.go

Add function to extract a band into a new image instead of replacing the current image ref

* Update image.go

* add test

Co-authored-by: Roffe <roffe@roffe.nu>

vips/image.go

@@ -1060,6 +1060,15 @@ func (r *ImageRef) ExtractBand(band int, num int) error {
 	return nil
 }
 
+// ExtractBandToImage extracts one or more bands out of the image to a new image
+func (r *ImageRef) ExtractBandToImage(band int, num int) (*ImageRef, error) {
+	out, err := vipsExtractBand(r.image, band, num)
+	if err != nil {
+		return nil, err
+	}
+	return &ImageRef{image: out}, nil
+}
+
 // BandJoin joins a set of images together, bandwise.
 func (r *ImageRef) BandJoin(images ...*ImageRef) error {
 	vipsImages := []*C.VipsImage{r.image}

vips/image_golden_test.go

@@ -745,6 +745,13 @@ func TestImage_ExtractBand(t *testing.T) {
 	}, nil, nil)
 }
 
+func TestImage_ExtractBandToImage(t *testing.T) {
+	goldenTest(t, resources+"with_alpha.png", func(img *ImageRef) error {
+		_, err := img.ExtractBandToImage(2, 1)
+		return err
+	}, nil, nil)
+}
+
 func TestImage_Flatten(t *testing.T) {
 	goldenTest(t, resources+"with_alpha.png", func(img *ImageRef) error {
 		return img.Flatten(&Color{R: 32, G: 64, B: 128})