feautre/exr added AddAlpha method

image.go

@@ -196,6 +196,12 @@ func (i *Image) Gamma(exponent float64) ([]byte, error) {
 	return i.Process(options)
 }
 
+// AddGamma returns the alpha added image buffer.
+func (i *Image) AddAlpha() ([]byte, error) {
+	options := Options{AddAlpha: true}
+	return i.Process(options)
+}
+
 // Process processes the image based on the given transformation options,
 // talking with libvips bindings accordingly and returning the resultant
 // image buffer.

options.go

@@ -246,6 +246,7 @@ type Options struct {
 	OutputICC      string
 	InputICC       string
 	Palette        bool
+	AddAlpha       bool
 	// Speed defines the AVIF encoders CPU effort. Valid values are:
 	// 0-8 for AVIF encoding.
 	// 0-9 for PNG encoding.

resizer.go

@@ -154,6 +154,12 @@ func resizer(buf []byte, o Options) ([]byte, error) {
 		return nil, err
 	}
 
+	// Add Alpha, if necessary
+	image, err = addAlpha(image, o)
+	if err != nil {
+		return nil, err
+	}
+
 	return saveImage(image, o)
 }
 
@@ -437,6 +443,17 @@ func applyGamma(image *C.VipsImage, o Options) (*C.VipsImage, error) {
 	return image, nil
 }
 
+func addAlpha(image *C.VipsImage, o Options) (*C.VipsImage, error) {
+	var err error
+	if o.AddAlpha {
+		image, err = vipsAddAlpha(image, 255.0)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return image, nil
+}
+
 func zoomImage(image *C.VipsImage, zoom int) (*C.VipsImage, error) {
 	if zoom == 0 {
 		return image, nil

vips.go

@@ -825,6 +825,17 @@ func max(x int) int {
 	return int(math.Max(float64(x), 0))
 }
 
+func vipsAddAlpha(image *C.VipsImage, alpha float64) (*C.VipsImage, error) {
+	var out *C.VipsImage
+	defer C.g_object_unref(C.gpointer(image))
+
+	err := C.vips_add_band(image, &out, C.double(alpha))
+	if err != 0 {
+		return nil, catchVipsError()
+	}
+	return out, nil
+}
+
 func vipsDrawWatermark(image *C.VipsImage, o WatermarkImage) (*C.VipsImage, error) {
 	var out *C.VipsImage