Premultiply alpha channel on resize

process.go

@@ -275,9 +275,25 @@ func processImage(data []byte, imgtype imageType, po processingOptions, t *timer
 				}
 			}
 
+			have_premultiplied := false
+			var bandFormat C.VipsBandFormat
+
+			if vipsImageHasAlpha(img) {
+				if bandFormat, err = vipsPremultiply(&img); err != nil {
+					return nil, err
+				}
+				have_premultiplied = true
+			}
+
 			if err = vipsResize(&img, scale); err != nil {
 				return nil, err
 			}
+
+			if have_premultiplied {
+				if err = vipsUnpremultiply(&img, bandFormat); err != nil {
+					return nil, err
+				}
+			}
 		}
 
 		if err = vipsFixColourspace(&img); err != nil {
@@ -359,6 +375,39 @@ func vipsSaveImage(img *C.struct__VipsImage, imgtype imageType) ([]byte, error)
 	return C.GoBytes(ptr, C.int(imgsize)), nil
 }
 
+func vipsImageHasAlpha(img *C.struct__VipsImage) bool {
+	return C.vips_image_hasalpha(img) > 0
+}
+
+func vipsPremultiply(img **C.struct__VipsImage) (C.VipsBandFormat, error) {
+	var tmp *C.struct__VipsImage
+
+	format := C.vips_band_format(*img)
+
+	if C.vips_premultiply_go(*img, &tmp) != 0 {
+		return 0, vipsError()
+	}
+
+	C.swap_and_clear(img, tmp)
+	return format, nil
+}
+
+func vipsUnpremultiply(img **C.struct__VipsImage, format C.VipsBandFormat) error {
+	var tmp *C.struct__VipsImage
+
+	if C.vips_unpremultiply_go(*img, &tmp) != 0 {
+		return vipsError()
+	}
+	C.swap_and_clear(img, tmp)
+
+	if C.vips_cast_go(*img, &tmp, format) != 0 {
+		return vipsError()
+	}
+	C.swap_and_clear(img, tmp)
+
+	return nil
+}
+
 func vipsResize(img **C.struct__VipsImage, scale float64) error {
 	var tmp *C.struct__VipsImage
 

vips.h

@@ -114,6 +114,26 @@ vips_support_smartcrop() {
 #endif
 }
 
+VipsBandFormat
+vips_band_format(VipsImage *in) {
+  return in->BandFmt;
+}
+
+int
+vips_premultiply_go(VipsImage *in, VipsImage **out) {
+  return vips_premultiply(in, out, NULL);
+}
+
+int
+vips_unpremultiply_go(VipsImage *in, VipsImage **out) {
+  return vips_unpremultiply(in, out, NULL);
+}
+
+int
+vips_cast_go(VipsImage *in, VipsImage **out, VipsBandFormat format) {
+  return vips_cast(in, out, format, NULL);
+}
+
 int
 vips_resize_go(VipsImage *in, VipsImage **out, double scale) {
   return vips_resize(in, out, scale, NULL);