Add support for the vips_gamma() function via a new func Gamma() …

…on `ImageRef` (#433)

* Add support for vips_gamma() function for adjusting the gamma of the image

* Add support for vips_gamma() function for adjusting the gamma of the image

* Fix test for gamma

vips/conversion.c

@@ -235,7 +235,7 @@ int crop(VipsImage *in, VipsImage **out, int left, int top,
   if (n_pages <= 1) {
     return vips_crop(in, out, left, top, width, height, NULL);
   }
-  
+
   int in_width = in->Xsize;
   VipsObject *base = VIPS_OBJECT(vips_image_new());
   VipsImage **page = (VipsImage **) vips_object_local_array(base, n_pages);
@@ -374,3 +374,7 @@ int replicate(VipsImage *in, VipsImage **out, int across, int down) {
 int grid(VipsImage *in, VipsImage **out, int tileHeight, int across, int down){
   return vips_grid(in, out, tileHeight, across, down, NULL);
 }
+
+int adjust_gamma(VipsImage *in, VipsImage **out, double g) {
+  return vips_gamma(in, out, "exponent", g, NULL);
+}

vips/conversion.go

@@ -507,3 +507,14 @@ func vipsGrid(in *C.VipsImage, tileHeight, across, down int) (*C.VipsImage, erro
 	}
 	return out, nil
 }
+
+func vipsGamma(image *C.VipsImage, gamma float64) (*C.VipsImage, error) {
+	incOpCounter("gamma")
+	var out *C.VipsImage
+
+	if err := C.adjust_gamma(image, &out, C.double(gamma)); err != 0 {
+		return nil, handleImageError(out)
+	}
+
+	return out, nil
+}

vips/conversion.h

@@ -66,3 +66,5 @@ int is_16bit(VipsInterpretation interpretation);
 int replicate(VipsImage *in, VipsImage **out, int across, int down);
 
 int grid(VipsImage *in, VipsImage **out, int tileHeight, int across, int down);
+
+int adjust_gamma(VipsImage *in, VipsImage **out, double g);

vips/image.go

@@ -1304,6 +1304,17 @@ func (r *ImageRef) Linear1(a, b float64) error {
 	return nil
 }
 
+// Adjusts the image's gamma value.
+// See https://www.libvips.org/API/current/libvips-conversion.html#vips-gamma
+func (r *ImageRef) Gamma(gamma float64) error {
+	out, err := vipsGamma(r.image, gamma)
+	if err != nil {
+		return err
+	}
+	r.setImage(out)
+	return nil
+}
+
 // GetRotationAngleFromExif returns the angle which the image is currently rotated in.
 // First returned value is the angle and second is a boolean indicating whether image is flipped.
 // This is based on the EXIF orientation tag standard.

vips/image_test.go

@@ -1180,6 +1180,16 @@ func TestImageRef_SetPages(t *testing.T) {
 	require.Equal(t, 3, image.Pages())
 }
 
+func TestImageRef_SetGamma(t *testing.T) {
+	Startup(nil)
+
+	image, err := NewImageFromFile(resources + "png-24bit.png")
+	require.NoError(t, err)
+
+	err = image.Gamma(1.0 / 2.4)
+	require.NoError(t, err)
+}
+
 // TODO unit tests to cover:
 // NewImageFromReader failing test
 // NewImageFromFile failing test