Adding trim operation.

Closes #22

Signed-off-by: Yoan Blanc <yoan@dosimple.ch>

README.md

@@ -35,7 +35,7 @@ If you're using `gopkg.in`, you can still rely in the `v0` without worrying abou
 
 - Resize
 - Enlarge
-- Crop (including smart crop support)
+- Crop (including smart crop support, libvips 8.5+)
 - Rotate (with auto-rotate based on EXIF orientation)
 - Flip (with auto-flip based on EXIF metadata)
 - Flop
@@ -47,6 +47,7 @@ If you're using `gopkg.in`, you can still rely in the `v0` without worrying abou
 - Custom output color space (RGB, grayscale...)
 - Format conversion (with additional quality/compression settings)
 - EXIF metadata (size, alpha channel, profile, orientation...)
+- Trim (libvips 8.6+)
 
 ## Prerequisites
 

image.go

@@ -178,6 +178,13 @@ func (i *Image) Colourspace(c Interpretation) ([]byte, error) {
 	return i.Process(options)
 }
 
+// Trim removes the background from the picture. It can result in a 0x0 output
+// if the image is all background.
+func (i *Image) Trim() ([]byte, error) {
+	options := Options{Trim: 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.

image_test.go

@@ -457,8 +457,8 @@ func TestFluentInterface(t *testing.T) {
 
 func TestImageSmartCrop(t *testing.T) {
 
-	if !(VipsMajorVersion >= 8 && VipsMinorVersion > 4) {
-		t.Skipf("Skipping this test, libvips doesn't meet version requirement %s > 8.4", VipsVersion)
+	if !(VipsMajorVersion >= 8 && VipsMinorVersion >= 5) {
+		t.Skipf("Skipping this test, libvips doesn't meet version requirement %s >= 8.5", VipsVersion)
 	}
 
 	i := initImage("northern_cardinal_bird.jpg")
@@ -475,6 +475,26 @@ func TestImageSmartCrop(t *testing.T) {
 	Write("fixtures/test_smart_crop.jpg", buf)
 }
 
+func TestImageTrim(t *testing.T) {
+
+	if !(VipsMajorVersion >= 8 && VipsMinorVersion >= 6) {
+		t.Skipf("Skipping this test, libvips doesn't meet version requirement %s >= 8.6", VipsVersion)
+	}
+
+	i := initImage("transparent.png")
+	buf, err := i.Trim()
+	if err != nil {
+		t.Errorf("Cannot process the image: %#v", err)
+	}
+
+	err = assertSize(buf, 250, 208)
+	if err != nil {
+		t.Errorf("The image wasn't trimmed.")
+	}
+
+	Write("fixtures/transparent_trim.png", buf)
+}
+
 func TestImageLength(t *testing.T) {
 	i := initImage("test.jpg")
 

options.go

@@ -205,6 +205,7 @@ type Options struct {
 	NoProfile      bool
 	Interlace      bool
 	StripMetadata  bool
+	Trim           bool
 	Extend         Extend
 	Rotate         Angle
 	Background     Color

resizer.go

@@ -175,7 +175,8 @@ func normalizeOperation(o *Options, inWidth, inHeight int) {
 
 func shouldTransformImage(o Options, inWidth, inHeight int) bool {
 	return o.Force || (o.Width > 0 && o.Width != inWidth) ||
-		(o.Height > 0 && o.Height != inHeight) || o.AreaWidth > 0 || o.AreaHeight > 0
+		(o.Height > 0 && o.Height != inHeight) || o.AreaWidth > 0 || o.AreaHeight > 0 ||
+		o.Trim
 }
 
 func shouldApplyEffects(o Options) bool {
@@ -268,7 +269,12 @@ func extractOrEmbedImage(image *C.VipsImage, o Options) (*C.VipsImage, error) {
 		left, top := (o.Width-inWidth)/2, (o.Height-inHeight)/2
 		image, err = vipsEmbed(image, left, top, o.Width, o.Height, o.Extend, o.Background)
 		break
-
+	case o.Trim:
+		left, top, width, height, err := vipsTrim(image)
+		if err == nil {
+			image, err = vipsExtract(image, left, top, width, height)
+		}
+		break
 	case o.Top != 0 || o.Left != 0 || o.AreaWidth != 0 || o.AreaHeight != 0:
 		if o.AreaWidth == 0 {
 			o.AreaHeight = o.Width

vips.go

@@ -503,6 +503,22 @@ func vipsSmartCrop(image *C.VipsImage, width, height int) (*C.VipsImage, error)
 	return buf, nil
 }
 
+func vipsTrim(image *C.VipsImage) (int, int, int, int, error) {
+	defer C.g_object_unref(C.gpointer(image))
+
+	top := C.int(0)
+	left := C.int(0)
+	width := C.int(0)
+	height := C.int(0)
+
+	err := C.vips_find_trim_bridge(image, &top, &left, &width, &height)
+	if err != 0 {
+		return 0, 0, 0, 0, catchVipsError()
+	}
+
+	return int(top), int(left), int(width), int(height), nil
+}
+
 func vipsShrinkJpeg(buf []byte, input *C.VipsImage, shrink int) (*C.VipsImage, error) {
 	var image *C.VipsImage
 	var ptr = unsafe.Pointer(&buf[0])

vips.h

@@ -539,3 +539,11 @@ vips_smartcrop_bridge(VipsImage *in, VipsImage **out, int width, int height) {
 	return 0;
 #endif
 }
+
+int vips_find_trim_bridge(VipsImage *in, int *top, int *left, int *width, int *height) {
+#if (VIPS_MAJOR_VERSION >= 8 && VIPS_MINOR_VERSION >= 6)
+	return vips_find_trim(in, top, left, width, height, NULL);
+#else
+	return 0;
+#endif
+}