feat: Implement AddText function, test and example

Author: GhiaC

README.md

@@ -41,6 +41,7 @@ If you're using `gopkg.in`, you can still rely in the `v0` without worrying abou
 - Thumbnail
 - Extract area
 - Watermark (using text or image)
+- AddText (support Persian)
 - Gaussian blur effect
 - Custom output color space (RGB, grayscale...)
 - Format conversion (with additional quality/compression settings)
@@ -265,6 +266,34 @@ if err != nil {
 bimg.Write("new.jpg", newImage)
 ```
 
+#### AddText
+
+```go
+buffer, err := bimg.Read("image.jpg")
+if err != nil {
+  fmt.Fprintln(os.Stderr, err)
+}
+
+addText := bimg.AddText{
+  Text:       "Hello | سلام",
+  Top:        50,
+  Left:       50,
+  Opacity:    0.25,
+  Width:      200,
+  DPI:        100,
+  Margin:     150,
+  Font:       "sans bold 12",
+  Background: bimg.Color{255, 255, 255},
+}
+
+newImage, err := bimg.NewImage(buffer).AddText(addText)
+if err != nil {
+  fmt.Fprintln(os.Stderr, err)
+}
+
+bimg.Write("new.jpg", newImage)
+```
+
 #### Fluent interface
 
 ```go

image.go

@@ -135,6 +135,12 @@ func (i *Image) Watermark(w Watermark) ([]byte, error) {
 	return i.Process(options)
 }
 
+// adds text on the given image.
+func (i *Image) AddText(a AddText) ([]byte, error) {
+	options := Options{AddText: a}
+	return i.Process(options)
+}
+
 // WatermarkImage adds image as watermark on the given image.
 func (i *Image) WatermarkImage(w WatermarkImage) ([]byte, error) {
 	options := Options{WatermarkImage: w}

image_test.go

@@ -241,6 +241,38 @@ func TestImageWatermark(t *testing.T) {
 
 	Write("testdata/test_watermark_text_out.jpg", buf)
 }
+func TestImageAddText(t *testing.T) {
+	image := initImage("test.jpg")
+	_, err := image.Crop(800, 600, GravityNorth)
+	if err != nil {
+		t.Errorf("Cannot process the image: %#v", err)
+	}
+
+	buf, err := image.AddText(AddText{
+		Width:      800,
+		Height:     600,
+		DPI:        100,
+		Top:        10,
+		Left:       500,
+		Text:       "Copy me if you can",
+		Background: Color{255, 255, 255},
+		Opacity:    1,
+	})
+	if err != nil {
+		t.Error(err)
+	}
+
+	err = assertSize(buf, 800, 600)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if DetermineImageType(buf) != JPEG {
+		t.Fatal("Image is not jpeg")
+	}
+
+	Write("test_add_text_out.jpg", buf)
+}
 
 func TestImageWatermarkWithImage(t *testing.T) {
 	image := initImage("test.jpg")

options.go

@@ -162,6 +162,19 @@ type Watermark struct {
 	Background  Color
 }
 
+// AddText represents the add-text options.
+type AddText struct {
+	Width      int
+	Height     int
+	DPI        int
+	Top        int
+	Left       int
+	Text       string
+	Font       string
+	Background Color
+	Opacity    float32
+}
+
 // WatermarkImage represents the image-based watermark supported options.
 type WatermarkImage struct {
 	Left    int
@@ -214,6 +227,7 @@ type Options struct {
 	Rotate         Angle
 	Background     Color
 	Gravity        Gravity
+	AddText        AddText
 	Watermark      Watermark
 	WatermarkImage WatermarkImage
 	Type           ImageType

resizer.go

@@ -113,6 +113,12 @@ func resizer(buf []byte, o Options) ([]byte, error) {
 		return nil, err
 	}
 
+	// Add text, if necessary
+	image, err = AddTextToImage(image, o.AddText)
+	if err != nil {
+		return nil, err
+	}
+
 	// Add watermark, if necessary
 	image, err = watermarkImageWithAnotherImage(image, o.WatermarkImage)
 	if err != nil {
@@ -355,6 +361,35 @@ func watermarkImageWithText(image *C.VipsImage, w Watermark) (*C.VipsImage, erro
 	return image, nil
 }
 
+func AddTextToImage(image *C.VipsImage, a AddText) (*C.VipsImage, error) {
+	if a.Text == "" {
+		return image, nil
+	}
+
+	// Defaults
+	if a.Font == "" {
+		a.Font = WatermarkFont
+	}
+	if a.Width == 0 {
+		a.Width = int(math.Floor(float64(image.Xsize / 6)))
+	}
+	if a.DPI == 0 {
+		a.DPI = 150
+	}
+	if a.Opacity == 0 {
+		a.Opacity = 0.25
+	} else if a.Opacity > 1 {
+		a.Opacity = 1
+	}
+
+	image, err := vipsAddText(image, a)
+	if err != nil {
+		return nil, err
+	}
+
+	return image, nil
+}
+
 func watermarkImageWithAnotherImage(image *C.VipsImage, w WatermarkImage) (*C.VipsImage, error) {
 
 	if len(w.Buf) == 0 {

vips.go

@@ -65,6 +65,16 @@ type vipsWatermarkOptions struct {
 	Background  [3]C.double
 }
 
+type vipsAddTextOptions struct {
+	Width      C.int
+	Height     C.int
+	DPI        C.int
+	Top        C.int
+	Left       C.int
+	Background [3]C.double
+	Opacity    C.float
+}
+
 type vipsWatermarkImageOptions struct {
 	Left    C.int
 	Top     C.int
@@ -292,6 +302,27 @@ func vipsWatermark(image *C.VipsImage, w Watermark) (*C.VipsImage, error) {
 	return out, nil
 }
 
+func vipsAddText(image *C.VipsImage, w AddText) (*C.VipsImage, error) {
+	var out *C.VipsImage
+
+	text := C.CString(w.Text)
+	font := C.CString(w.Font)
+	background := [3]C.double{C.double(w.Background.R), C.double(w.Background.G), C.double(w.Background.B)}
+
+	textOpts := vipsWatermarkTextOptions{text, font}
+	opts := vipsAddTextOptions{C.int(w.Width), C.int(w.Height), C.int(w.DPI), C.int(w.Top), C.int(w.Left), background, C.float(w.Opacity)}
+
+	defer C.free(unsafe.Pointer(text))
+	defer C.free(unsafe.Pointer(font))
+
+	err := C.vips_add_text(image, &out, (*C.WatermarkTextOptions)(unsafe.Pointer(&textOpts)), (*C.AddTextOptions)(unsafe.Pointer(&opts)))
+	if err != 0 {
+		return nil, catchVipsError()
+	}
+
+	return out, nil
+}
+
 func vipsRead(buf []byte) (*C.VipsImage, ImageType, error) {
 	var image *C.VipsImage
 	imageType := vipsImageType(buf)

vips.h

@@ -49,6 +49,16 @@ typedef struct {
 	double Background[3];
 } WatermarkOptions;
 
+typedef struct {
+	int    Width;
+	int    Height;
+	int    DPI;
+	int    Top;
+	int    Left;
+	double Background[3];
+	float    Opacity;
+} AddTextOptions;
+
 typedef struct {
 	int    Left;
 	int    Top;
@@ -451,6 +461,51 @@ vips_watermark(VipsImage *in, VipsImage **out, WatermarkTextOptions *to, Waterma
 	return 0;
 }
 
+int
+vips_add_text(VipsImage *in, VipsImage **out, WatermarkTextOptions *to, AddTextOptions *o) {
+	double ones[3] = { 1, 1, 1 };
+
+	VipsImage *base = vips_image_new();
+	VipsImage **t = (VipsImage **) vips_object_local_array(VIPS_OBJECT(base), 10);
+	t[0] = in;
+
+	// Make the mask.
+	if (
+		vips_text(&t[1], to->Text,
+			"width", o->Width,
+			"dpi", o->DPI,
+			"font", to->Font,
+			NULL) ||
+		vips_linear1(t[1], &t[2], o->Opacity, 0.0, NULL) ||
+		vips_cast(t[2], &t[3], VIPS_FORMAT_UCHAR, NULL) ||
+		vips_embed(t[3], &t[4], o->Left, o->Top, o->Width, o->Height, NULL)
+		) {
+		g_object_unref(base);
+		return 1;
+	}
+
+	// Make the constant image to paint the text with.
+	if (
+		vips_black(&t[5], 1, 1, NULL) ||
+		vips_linear(t[5], &t[6], ones, o->Background, 3, NULL) ||
+		vips_cast(t[6], &t[7], VIPS_FORMAT_UCHAR, NULL) ||
+		vips_copy(t[7], &t[8], "interpretation", t[0]->Type, NULL) ||
+		vips_embed(t[8], &t[9], 0, 0, t[0]->Xsize, t[0]->Ysize, "extend", VIPS_EXTEND_COPY, NULL)
+		) {
+		g_object_unref(base);
+		return 1;
+	}
+
+	// Blend the mask and text and write to output.
+	if (vips_ifthenelse(t[4], t[9], t[0], out, "blend", TRUE, NULL)) {
+		g_object_unref(base);
+		return 1;
+	}
+
+	g_object_unref(base);
+	return 0;
+}
+
 int
 vips_gaussblur_bridge(VipsImage *in, VipsImage **out, double sigma, double min_ampl) {
 #if (VIPS_MAJOR_VERSION == 7 && VIPS_MINOR_VERSION < 41)