imgcodecs: Add IMDecodeIntoMat to reduce heap allocations (#1035)

* imgcodecs: add IMDecodeIntoMat which allows to imdecode into existing Mat for performance

imgcodecs.cpp

@@ -42,3 +42,8 @@ Mat Image_IMDecode(ByteArray buf, int flags) {
     cv::Mat img = cv::imdecode(data, flags);
     return new cv::Mat(img);
 }
+
+void Image_IMDecodeIntoMat(ByteArray buf, int flags, Mat dest) {
+    std::vector<uchar> data(buf.data, buf.data + buf.length);
+    cv::imdecode(data, flags, dest);
+}

imgcodecs.go

@@ -135,7 +135,6 @@ const (
 //
 // For further details, please see:
 // http://docs.opencv.org/master/d4/da8/group__imgcodecs.html#ga288b8b3da0892bd651fce07b3bbd3a56
-//
 func IMRead(name string, flags IMReadFlag) Mat {
 	cName := C.CString(name)
 	defer C.free(unsafe.Pointer(cName))
@@ -147,7 +146,6 @@ func IMRead(name string, flags IMReadFlag) Mat {
 //
 // For further details, please see:
 // http://docs.opencv.org/master/d4/da8/group__imgcodecs.html#gabbc7ef1aa2edfaa87772f1202d67e0ce
-//
 func IMWrite(name string, img Mat) bool {
 	cName := C.CString(name)
 	defer C.free(unsafe.Pointer(cName))
@@ -160,7 +158,6 @@ func IMWrite(name string, img Mat) bool {
 //
 // For further details, please see:
 // http://docs.opencv.org/master/d4/da8/group__imgcodecs.html#gabbc7ef1aa2edfaa87772f1202d67e0ce
-//
 func IMWriteWithParams(name string, img Mat, params []int) bool {
 	cName := C.CString(name)
 	defer C.free(unsafe.Pointer(cName))
@@ -196,7 +193,6 @@ const (
 //
 // For further details, please see:
 // http://docs.opencv.org/master/d4/da8/group__imgcodecs.html#ga461f9ac09887e47797a54567df3b8b63
-//
 func IMEncode(fileExt FileExt, img Mat) (buf *NativeByteBuffer, err error) {
 	cfileExt := C.CString(string(fileExt))
 	defer C.free(unsafe.Pointer(cfileExt))
@@ -211,11 +207,11 @@ func IMEncode(fileExt FileExt, img Mat) (buf *NativeByteBuffer, err error) {
 // using the image format passed in in the form of a file extension string.
 //
 // Usage example:
+//
 //	buffer, err := gocv.IMEncodeWithParams(gocv.JPEGFileExt, img, []int{gocv.IMWriteJpegQuality, quality})
 //
 // For further details, please see:
 // http://docs.opencv.org/master/d4/da8/group__imgcodecs.html#ga461f9ac09887e47797a54567df3b8b63
-//
 func IMEncodeWithParams(fileExt FileExt, img Mat, params []int) (buf *NativeByteBuffer, err error) {
 	cfileExt := C.CString(string(fileExt))
 	defer C.free(unsafe.Pointer(cfileExt))
@@ -242,7 +238,6 @@ func IMEncodeWithParams(fileExt FileExt, img Mat, params []int) (buf *NativeByte
 //
 // For further details, please see:
 // https://docs.opencv.org/master/d4/da8/group__imgcodecs.html#ga26a67788faa58ade337f8d28ba0eb19e
-//
 func IMDecode(buf []byte, flags IMReadFlag) (Mat, error) {
 	data, err := toByteArray(buf)
 	if err != nil {
@@ -250,3 +245,19 @@ func IMDecode(buf []byte, flags IMReadFlag) (Mat, error) {
 	}
 	return newMat(C.Image_IMDecode(*data, C.int(flags))), nil
 }
+
+// IMDecodeIntoMat reads an image from a buffer in memory into a matrix.
+// The function IMDecodeIntoMat reads an image from the specified buffer in memory.
+// If the buffer is too short or contains invalid data, the function
+// returns an error
+//
+// For further details, please see:
+// https://docs.opencv.org/4.x/d4/da8/group__imgcodecs.html#ga5a0acefe5cbe0a81e904e452ec7ca733
+func IMDecodeIntoMat(buf []byte, flags IMReadFlag, dest *Mat) error {
+	data, err := toByteArray(buf)
+	if err != nil {
+		return err
+	}
+	C.Image_IMDecodeIntoMat(*data, C.int(flags), dest.p)
+	return nil
+}

imgcodecs.h

@@ -17,6 +17,7 @@ void Image_IMEncode(const char* fileExt, Mat img, void* vector);
 
 void Image_IMEncode_WithParams(const char* fileExt, Mat img, IntVector params, void* vector);
 Mat Image_IMDecode(ByteArray buf, int flags);
+void Image_IMDecodeIntoMat(ByteArray buf, int flag, Mat dest);
 
 #ifdef __cplusplus
 }

imgcodecs_test.go

@@ -149,6 +149,25 @@ func TestIMDecode(t *testing.T) {
 		t.Error("Should not decode empty array")
 	}
 }
+
+func TestIMDecodeIntoMat(t *testing.T) {
+	mat := NewMat()
+	defer mat.Close()
+	content, err := ioutil.ReadFile("images/face-detect.jpg")
+	if err != nil {
+		t.Error("Invalid ReadFile in IMDecode")
+	}
+
+	err = IMDecodeIntoMat(content, IMReadColor, &mat)
+	if err != nil {
+		t.Error(err.Error())
+	}
+	if mat.Empty() {
+		t.Error("Invalid Mat in IMDecode")
+	}
+
+}
+
 func TestIMDecodeWebp(t *testing.T) {
 	content, err := ioutil.ReadFile("images/sample.webp")
 	if err != nil {