added tutorial on custom sources and sinks

modules/viz2d/samples/cpp/custom_source_and_sink.cpp

@@ -0,0 +1,69 @@
+#include <opencv2/viz2d/viz2d.hpp>
+#include <opencv2/viz2d/nvg.hpp>
+#include <opencv2/imgcodecs.hpp>
+
+using namespace cv;
+using namespace cv::viz;
+
+constexpr int WIDTH = 1280;
+constexpr int HEIGHT = 720;
+
+int main(int argc, char** argv) {
+	string hv = "Hello Rainbow!";
+	Ptr<Viz2D> v2d = Viz2D::make(Size(WIDTH, HEIGHT), "Video Editing");
+	//Make a Source that generates rainbow frames.
+	Source src([=](cv::UMat& frame){
+        static long cnt = 0;
+
+	    if(frame.empty())
+	        frame.create(v2d->getFrameBufferSize(), CV_8UC3);
+	    frame = colorConvert(Scalar(cnt % 180, 128, 128, 255), COLOR_HLS2BGR);
+
+	    ++cnt;
+	    if(cnt > std::numeric_limits<long>().max() / 2.0)
+	        cnt = 0;
+	    return true;
+	}, 60.0f);
+
+	//Make a Sink the saves each frame to a PNG file.
+	Sink sink([](const cv::UMat& frame){
+        static long cnt = 0;
+
+	    try {
+	        imwrite(std::to_string(cnt) + ".png", frame);
+	    } catch(std::exception& ex) {
+	        cerr << "Unable to write frame: " << ex.what() << endl;
+	        return false;
+	    }
+
+        ++cnt;
+        if(cnt > std::numeric_limits<long>().max() / 2.0)
+            cnt = 0;
+
+	    return true;
+	});
+
+	//Attach source and sink
+	v2d->setSource(src);
+	v2d->setSink(sink);
+
+	v2d->run([=]() {
+	    //Capture video from the Source
+		if(!v2d->capture())
+			return false; //end of input video
+
+		//Render "Hello Rainbow!" over the frame
+		v2d->nvg([=](const Size& sz) {
+			using namespace cv::viz::nvg;
+
+			fontSize(40.0f);
+			fontFace("sans-bold");
+			fillColor(Scalar(255, 0, 0, 255));
+			textAlign(NVG_ALIGN_CENTER | NVG_ALIGN_TOP);
+			text(sz.width / 2.0, sz.height / 2.0, hv.c_str(), hv.c_str() + hv.size());
+		});
+		v2d->write(); //Write video to the Sink
+		return v2d->display(); //Display the framebuffer in the native window
+	});
+}
+

modules/viz2d/tutorials/06-custom_source_and_sink.markdown

@@ -9,9 +9,9 @@
 | Compatibility | OpenCV >= 4.7 |
 
 ## Reading and writing to Viz2D using custom Sources and Sinks
-In the previous tutorial we used a default video source and a video sink to stream a video through Viz2D's framebuffer which can be manipulated using OpenGL or NanoVG - hence video editing.
+In the previous tutorial we used a default video source and a video sink to stream a video through Viz2D which can be manipulated using OpenGL or NanoVG. In this example we are creating a custom Source that generates rainbow frames. For each time the Source is invoked the frame is colored a slightly different color. Additionally the custom Sink saves individual images instead of a video.
 
 @include samples/cpp/custom_source_and_sink.cpp
 
-![The result](doc/video_editing.png)
+![The result](doc/custom_source_and_sink.png)