Display an image using direct framebuffer access

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Original author	Amir Hassan (kallaballa) amir@.nosp@m.viel.nosp@m.-zu.o.nosp@m.rg
Compatibility	OpenCV >= 4.7

Using direct framebuffer access

Instead of feeding to the video pipeline we can request the framebuffer in a fb context and copy the image to it. But first we must manually resize and color convert the image.

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#include <opencv2/v4d/v4d.hpp>
#include <opencv2/imgcodecs.hpp>
 
using namespace cv;
using namespace cv::v4d;
 
class DisplayImageFB : public Plan {
    UMat image_;
    UMat converted_;
public:
    void setup(cv::Ptr<V4D> win) override {
        win->parallel([](cv::UMat& image, cv::UMat& converted, const cv::Size& sz){
            //Loads an image as a UMat (just in case we have hardware acceleration available)
#ifdef __EMSCRIPTEN__
            image = read_embedded_image("doc/lena.png").getUMat(ACCESS_READ);
#else
            image = imread(samples::findFile("lena.jpg")).getUMat(ACCESS_READ);
#endif
            //We have to manually resize and color convert the image when using direct frambuffer access.
            resize(image, converted, sz);
            cvtColor(converted, converted, COLOR_RGB2BGRA);
        }, image_, converted_, win->fbSize());
    }
 
    void infer(Ptr<V4D> win) override {
        //Create a fb context and copy the prepared image to the framebuffer. The fb context
        //takes care of retrieving and storing the data on the graphics card (using CL-GL
        //interop if available), ready for other contexts to use
        win->fb([](UMat& framebuffer, const cv::UMat& c){
            c.copyTo(framebuffer);
        }, converted_);
    }
};
 
int main() {
    //Creates a V4D object
    Ptr<V4D> window = V4D::make(960, 960, "Display an Image through direct FB access");
    window->run<DisplayImageFB>(0);
}