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;
 
int main() {
    //Creates a V4D object
    Ptr<V4D> window = V4D::make(960, 960, "Display an Image through direct FB access", false, false, 0);
 
    //Loads an image as a UMat (just in case we have hardware acceleration available)
#ifdef __EMSCRIPTEN__
    UMat image = read_embedded_image("doc/lena.png").getUMat(ACCESS_READ);
#else
    UMat 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.
    UMat resized;
    UMat converted;
    resize(image, resized, window->framebufferSize());
    cvtColor(resized, converted, COLOR_RGB2BGRA);
 
    window->run([converted](Ptr<V4D> win){
        //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){
            converted.copyTo(framebuffer);
        });
        return win->display();
    });
}