Form based GUI

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

Font rendering with form based GUI

Draws "Hello World" to the screen and let's you control the font size and color with a GUI.

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#include <opencv2/v4d/v4d.hpp>
 
using namespace cv;
using namespace cv::v4d;
 
int main() {
    Ptr<V4D> window = V4D::make(960, 960, "Font Rendering with GUI", false, false, 0);
 
    //The font size
    float size = 40.0f;
    //The text hue
    float color[3] = {1.0f, 0.0f, 0.0f};
    //The text
    string hw = "hello world";
    //Setup the GUI
    window->imgui([&](ImGuiContext* ctx) {
        using namespace ImGui;
        SetCurrentContext(ctx);
        Begin("Settings");
        SliderFloat("Font Size", &size, 1.0f, 100.0f);
        ColorPicker3("Text Color", color);
        End();
    });
 
    window->run([&](Ptr<V4D> win) {
        //Render the text at the center of the screen using parameters from the GUI.
        win->nvg([&](const Size& sz) {
            using namespace cv::v4d::nvg;
            clear();
            fontSize(size);
            fontFace("sans-bold");
            fillColor(Scalar(color[2] * 255, color[1] * 255, color[0] * 255, 255));
            textAlign(NVG_ALIGN_CENTER | NVG_ALIGN_TOP);
            text(sz.width / 2.0, sz.height / 2.0, hw.c_str(), hw.c_str() + hw.size());
        });
 
        return win->display();
    });
}