Render vector graphics and manipulate the framebuffer

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

Vector graphics and framebuffer manipulation

The framebuffer can be accessed directly to manipulate data created in other contexts. In this case vector graphics is rendered to the framebuffer through NanoVG and then blurred using an `fb context.

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#include <opencv2/v4d/v4d.hpp>
#include <opencv2/v4d/util.hpp>
 
using namespace cv;
using namespace cv::v4d;
 
int main() {
    Ptr<V4D> window = V4D::make(960, 960, "Vector Graphics and Framebuffer", false, false, 0);
 
    window->run([](Ptr<V4D> win) {
        //Again creates a NanoVG context and draws googly eyes
        win->nvg([](const Size& sz) {
            //Calls from this namespace may only be used inside a nvg context
            using namespace cv::v4d::nvg;
            clear();
 
            static long start = cv::getTickCount() / cv::getTickFrequency();
            float t = cv::getTickCount() / cv::getTickFrequency() - start;
            float x = 0;
            float y = 0;
            float w = sz.width / 4;
            float h = sz.height / 4;
            translate((sz.width / 2.0f) - (w / 2.0f), (sz.height / 2.0f) - (h / 2.0f));
            float mx = w / 2.0;
            float my = h / 2.0;
            Paint gloss, bg;
            float ex = w * 0.23f;
            float ey = h * 0.5f;
            float lx = x + ex;
            float ly = y + ey;
            float rx = x + w - ex;
            float ry = y + ey;
            float dx, dy, d;
            float br = (ex < ey ? ex : ey) * 0.5f;
            float blink = 1 - pow(sinf(t * 0.5f), 200) * 0.8f;
 
            bg = linearGradient(x, y + h * 0.5f, x + w * 0.1f, y + h, cv::Scalar(0, 0, 0, 32), cv::Scalar(0,0,0,16));
            beginPath();
            ellipse(lx + 3.0f, ly + 16.0f, ex, ey);
            ellipse(rx + 3.0f, ry + 16.0f, ex, ey);
            fillPaint(bg);
            fill();
 
            bg = linearGradient(x, y + h * 0.25f, x + w * 0.1f, y + h,
                    cv::Scalar(220, 220, 220, 255), cv::Scalar(128, 128, 128, 255));
            beginPath();
            ellipse(lx, ly, ex, ey);
            ellipse(rx, ry, ex, ey);
            fillPaint(bg);
            fill();
 
            dx = (mx - rx) / (ex * 10);
            dy = (my - ry) / (ey * 10);
            d = sqrtf(dx * dx + dy * dy);
            if (d > 1.0f) {
                dx /= d;
                dy /= d;
            }
            dx *= ex * 0.4f;
            dy *= ey * 0.5f;
            beginPath();
            ellipse(lx + dx, ly + dy + ey * 0.25f * (1 - blink), br, br * blink);
            fillColor(cv::Scalar(32, 32, 32, 255));
            fill();
 
            dx = (mx - rx) / (ex * 10);
            dy = (my - ry) / (ey * 10);
            d = sqrtf(dx * dx + dy * dy);
            if (d > 1.0f) {
                dx /= d;
                dy /= d;
            }
            dx *= ex * 0.4f;
            dy *= ey * 0.5f;
            beginPath();
            ellipse(rx + dx, ry + dy + ey * 0.25f * (1 - blink), br, br * blink);
            fillColor(cv::Scalar(32, 32, 32, 255));
            fill();
 
            gloss = radialGradient(lx - ex * 0.25f, ly - ey * 0.5f, ex * 0.1f, ex * 0.75f,
                    cv::Scalar(255, 255, 255, 128), cv::Scalar(255, 255, 255, 0));
            beginPath();
            ellipse(lx, ly, ex, ey);
            fillPaint(gloss);
            fill();
 
            gloss = radialGradient(rx - ex * 0.25f, ry - ey * 0.5f, ex * 0.1f, ex * 0.75f,
                    cv::Scalar(255, 255, 255, 128), cv::Scalar(255, 255, 255, 0));
            beginPath();
            ellipse(rx, ry, ex, ey);
            fillPaint(gloss);
            fill();
        });
 
        //Provides the framebuffer as left-off by the nvg context.
        win->fb([](UMat& framebuffer) {
            //Heavily blurs the eyes using a cheap boxFilter
            boxFilter(framebuffer, framebuffer, -1, Size(15, 15), Point(-1,-1), true, BORDER_REPLICATE);
        });
 
        return win->display();
    });
}