/*

        If the starting point is (x0, y0), then this curve is defined as the

        paramentric curve as `t` goes from 0 to 1:

          s := 1 - t

          x := (s * s * s * x0) +

               (3 * s * s * t * x1) +

               (3 * s * t * t * x2) +

               (t * t * t * x3)

          y := (s * s * s * y0) +

               (3 * s * s * t * y1) +

               (3 * s * t * t * y2) +

               (t * t * t * y3)

​

*/

​

SkPoint cubic(SkPoint p0, SkPoint p1, SkPoint p2, SkPoint p3, float t) {

    float s = 1 - t;

    return {(s * s * s * p0.x()) + (3 * s * s * t * p1.x()) + (3 * s * t * t * p2.x()) +

                    (t * t * t * p3.x()),

            (s * s * s * p0.y()) + (3 * s * s * t * p1.y()) + (3 * s * t * t * p2.y()) +

                    (t * t * t * p3.y())};

}

​

void draw(SkCanvas* canvas) {

    canvas->clear(SkColorSetARGB(255, 255, 255, 255));

    SkFont font(fontMgr->matchFamilyStyle(nullptr, {}), 32);

​

    SkPaint paint;

    paint.setAntiAlias(true);

    paint.setStyle(SkPaint::kStroke_Style);

    paint.setStrokeWidth(5);

​

    SkPoint a{136, 64};

    SkPoint b{448, 448};

    SkPoint c{64, 448};

    SkPoint d{376, 64};

​

    SkPath threeSegments;

    threeSegments.moveTo(a);

    threeSegments.lineTo(b);

    threeSegments.lineTo(c);

    threeSegments.lineTo(d);

​

    canvas->drawPath(threeSegments, paint);

​

    paint.setColor(SkColorSetARGB(255, 0, 0, 255));

    SkPath cubicCurve;

    cubicCurve.moveTo(a);

    cubicCurve.cubicTo(b, c, d);

    canvas->drawPath(cubicCurve, paint);

​

    SkPaint textPaint;

    textPaint.setColor(SkColorSetARGB(255, 0, 255, 0));

    textPaint.setAntiAlias(true);

    canvas->drawString("a", a.x(), a.y(), font, textPaint);

    canvas->drawString("b", b.x(), b.y(), font, textPaint);

    canvas->drawString("c", c.x() - 20, c.y(), font, textPaint);

    canvas->drawString("d", d.x(), d.y(), font, textPaint);

​

    SkPaint pointPaint;

    pointPaint.setAntiAlias(true);

    pointPaint.setStrokeWidth(8);

    pointPaint.setStrokeCap(SkPaint::kRound_Cap);

    pointPaint.setColor(SkColorSetARGB(255, 0, 255, 0));

    const int N = 16;

    for (int i = 0; i <= N; ++i) {

        SkPoint p = cubic(a, b, c, d, (float)i / N);

        canvas->drawPoint(p.x(), p.y(), pointPaint);

    }

    pointPaint.setColor(SkColorSetARGB(255, 255, 0, 0));

}