/*

        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) {

    // a simple mathematical definition of cubic curve.

    // There are faster ways to calculate this.

    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())};

}

​

static SkPoint interpolate(SkPoint a, SkPoint b, float t) {

   return {SkScalarInterp(a.x(), b.x(), t),

           SkScalarInterp(a.y(), b.y(), t)};

}

​

void draw(SkCanvas* canvas) {

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

​

    SkPoint a{136,  64};

    SkPoint b{448, 448};

    SkPoint c{64,  448};

    SkPoint d{376, 64};

​

    SkPoint ab = interpolate(a, b, frame);

    SkPoint bc = interpolate(b, c, frame);

    SkPoint cd = interpolate(c, d, frame);

    SkPoint abc = interpolate(ab, bc, frame);

    SkPoint bcd = interpolate(bc, cd, frame);

​

    SkPaint paint;

    paint.setAntiAlias(true);

    paint.setStyle(SkPaint::kStroke_Style);

    paint.setStrokeWidth(1);

​

    canvas->drawLine(ab, bc, paint);

    canvas->drawLine(bc, cd, paint);

    canvas->drawLine(abc, bcd, paint);

​

    paint.setStrokeWidth(3);

    canvas->drawLine(a, b, paint);

    canvas->drawLine(b, c, paint);

    canvas->drawLine(c, d, paint);

​

    paint.setStrokeWidth(5);

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

    SkPath cubicCurve;

    cubicCurve.moveTo(a);

    cubicCurve.cubicTo(b, c, d);

    canvas->drawPath(cubicCurve, paint);

​

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

    SkPaint textPaint;

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

    textPaint.setAntiAlias(true);

​

    sk_sp<SkTypeface> face(fontMgr->matchFamilyStyle("DejaVu Sans Mono", SkFontStyle()));

    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);

    SkString msg = SkStringPrintf("%.4f", frame);

    textPaint.setColor(SkColorSetARGB(255, 204, 204, 204));

    canvas->drawString(msg.c_str(), 4, 36, font, textPaint);

​

    SkPaint pointPaint;

    pointPaint.setAntiAlias(true);

    pointPaint.setStrokeWidth(8);

    pointPaint.setStrokeCap(SkPaint::kRound_Cap);

​

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

    canvas->drawPoint(interpolate(abc, bcd, frame), pointPaint);

​

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

    pointPaint.setStrokeWidth(7);

    canvas->drawPoint(cubic(a, b, c, d, frame), pointPaint);

}