docs/examples/SkPath_quadTo_example_parametric.cpp

// Copyright 2020 Google LLC.
// Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
#include "tools/fiddle/examples.h"
REG_FIDDLE(SkPath_quadTo_example_parametric, 512, 512, false, 0) {
SkPoint quad(SkPoint p0, SkPoint p1, SkPoint p2, float t) {
    float s = 1 - t;
    return {(s * s * p0.x()) + (2 * s * t * p1.x()) + (t * t * p2.x()),
            (s * s * p0.y()) + (2 * s * t * p1.y()) + (t * t * p2.y())};
}

/*

        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 * x0) + (2 * s * t * x1) + (t * t * x2)
          y := (s * s * y0) + (2 * s * t * y1) + (t * t * y2)

*/

void draw(SkCanvas* canvas) {
    canvas->clear(SkColorSetARGB(255, 255, 255, 255));

    SkPaint paint;
    paint.setAntiAlias(true);
    paint.setStyle(SkPaint::kStroke_Style);
    paint.setStrokeWidth(5);

    SkPoint a{100, 100};
    SkPoint b{200, 400};
    SkPoint c{300, 100};

    SkPath twoSegments;
    twoSegments.moveTo(a);
    twoSegments.lineTo(b);
    twoSegments.lineTo(c);

    canvas->drawPath(twoSegments, paint);

    paint.setColor(SkColorSetARGB(255, 0, 0, 255));
    SkPath quadraticCurve;
    quadraticCurve.moveTo(a);
    quadraticCurve.quadTo(b, c);
    canvas->drawPath(quadraticCurve, paint);

    SkFont font(nullptr, 32);
    SkPaint textPaint;
    textPaint.setAntiAlias(true);
    canvas->drawString("a", a.x(), a.y(), font, textPaint);
    canvas->drawString("b", b.x() + 20, b.y() + 20, font, textPaint);
    canvas->drawString("c", c.x(), c.y(), font, textPaint);

    SkPaint pointPaint;
    pointPaint.setAntiAlias(true);
    pointPaint.setStrokeWidth(8);
    pointPaint.setStrokeCap(SkPaint::kRound_Cap);
    pointPaint.setColor(SkColorSetARGB(255, 0, 255, 0));
    int N = 15;
    for (int i = 0; i <= N; ++i) {
        SkPoint p = quad(a, b, c, (float)i / N);
        canvas->drawPoint(p.x(), p.y(), pointPaint);
    }
}
}  // END FIDDLE