skia2/docs/examples/SkPath_quadTo_example_parametric.cpp

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// 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