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278 lines
9.6 KiB
C++
278 lines
9.6 KiB
C++
/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#include "PathHelpers.h"
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namespace mozilla {
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namespace gfx {
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UserDataKey sDisablePixelSnapping;
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void
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AppendRectToPath(PathBuilder* aPathBuilder,
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const Rect& aRect,
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bool aDrawClockwise)
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{
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if (aDrawClockwise) {
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aPathBuilder->MoveTo(aRect.TopLeft());
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aPathBuilder->LineTo(aRect.TopRight());
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aPathBuilder->LineTo(aRect.BottomRight());
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aPathBuilder->LineTo(aRect.BottomLeft());
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} else {
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aPathBuilder->MoveTo(aRect.TopRight());
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aPathBuilder->LineTo(aRect.TopLeft());
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aPathBuilder->LineTo(aRect.BottomLeft());
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aPathBuilder->LineTo(aRect.BottomRight());
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}
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aPathBuilder->Close();
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}
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void
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AppendRoundedRectToPath(PathBuilder* aPathBuilder,
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const Rect& aRect,
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const RectCornerRadii& aRadii,
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bool aDrawClockwise)
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{
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// For CW drawing, this looks like:
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//
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// ...******0** 1 C
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// ****
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// *** 2
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// **
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// *
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// *
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// 3
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// *
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// *
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//
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// Where 0, 1, 2, 3 are the control points of the Bezier curve for
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// the corner, and C is the actual corner point.
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//
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// At the start of the loop, the current point is assumed to be
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// the point adjacent to the top left corner on the top
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// horizontal. Note that corner indices start at the top left and
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// continue clockwise, whereas in our loop i = 0 refers to the top
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// right corner.
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//
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// When going CCW, the control points are swapped, and the first
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// corner that's drawn is the top left (along with the top segment).
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//
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// There is considerable latitude in how one chooses the four
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// control points for a Bezier curve approximation to an ellipse.
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// For the overall path to be continuous and show no corner at the
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// endpoints of the arc, points 0 and 3 must be at the ends of the
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// straight segments of the rectangle; points 0, 1, and C must be
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// collinear; and points 3, 2, and C must also be collinear. This
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// leaves only two free parameters: the ratio of the line segments
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// 01 and 0C, and the ratio of the line segments 32 and 3C. See
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// the following papers for extensive discussion of how to choose
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// these ratios:
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//
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// Dokken, Tor, et al. "Good approximation of circles by
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// curvature-continuous Bezier curves." Computer-Aided
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// Geometric Design 7(1990) 33--41.
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// Goldapp, Michael. "Approximation of circular arcs by cubic
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// polynomials." Computer-Aided Geometric Design 8(1991) 227--238.
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// Maisonobe, Luc. "Drawing an elliptical arc using polylines,
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// quadratic, or cubic Bezier curves."
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// http://www.spaceroots.org/documents/ellipse/elliptical-arc.pdf
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//
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// We follow the approach in section 2 of Goldapp (least-error,
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// Hermite-type approximation) and make both ratios equal to
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//
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// 2 2 + n - sqrt(2n + 28)
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// alpha = - * ---------------------
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// 3 n - 4
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//
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// where n = 3( cbrt(sqrt(2)+1) - cbrt(sqrt(2)-1) ).
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//
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// This is the result of Goldapp's equation (10b) when the angle
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// swept out by the arc is pi/2, and the parameter "a-bar" is the
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// expression given immediately below equation (21).
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//
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// Using this value, the maximum radial error for a circle, as a
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// fraction of the radius, is on the order of 0.2 x 10^-3.
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// Neither Dokken nor Goldapp discusses error for a general
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// ellipse; Maisonobe does, but his choice of control points
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// follows different constraints, and Goldapp's expression for
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// 'alpha' gives much smaller radial error, even for very flat
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// ellipses, than Maisonobe's equivalent.
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//
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// For the various corners and for each axis, the sign of this
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// constant changes, or it might be 0 -- it's multiplied by the
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// appropriate multiplier from the list before using.
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const Float alpha = Float(0.55191497064665766025);
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typedef struct { Float a, b; } twoFloats;
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twoFloats cwCornerMults[4] = { { -1, 0 }, // cc == clockwise
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{ 0, -1 },
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{ +1, 0 },
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{ 0, +1 } };
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twoFloats ccwCornerMults[4] = { { +1, 0 }, // ccw == counter-clockwise
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{ 0, -1 },
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{ -1, 0 },
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{ 0, +1 } };
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twoFloats *cornerMults = aDrawClockwise ? cwCornerMults : ccwCornerMults;
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Point cornerCoords[] = { aRect.TopLeft(), aRect.TopRight(),
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aRect.BottomRight(), aRect.BottomLeft() };
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Point pc, p0, p1, p2, p3;
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if (aDrawClockwise) {
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aPathBuilder->MoveTo(Point(aRect.X() + aRadii[RectCorner::TopLeft].width,
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aRect.Y()));
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} else {
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aPathBuilder->MoveTo(Point(aRect.X() + aRect.Width() - aRadii[RectCorner::TopRight].width,
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aRect.Y()));
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}
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for (int i = 0; i < 4; ++i) {
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// the corner index -- either 1 2 3 0 (cw) or 0 3 2 1 (ccw)
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int c = aDrawClockwise ? ((i+1) % 4) : ((4-i) % 4);
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// i+2 and i+3 respectively. These are used to index into the corner
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// multiplier table, and were deduced by calculating out the long form
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// of each corner and finding a pattern in the signs and values.
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int i2 = (i+2) % 4;
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int i3 = (i+3) % 4;
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pc = cornerCoords[c];
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if (aRadii[c].width > 0.0 && aRadii[c].height > 0.0) {
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p0.x = pc.x + cornerMults[i].a * aRadii[c].width;
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p0.y = pc.y + cornerMults[i].b * aRadii[c].height;
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p3.x = pc.x + cornerMults[i3].a * aRadii[c].width;
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p3.y = pc.y + cornerMults[i3].b * aRadii[c].height;
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p1.x = p0.x + alpha * cornerMults[i2].a * aRadii[c].width;
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p1.y = p0.y + alpha * cornerMults[i2].b * aRadii[c].height;
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p2.x = p3.x - alpha * cornerMults[i3].a * aRadii[c].width;
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p2.y = p3.y - alpha * cornerMults[i3].b * aRadii[c].height;
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aPathBuilder->LineTo(p0);
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aPathBuilder->BezierTo(p1, p2, p3);
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} else {
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aPathBuilder->LineTo(pc);
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}
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}
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aPathBuilder->Close();
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}
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void
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AppendEllipseToPath(PathBuilder* aPathBuilder,
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const Point& aCenter,
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const Size& aDimensions)
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{
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Size halfDim = aDimensions / 2.f;
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Rect rect(aCenter - Point(halfDim.width, halfDim.height), aDimensions);
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RectCornerRadii radii(halfDim.width, halfDim.height);
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AppendRoundedRectToPath(aPathBuilder, rect, radii);
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}
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bool
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SnapLineToDevicePixelsForStroking(Point& aP1, Point& aP2,
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const DrawTarget& aDrawTarget,
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Float aLineWidth)
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{
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Matrix mat = aDrawTarget.GetTransform();
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if (mat.HasNonTranslation()) {
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return false;
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}
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if (aP1.x != aP2.x && aP1.y != aP2.y) {
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return false; // not a horizontal or vertical line
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}
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Point p1 = aP1 + mat.GetTranslation(); // into device space
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Point p2 = aP2 + mat.GetTranslation();
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p1.Round();
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p2.Round();
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p1 -= mat.GetTranslation(); // back into user space
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p2 -= mat.GetTranslation();
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aP1 = p1;
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aP2 = p2;
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bool lineWidthIsOdd = (int(aLineWidth) % 2) == 1;
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if (lineWidthIsOdd) {
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if (aP1.x == aP2.x) {
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// snap vertical line, adding 0.5 to align it to be mid-pixel:
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aP1 += Point(0.5, 0);
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aP2 += Point(0.5, 0);
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} else {
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// snap horizontal line, adding 0.5 to align it to be mid-pixel:
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aP1 += Point(0, 0.5);
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aP2 += Point(0, 0.5);
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}
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}
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return true;
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}
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void
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StrokeSnappedEdgesOfRect(const Rect& aRect, DrawTarget& aDrawTarget,
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const ColorPattern& aColor,
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const StrokeOptions& aStrokeOptions)
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{
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if (aRect.IsEmpty()) {
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return;
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}
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Point p1 = aRect.TopLeft();
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Point p2 = aRect.BottomLeft();
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SnapLineToDevicePixelsForStroking(p1, p2, aDrawTarget,
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aStrokeOptions.mLineWidth);
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aDrawTarget.StrokeLine(p1, p2, aColor, aStrokeOptions);
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p1 = aRect.BottomLeft();
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p2 = aRect.BottomRight();
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SnapLineToDevicePixelsForStroking(p1, p2, aDrawTarget,
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aStrokeOptions.mLineWidth);
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aDrawTarget.StrokeLine(p1, p2, aColor, aStrokeOptions);
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p1 = aRect.TopLeft();
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p2 = aRect.TopRight();
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SnapLineToDevicePixelsForStroking(p1, p2, aDrawTarget,
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aStrokeOptions.mLineWidth);
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aDrawTarget.StrokeLine(p1, p2, aColor, aStrokeOptions);
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p1 = aRect.TopRight();
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p2 = aRect.BottomRight();
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SnapLineToDevicePixelsForStroking(p1, p2, aDrawTarget,
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aStrokeOptions.mLineWidth);
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aDrawTarget.StrokeLine(p1, p2, aColor, aStrokeOptions);
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}
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// The logic for this comes from _cairo_stroke_style_max_distance_from_path
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Margin
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MaxStrokeExtents(const StrokeOptions& aStrokeOptions,
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const Matrix& aTransform)
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{
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double styleExpansionFactor = 0.5f;
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if (aStrokeOptions.mLineCap == CapStyle::SQUARE) {
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styleExpansionFactor = M_SQRT1_2;
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}
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if (aStrokeOptions.mLineJoin == JoinStyle::MITER &&
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styleExpansionFactor < M_SQRT2 * aStrokeOptions.mMiterLimit) {
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styleExpansionFactor = M_SQRT2 * aStrokeOptions.mMiterLimit;
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}
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styleExpansionFactor *= aStrokeOptions.mLineWidth;
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double dx = styleExpansionFactor * hypot(aTransform._11, aTransform._21);
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double dy = styleExpansionFactor * hypot(aTransform._22, aTransform._12);
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return Margin(dy, dx, dy, dx);
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}
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} // namespace gfx
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} // namespace mozilla
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