SkQuadClipper.cpp revision 0910916c0f7b951ee55c4b7c6358295b9bca0565 
1/*
2 * Copyright (C) 2009 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#include "SkQuadClipper.h"
18#include "SkGeometry.h"
19
20static bool chopMonoQuadAtY(SkPoint pts[3], SkScalar y, SkScalar* t) {
21    /* Solve F(t) = y where F(t) := [0](1-t)^2 + 2[1]t(1-t) + [2]t^2
22     *  We solve for t, using quadratic equation, hence we have to rearrange
23     * our cooefficents to look like At^2 + Bt + C
24     */
25    SkScalar A = pts[0].fY - pts[1].fY - pts[1].fY + pts[2].fY;
26    SkScalar B = 2*(pts[1].fY - pts[0].fY);
27    SkScalar C = pts[0].fY - y;
28
29    SkScalar roots[2];  // we only expect one, but make room for 2 for safety
30    int count = SkFindUnitQuadRoots(A, B, C, roots);
31    if (count) {
32        *t = roots[0];
33        return true;
34    }
35    return false;
36}
37
38SkQuadClipper::SkQuadClipper() {}
39
40void SkQuadClipper::setClip(const SkIRect& clip) {
41    // conver to scalars, since that's where we'll see the points
42    fClip.set(clip);
43}
44
45/*  If we somehow returned the fact that we had to flip the pts in Y, we could
46    communicate that to setQuadratic, and then avoid having to flip it back
47    here (only to have setQuadratic do the flip again)
48 */
49bool SkQuadClipper::clipQuad(const SkPoint srcPts[3], SkPoint dst[3]) {
50    bool reverse;
51
52    // we need the data to be monotonically descending in Y
53    if (srcPts[0].fY > srcPts[2].fY) {
54        dst[0] = srcPts[2];
55        dst[1] = srcPts[1];
56        dst[2] = srcPts[0];
57        reverse = true;
58    } else {
59        memcpy(dst, srcPts, 3 * sizeof(SkPoint));
60        reverse = false;
61    }
62
63    // are we completely above or below
64    const SkScalar ctop = fClip.fTop;
65    const SkScalar cbot = fClip.fBottom;
66    if (dst[2].fY <= ctop || dst[0].fY >= cbot) {
67        return false;
68    }
69
70    SkScalar t;
71    SkPoint tmp[5]; // for SkChopQuadAt
72
73    // are we partially above
74    if (dst[0].fY < ctop && chopMonoQuadAtY(dst, ctop, &t)) {
75        SkChopQuadAt(dst, tmp, t);
76        dst[0] = tmp[2];
77        dst[1] = tmp[3];
78    }
79
80    // are we partially below
81    if (dst[2].fY > cbot && chopMonoQuadAtY(dst, cbot, &t)) {
82        SkChopQuadAt(dst, tmp, t);
83        dst[1] = tmp[1];
84        dst[2] = tmp[2];
85    }
86
87    if (reverse) {
88        SkTSwap<SkPoint>(dst[0], dst[2]);
89    }
90    return true;
91}
92
93