1
2/*
3 * Copyright 2011 Google Inc.
4 *
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
7 */
8#include "Test.h"
9#include "SkColorPriv.h"
10#include "SkXfermode.h"
11
12// our std SkAlpha255To256
13static int test_srcover0(unsigned dst, unsigned alpha) {
14    return alpha + SkAlphaMul(dst, SkAlpha255To256(255 - alpha));
15}
16
17// faster hack +1
18static int test_srcover1(unsigned dst, unsigned alpha) {
19    return alpha + SkAlphaMul(dst, 256 - alpha);
20}
21
22// slower "correct"
23static int test_srcover2(unsigned dst, unsigned alpha) {
24    return alpha + SkMulDiv255Round(dst, 255 - alpha);
25}
26
27static void test_srcover_hack(skiatest::Reporter* reporter) {
28    /*  Here's the idea. Can we ensure that when we blend on top of an opaque
29        dst, that the result always stay's opaque (i.e. exactly 255)?
30     */
31
32    unsigned i;
33    int opaqueCounter0 = 0;
34    int opaqueCounter1 = 0;
35    int opaqueCounter2 = 0;
36    for (i = 0; i <= 255; i++) {
37        unsigned result0 = test_srcover0(0xFF, i);
38        unsigned result1 = test_srcover1(0xFF, i);
39        unsigned result2 = test_srcover2(0xFF, i);
40        opaqueCounter0 += (result0 == 0xFF);
41        opaqueCounter1 += (result1 == 0xFF);
42        opaqueCounter2 += (result2 == 0xFF);
43    }
44#if 0
45    SkDebugf("---- opaque test: [%d %d %d]\n",
46             opaqueCounter0, opaqueCounter1, opaqueCounter2);
47#endif
48    // we acknowledge that technique0 does not always return opaque
49    REPORTER_ASSERT(reporter, opaqueCounter0 == 256);
50    REPORTER_ASSERT(reporter, opaqueCounter1 == 256);
51    REPORTER_ASSERT(reporter, opaqueCounter2 == 256);
52
53    // Now ensure that we never over/underflow a byte
54    for (i = 0; i <= 255; i++) {
55        for (unsigned dst = 0; dst <= 255; dst++) {
56            unsigned r0 = test_srcover0(dst, i);
57            unsigned r1 = test_srcover1(dst, i);
58            unsigned r2 = test_srcover2(dst, i);
59            unsigned max = SkMax32(dst, i);
60            // ignore the known failure
61            if (dst != 255) {
62                REPORTER_ASSERT(reporter, r0 <= 255 && r0 >= max);
63            }
64            REPORTER_ASSERT(reporter, r1 <= 255 && r1 >= max);
65            REPORTER_ASSERT(reporter, r2 <= 255 && r2 >= max);
66
67#if 0
68            // this shows where r1 (faster) differs from r2 (more exact)
69            if (r1 != r2) {
70                SkDebugf("--- dst=%d i=%d r1=%d r2=%d exact=%g\n",
71                         dst, i, r1, r2, i + dst - dst*i/255.0f);
72            }
73#endif
74        }
75    }
76}
77
78#include "TestClassDef.h"
79DEFINE_TESTCLASS("SrcOver", SrcOverTestClass, test_srcover_hack)
80