SrcOverTest.cpp revision d66efc273e4996fb18f343bd30dcd506a1698b5b 
1#include "Test.h"
2#include "SkColorPriv.h"
3#include "SkXfermode.h"
4
5// our std SkAlpha255To256
6static int test_srcover0(unsigned dst, unsigned alpha) {
7    return alpha + SkAlphaMul(dst, SkAlpha255To256(255 - alpha));
8}
9
10// faster hack +1
11static int test_srcover1(unsigned dst, unsigned alpha) {
12    return alpha + SkAlphaMul(dst, 256 - alpha);
13}
14
15// slower "correct"
16static int test_srcover2(unsigned dst, unsigned alpha) {
17    return alpha + SkMulDiv255Round(dst, 255 - alpha);
18}
19
20static void test_srcover_hack(skiatest::Reporter* reporter) {
21    /*  Here's the idea. Can we ensure that when we blend on top of an opaque
22        dst, that the result always stay's opaque (i.e. exactly 255)?
23     */
24
25    unsigned i;
26    int opaqueCounter0 = 0;
27    int opaqueCounter1 = 0;
28    int opaqueCounter2 = 0;
29    for (i = 0; i <= 255; i++) {
30        unsigned result0 = test_srcover0(0xFF, i);
31        unsigned result1 = test_srcover1(0xFF, i);
32        unsigned result2 = test_srcover2(0xFF, i);
33        opaqueCounter0 += (result0 == 0xFF);
34        opaqueCounter1 += (result1 == 0xFF);
35        opaqueCounter2 += (result2 == 0xFF);
36    }
37#if 0
38    SkDebugf("---- opaque test: [%d %d %d]\n",
39             opaqueCounter0, opaqueCounter1, opaqueCounter2);
40#endif
41    // we acknowledge that technique0 does not always return opaque
42    REPORTER_ASSERT(reporter, opaqueCounter0 == 129);
43    REPORTER_ASSERT(reporter, opaqueCounter1 == 256);
44    REPORTER_ASSERT(reporter, opaqueCounter2 == 256);
45
46    // Now ensure that we never over/underflow a byte
47    for (i = 0; i <= 255; i++) {
48        for (unsigned dst = 0; dst <= 255; dst++) {
49            unsigned r0 = test_srcover0(dst, i);
50            unsigned r1 = test_srcover1(dst, i);
51            unsigned r2 = test_srcover2(dst, i);
52            unsigned max = SkMax32(dst, i);
53            // ignore the known failure
54            if (dst != 255) {
55                REPORTER_ASSERT(reporter, r0 <= 255 && r0 >= max);
56            }
57            REPORTER_ASSERT(reporter, r1 <= 255 && r1 >= max);
58            REPORTER_ASSERT(reporter, r2 <= 255 && r2 >= max);
59
60#if 0
61            // this shows where r1 (faster) differs from r2 (more exact)
62            if (r1 != r2) {
63                SkDebugf("--- dst=%d i=%d r1=%d r2=%d exact=%g\n", dst, i, r1, r2, i + dst - dst*i/255.0f);
64            }
65#endif
66        }
67    }
68}
69
70#include "TestClassDef.h"
71DEFINE_TESTCLASS("SrcOver", SrcOverTestClass, test_srcover_hack)
72