tests/BlendTest.cpp

#include "Test.h"
#include "SkColor.h"

#define ASSERT(x) REPORTER_ASSERT(r, x)

static uint8_t double_to_u8(double d) {
    SkASSERT(d >= 0);
    SkASSERT(d < 256);
    return uint8_t(d);
}

// All algorithms we're testing have this interface.
// We want a single channel blend, src over dst, assuming src is premultiplied by srcAlpha.
typedef uint8_t(*Blend)(uint8_t dst, uint8_t src, uint8_t srcAlpha);

// This is our golden algorithm.
static uint8_t blend_double_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    SkASSERT(src <= srcAlpha);
    return double_to_u8(0.5 + src + dst * (255.0 - srcAlpha) / 255.0);
}

static uint8_t abs_diff(uint8_t a, uint8_t b) {
    const int diff = a - b;
    return diff > 0 ? diff : -diff;
}

static void test(skiatest::Reporter* r, int maxDiff, Blend algorithm,
                 uint8_t dst, uint8_t src, uint8_t alpha) {
    const uint8_t golden = blend_double_round(dst, src, alpha);
    const uint8_t  blend =          algorithm(dst, src, alpha);
    if (abs_diff(blend, golden) > maxDiff) {
        SkDebugf("dst %02x, src %02x, alpha %02x, |%02x - %02x| > %d\n",
                 dst, src, alpha, blend, golden, maxDiff);
        ASSERT(abs_diff(blend, golden) <= maxDiff);
    }
}

// Exhaustively compare an algorithm against our golden, for a given alpha.
static void test_alpha(skiatest::Reporter* r, uint8_t alpha, int maxDiff, Blend algorithm) {
    SkASSERT(maxDiff >= 0);

    for (unsigned src = 0; src <= alpha; src++) {
        for (unsigned dst = 0; dst < 256; dst++) {
            test(r, maxDiff, algorithm, dst, src, alpha);
        }
    }
}

// Exhaustively compare an algorithm against our golden, for a given dst.
static void test_dst(skiatest::Reporter* r, uint8_t dst, int maxDiff, Blend algorithm) {
    SkASSERT(maxDiff >= 0);

    for (unsigned alpha = 0; alpha < 256; alpha++) {
        for (unsigned src = 0; src <= alpha; src++) {
            test(r, maxDiff, algorithm, dst, src, alpha);
        }
    }
}

static uint8_t blend_double_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    return double_to_u8(src + dst * (255.0 - srcAlpha) / 255.0);
}

static uint8_t blend_float_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    return double_to_u8(src + dst * (255.0f - srcAlpha) / 255.0f);
}

static uint8_t blend_float_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    return double_to_u8(0.5f + src + dst * (255.0f - srcAlpha) / 255.0f);
}

static uint8_t blend_255_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 255 - srcAlpha;
    const uint16_t product = dst * invAlpha;
    return src + (product >> 8);
}

static uint8_t blend_255_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 255 - srcAlpha;
    const uint16_t product = dst * invAlpha + 128;
    return src + (product >> 8);
}

static uint8_t blend_256_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 256 - (srcAlpha + (srcAlpha >> 7));
    const uint16_t product = dst * invAlpha;
    return src + (product >> 8);
}

static uint8_t blend_256_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 256 - (srcAlpha + (srcAlpha >> 7));
    const uint16_t product = dst * invAlpha + 128;
    return src + (product >> 8);
}

static uint8_t blend_256_round_alt(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint8_t invAlpha8 = 255 - srcAlpha;
    const uint16_t invAlpha = invAlpha8 + (invAlpha8 >> 7);
    const uint16_t product = dst * invAlpha + 128;
    return src + (product >> 8);
}

static uint8_t blend_256_plus1_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 256 - (srcAlpha + 1);
    const uint16_t product = dst * invAlpha;
    return src + (product >> 8);
}

static uint8_t blend_256_plus1_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint16_t invAlpha = 256 - (srcAlpha + 1);
    const uint16_t product = dst * invAlpha + 128;
    return src + (product >> 8);
}

static uint8_t blend_perfect(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
    const uint8_t invAlpha = 255 - srcAlpha;
    const uint16_t product = dst * invAlpha + 128;
    return src + ((product + (product >> 8)) >> 8);
}


// We want 0 diff whenever src is fully transparent.
DEF_TEST(Blend_alpha_0x00, r) {
    const uint8_t alpha = 0x00;

    // GOOD
    test_alpha(r, alpha, 0, blend_256_round);
    test_alpha(r, alpha, 0, blend_256_round_alt);
    test_alpha(r, alpha, 0, blend_256_trunc);
    test_alpha(r, alpha, 0, blend_double_trunc);
    test_alpha(r, alpha, 0, blend_float_round);
    test_alpha(r, alpha, 0, blend_float_trunc);
    test_alpha(r, alpha, 0, blend_perfect);

    // BAD
    test_alpha(r, alpha, 1, blend_255_round);
    test_alpha(r, alpha, 1, blend_255_trunc);
    test_alpha(r, alpha, 1, blend_256_plus1_round);
    test_alpha(r, alpha, 1, blend_256_plus1_trunc);
}

// We want 0 diff whenever dst is 0.
DEF_TEST(Blend_dst_0x00, r) {
    const uint8_t dst = 0x00;

    // GOOD
    test_dst(r, dst, 0, blend_255_round);
    test_dst(r, dst, 0, blend_255_trunc);
    test_dst(r, dst, 0, blend_256_plus1_round);
    test_dst(r, dst, 0, blend_256_plus1_trunc);
    test_dst(r, dst, 0, blend_256_round);
    test_dst(r, dst, 0, blend_256_round_alt);
    test_dst(r, dst, 0, blend_256_trunc);
    test_dst(r, dst, 0, blend_double_trunc);
    test_dst(r, dst, 0, blend_float_round);
    test_dst(r, dst, 0, blend_float_trunc);
    test_dst(r, dst, 0, blend_perfect);

    // BAD
}

// We want 0 diff whenever src is fully opaque.
DEF_TEST(Blend_alpha_0xFF, r) {
    const uint8_t alpha = 0xFF;

    // GOOD
    test_alpha(r, alpha, 0, blend_255_round);
    test_alpha(r, alpha, 0, blend_255_trunc);
    test_alpha(r, alpha, 0, blend_256_plus1_round);
    test_alpha(r, alpha, 0, blend_256_plus1_trunc);
    test_alpha(r, alpha, 0, blend_256_round);
    test_alpha(r, alpha, 0, blend_256_round_alt);
    test_alpha(r, alpha, 0, blend_256_trunc);
    test_alpha(r, alpha, 0, blend_double_trunc);
    test_alpha(r, alpha, 0, blend_float_round);
    test_alpha(r, alpha, 0, blend_float_trunc);
    test_alpha(r, alpha, 0, blend_perfect);

    // BAD
}

// We want 0 diff whenever dst is 0xFF.
DEF_TEST(Blend_dst_0xFF, r) {
    const uint8_t dst = 0xFF;

    // GOOD
    test_dst(r, dst, 0, blend_256_round);
    test_dst(r, dst, 0, blend_256_round_alt);
    test_dst(r, dst, 0, blend_double_trunc);
    test_dst(r, dst, 0, blend_float_round);
    test_dst(r, dst, 0, blend_float_trunc);
    test_dst(r, dst, 0, blend_perfect);

    // BAD
    test_dst(r, dst, 1, blend_255_round);
    test_dst(r, dst, 1, blend_255_trunc);
    test_dst(r, dst, 1, blend_256_plus1_round);
    test_dst(r, dst, 1, blend_256_plus1_trunc);
    test_dst(r, dst, 1, blend_256_trunc);
}

// We'd like diff <= 1 everywhere.
DEF_TEST(Blend_alpha_Exhaustive, r) {
    for (unsigned alpha = 0; alpha < 256; alpha++) {
        // PERFECT
        test_alpha(r, alpha, 0, blend_float_round);
        test_alpha(r, alpha, 0, blend_perfect);

        // GOOD
        test_alpha(r, alpha, 1, blend_255_round);
        test_alpha(r, alpha, 1, blend_256_plus1_round);
        test_alpha(r, alpha, 1, blend_256_round);
        test_alpha(r, alpha, 1, blend_256_round_alt);
        test_alpha(r, alpha, 1, blend_256_trunc);
        test_alpha(r, alpha, 1, blend_double_trunc);
        test_alpha(r, alpha, 1, blend_float_trunc);

        // BAD
        test_alpha(r, alpha, 2, blend_255_trunc);
        test_alpha(r, alpha, 2, blend_256_plus1_trunc);
    }
}

// We'd like diff <= 1 everywhere.
DEF_TEST(Blend_dst_Exhaustive, r) {
    for (unsigned dst = 0; dst < 256; dst++) {
        // PERFECT
        test_dst(r, dst, 0, blend_float_round);
        test_dst(r, dst, 0, blend_perfect);

        // GOOD
        test_dst(r, dst, 1, blend_255_round);
        test_dst(r, dst, 1, blend_256_plus1_round);
        test_dst(r, dst, 1, blend_256_round);
        test_dst(r, dst, 1, blend_256_round_alt);
        test_dst(r, dst, 1, blend_256_trunc);
        test_dst(r, dst, 1, blend_double_trunc);
        test_dst(r, dst, 1, blend_float_trunc);

        // BAD
        test_dst(r, dst, 2, blend_255_trunc);
        test_dst(r, dst, 2, blend_256_plus1_trunc);
    }
}
// Overall summary:
// PERFECT
//  blend_double_round
//  blend_float_round
//  blend_perfect
// GOOD ENOUGH
//  blend_double_trunc
//  blend_float_trunc
//  blend_256_round
//  blend_256_round_alt
// NOT GOOD ENOUGH
//  all others
//
//  Algorithms that make sense to use in Skia: blend_256_round, blend_256_round_alt, blend_perfect