Check in today's exhaustive blend experiments.

tests/BlendTest.cpp

+#include "Test.h"
+#include "SkColor.h"
+
+#define ASSERT(x) REPORTER_ASSERT(r, x)
+
+// 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 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 src + dst * (255.0 - srcAlpha) / 255.0;
+}
+
+static uint8_t blend_float_trunc(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
+    return src + dst * (255.0f - srcAlpha) / 255.0f;
+}
+
+static uint8_t blend_float_round(uint8_t dst, uint8_t src, uint8_t srcAlpha) {
+    return 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