[Pixman] [PATCH 5/5] Make code for color burn follow the math more closely

[Søren Sandmann Pedersen]

For superluminescent destinations, the old code could underflow in

    uint32_t r = (ad - d) * as / s;

when (ad - d) was negative. The new code avoids this problem (and
therefore causes changes in the checksums of thread-test and
blitters-test), but it is likely still buggy due to the use of
unsigned variables and other issues in the blend mode code.
---
 pixman/pixman-combine32.c | 15 +++++++--------
 test/blitters-test.c      |  2 +-
 test/thread-test.c        |  2 +-
 3 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/pixman/pixman-combine32.c b/pixman/pixman-combine32.c
index af059eb..54946cc 100644
--- a/pixman/pixman-combine32.c
+++ b/pixman/pixman-combine32.c
@@ -777,15 +777,14 @@ PDF_SEPARABLE_BLEND_MODE (color_dodge)
 static inline uint32_t
 blend_color_burn (uint32_t d, uint32_t ad, uint32_t s, uint32_t as)
 {
-    if (s == 0)
-    {
-	return d < ad ? 0 : DIV_ONE_UN8 (as * ad);
-    }
+    if (d >= ad)
+	return DIV_ONE_UN8 (ad * as);
+    else if (as * ad - as * d >= ad * s)
+	return 0;
+    else if (s == 0)
+	return 0;
     else
-    {
-	uint32_t r = (ad - d) * as / s;
-	return DIV_ONE_UN8 (as * (MAX (r, ad) - r));
-    }
+	return DIV_ONE_UN8 (ad * as - (as * as * (ad - d)) / s);
 }
 
 PDF_SEPARABLE_BLEND_MODE (color_burn)
diff --git a/test/blitters-test.c b/test/blitters-test.c
index 396b5b5..ea03f47 100644
--- a/test/blitters-test.c
+++ b/test/blitters-test.c
@@ -394,6 +394,6 @@ main (int argc, const char *argv[])
     }
 
     return fuzzer_test_main("blitters", 2000000,
-			    0x6A783AD5,
+			    0xE0A07495,
 			    test_composite, argc, argv);
 }
diff --git a/test/thread-test.c b/test/thread-test.c
index a0c7819..71b84f0 100644
--- a/test/thread-test.c
+++ b/test/thread-test.c
@@ -181,7 +181,7 @@ main (void)
 
     crc32 = compute_crc32 (0, crc32s, sizeof crc32s);
 
-#define EXPECTED 0x12F4B484
+#define EXPECTED 0xE299B18E
 
     if (crc32 != EXPECTED)
     {
-- 
1.7.11.7