Fix mask gamma rounding issues on x87.

https://codereview.appspot.com/6497091/

May require rebaselines.


git-svn-id: http://skia.googlecode.com/svn/trunk@5446 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 13 insertions, 7 deletions

diff --git a/src/core/SkMaskGamma.cpp b/src/core/SkMaskGamma.cpp
index 449a78de93..15ecdf89e0 100644
--- a/src/core/SkMaskGamma.cpp
+++ b/src/core/SkMaskGamma.cpp
@@ -49,8 +49,8 @@ static float apply_contrast(float srca, float contrast) {
 }
 
 void SkTMaskGamma_build_correcting_lut(uint8_t table[256], U8CPU srcI, SkScalar contrast,
-                                const SkColorSpaceLuminance& srcConvert,
-                                const SkColorSpaceLuminance& dstConvert) {
+                                       const SkColorSpaceLuminance& srcConvert,
+                                       const SkColorSpaceLuminance& dstConvert) {
     const float src = (float)srcI / 255.0f;
     const float linSrc = srcConvert.toLuma(src);
     //Guess at the dst. The perceptual inverse provides smaller visual
@@ -62,19 +62,25 @@ void SkTMaskGamma_build_correcting_lut(uint8_t table[256], U8CPU srcI, SkScalar
 
     //Contrast value tapers off to 0 as the src luminance becomes white
     const float adjustedContrast = SkScalarToFloat(contrast) * linDst;
-    const float step = 1.0f / 255.0f;
 
     //Remove discontinuity and instability when src is close to dst.
     //The value 1/256 is arbitrary and appears to contain the instability.
     if (fabs(src - dst) < (1.0f / 256.0f)) {
-        float rawSrca = 0.0f;
-        for (int i = 0; i < 256; ++i, rawSrca += step) {
+        float ii = 0.0f;
+        for (int i = 0; i < 256; ++i, ii += 1.0f) {
+            float rawSrca = ii / 255.0f;
             float srca = apply_contrast(rawSrca, adjustedContrast);
             table[i] = SkToU8(sk_float_round2int(255.0f * srca));
         }
     } else {
-        float rawSrca = 0.0f;
-        for (int i = 0; i < 256; ++i, rawSrca += step) {
+        // Avoid slow int to float conversion.
+        float ii = 0.0f;
+        for (int i = 0; i < 256; ++i, ii += 1.0f) {
+            // 'rawSrca += 1.0f / 255.0f' and even
+            // 'rawSrca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
+            // When this happens the table[255] == 0x0 instead of 0xff.
+            // See http://code.google.com/p/chromium/issues/detail?id=146466
+            float rawSrca = ii / 255.0f;
             float srca = apply_contrast(rawSrca, adjustedContrast);
             SkASSERT(srca <= 1.0f);
             float dsta = 1.0f - srca;