commit 970fd1fbea16da013d5b103182d78499ed85f95f
author jvanverth <jvanverth@google.com> Wed Feb 24 08:33:10 2016 -0800
committer Commit bot <commit-bot@chromium.org> Wed Feb 24 08:33:10 2016 -0800
tree 0f35988aad810bbe331a6846fbc6a9c7ac76440f
parent c2d35d851365bb89f392fe2dd7af57e169de26b7

Use ARM fixes for SDF encoding.

This pulls out the encoding fixes by Joel Liang from
https://codereview.chromium.org/1643143002/
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1726763002

Review URL: https://codereview.chromium.org/1726763002

src/core/SkDistanceFieldGen.cpp

diff --git a/src/core/SkDistanceFieldGen.cpp b/src/core/SkDistanceFieldGen.cpp
index 147aefa..187a5bf 100755
--- a/src/core/SkDistanceFieldGen.cpp
+++ b/src/core/SkDistanceFieldGen.cpp
@@ -316,14 +316,20 @@
 #define DUMP_EDGE 0
 
 #if !DUMP_EDGE
-static unsigned char pack_distance_field_val(float dist, float distanceMagnitude) {
-    if (dist <= -distanceMagnitude) {
-        return 255;
-    } else if (dist > distanceMagnitude) {
-        return 0;
-    } else {
-        return (unsigned char)((distanceMagnitude-dist)*128.0f/distanceMagnitude);
-    }
+template <int distanceMagnitude>
+static unsigned char pack_distance_field_val(float dist) {
+    // The distance field is constructed as unsigned char values, so that the zero value is at 128,
+    // Beside 128, we have 128 values in range [0, 128), but only 127 values in range (128, 255].
+    // So we multiply distanceMagnitude by 127/128 at the latter range to avoid overflow.
+    dist = SkScalarPin(-dist, -distanceMagnitude, distanceMagnitude * 127.0f / 128.0f);
+
+    // Scale into the positive range for unsigned distance.
+    dist += distanceMagnitude;
+
+    // Scale into unsigned char range.
+    // Round to place negative and positive values as equally as possible around 128
+    // (which represents zero).
+    return (unsigned char)SkScalarRoundToInt(dist / (2 * distanceMagnitude) * 256.0f);
 }
 #endif
 
@@ -441,7 +447,7 @@
             } else {
                 dist = SkScalarSqrt(currData->fDistSq);
             }
-            *dfPtr++ = pack_distance_field_val(dist, (float)SK_DistanceFieldMagnitude);
+            *dfPtr++ = pack_distance_field_val<SK_DistanceFieldMagnitude>(dist);
 #endif
             ++currData;
             ++currEdge;

src/core/SkDistanceFieldGen.h

diff --git a/src/core/SkDistanceFieldGen.h b/src/core/SkDistanceFieldGen.h
index 4a09ace..5e7af52 100644
--- a/src/core/SkDistanceFieldGen.h
+++ b/src/core/SkDistanceFieldGen.h
@@ -10,7 +10,7 @@
 #include "SkTypes.h"
 
 // the max magnitude for the distance field
-// distance values are limited to the range [-SK_DistanceFieldMagnitude, SK_DistanceFieldMagnitude)
+// distance values are limited to the range (-SK_DistanceFieldMagnitude, SK_DistanceFieldMagnitude]
 #define SK_DistanceFieldMagnitude   4
 // we need to pad around the original glyph to allow our maximum distance of
 // SK_DistanceFieldMagnitude texels away from any edge
@@ -18,9 +18,10 @@
 // the rect we render with is inset from the distance field glyph size to allow for bilerp
 #define SK_DistanceFieldInset       2
 
-// for the fragment shader
-// The distance field is constructed as unsigned char values, so that the zero value is at 128,
-// and the range is [-4, 4 - 1/255). Hence our multiplier is 8 - 1/32 and zero threshold is 128/255.
+// For the fragment shader:
+//   The distance field is constructed as unsigned char values,
+//   so that the zero value is at 128, and the supported range of distances is [-4 * 127/128, 4].
+//   Hence our multiplier (width of the range) is 4 * 255/128 and zero threshold is 128/255.
 #define SK_DistanceFieldMultiplier   "7.96875"
 #define SK_DistanceFieldThreshold    "0.50196078431"