Add support for half float alpha textures.

This allows us to create distance field textures with better precision,
which may help text quality.

BUG=skia:3103

Review URL: https://codereview.chromium.org/762923003

2 files changed, 12 insertions, 1 deletions

diff --git a/include/gpu/GrColor.h b/include/gpu/GrColor.h
index f862ecc1d0..98bdb22840 100644
--- a/include/gpu/GrColor.h
+++ b/include/gpu/GrColor.h
@@ -168,6 +168,7 @@ static inline uint32_t GrPixelConfigComponentMask(GrPixelConfig config) {
         kA_GrColorComponentFlag,        // kR11_EAC_GrPixelConfig
         kRGBA_GrColorComponentFlags,    // kASTC_12x12_GrPixelConfig
         kRGBA_GrColorComponentFlags,    // kRGBA_float_GrPixelConfig
+        kA_GrColorComponentFlag,        // kAlpha_16_GrPixelConfig
     };
     return kFlags[config];
 
@@ -183,6 +184,7 @@ static inline uint32_t GrPixelConfigComponentMask(GrPixelConfig config) {
     GR_STATIC_ASSERT(9  == kR11_EAC_GrPixelConfig);
     GR_STATIC_ASSERT(10 == kASTC_12x12_GrPixelConfig);
     GR_STATIC_ASSERT(11 == kRGBA_float_GrPixelConfig);
+    GR_STATIC_ASSERT(12 == kAlpha_half_GrPixelConfig);
     GR_STATIC_ASSERT(SK_ARRAY_COUNT(kFlags) == kGrPixelConfigCnt);
 }
 
diff --git a/include/gpu/GrTypes.h b/include/gpu/GrTypes.h
index 0ef964dcc4..1ed7070d44 100644
--- a/include/gpu/GrTypes.h
+++ b/include/gpu/GrTypes.h
@@ -302,7 +302,13 @@ enum GrPixelConfig {
      * Byte order is r, g, b, a.  This color format is 32 bits per channel
      */
     kRGBA_float_GrPixelConfig,
-    kLast_GrPixelConfig = kRGBA_float_GrPixelConfig
+
+    /**
+     * This color format is a single 16 bit float channel
+     */
+    kAlpha_half_GrPixelConfig,
+
+    kLast_GrPixelConfig = kAlpha_half_GrPixelConfig
 };
 static const int kGrPixelConfigCnt = kLast_GrPixelConfig + 1;
 
@@ -364,6 +370,7 @@ static inline size_t GrBytesPerPixel(GrPixelConfig config) {
             return 1;
         case kRGB_565_GrPixelConfig:
         case kRGBA_4444_GrPixelConfig:
+        case kAlpha_half_GrPixelConfig:
             return 2;
         case kRGBA_8888_GrPixelConfig:
         case kBGRA_8888_GrPixelConfig:
@@ -382,6 +389,7 @@ static inline size_t GrUnpackAlignment(GrPixelConfig config) {
             return 1;
         case kRGB_565_GrPixelConfig:
         case kRGBA_4444_GrPixelConfig:
+        case kAlpha_half_GrPixelConfig:
             return 2;
         case kRGBA_8888_GrPixelConfig:
         case kBGRA_8888_GrPixelConfig:
@@ -408,6 +416,7 @@ static inline bool GrPixelConfigIsAlphaOnly(GrPixelConfig config) {
         case kLATC_GrPixelConfig:
         case kASTC_12x12_GrPixelConfig:
         case kAlpha_8_GrPixelConfig:
+        case kAlpha_half_GrPixelConfig:
             return true;
         default:
             return false;