SkRasterPipeline: implement SkLumaColorFilter

After getting discouraged by the non-separable xfermodes, I decided to look at filling out the color filters instead.  This one's nice and easy.

There's only 1 GM that exercises this color filter, and it's drawing noticeably lighter now in f16 and sRGB configs.  565 is unchanged.  This makes me think the diffs are due to lost precision in the previous method, which was going through the default fallback to 8888 filterSpan().

I double checked: the f16 config now draws nearly identically to the gpuf16 config.  It used to be quite different.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4183
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: Ic6feaecae5cf18493b5df89733f6a5ca362e9a75
Reviewed-on: https://skia-review.googlesource.com/4183
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

4 files changed, 29 insertions, 13 deletions

diff --git a/include/effects/SkLumaColorFilter.h b/include/effects/SkLumaColorFilter.h
index 9625435b25..a55e37ad35 100644
--- a/include/effects/SkLumaColorFilter.h
+++ b/include/effects/SkLumaColorFilter.h
@@ -11,6 +11,8 @@
 #include "SkColorFilter.h"
 #include "SkRefCnt.h"
 
+class SkRasterPipeline;
+
 /**
  *  Luminance-to-alpha color filter, as defined in
  *  http://www.w3.org/TR/SVG/masking.html#Masking
@@ -40,6 +42,7 @@ protected:
 
 private:
     SkLumaColorFilter();
+    bool onAppendStages(SkRasterPipeline*) const override;
 
     typedef SkColorFilter INHERITED;
 };
diff --git a/src/core/SkRasterPipeline.h b/src/core/SkRasterPipeline.h
index ccfdeb5949..6d8fef299e 100644
--- a/src/core/SkRasterPipeline.h
+++ b/src/core/SkRasterPipeline.h
@@ -54,19 +54,20 @@
 // TODO: There may be a better place to stuff tail, e.g. in the bottom alignment bits of
 // the Stage*.  This mostly matters on 64-bit Windows where every register is precious.
 
-#define SK_RASTER_PIPELINE_STAGES(M)                            \
-    M(swap_src_dst) M(constant_color) M(clamp_1)                \
-    M(load_s_565)  M(load_d_565)  M(store_565)                  \
-    M(load_s_srgb) M(load_d_srgb) M(store_srgb)                 \
-    M(load_s_f16)  M(load_d_f16)  M(store_f16)                  \
-    M(scale_u8)                                                 \
-    M(lerp_u8) M(lerp_565) M(lerp_constant_float)               \
-    M(dst)                                                      \
-    M(dstatop) M(dstin) M(dstout) M(dstover)                    \
-    M(srcatop) M(srcin) M(srcout) M(srcover)                    \
-    M(clear) M(modulate) M(multiply) M(plus_) M(screen) M(xor_) \
-    M(colorburn) M(colordodge) M(darken) M(difference)          \
-    M(exclusion) M(hardlight) M(lighten) M(overlay) M(softlight)
+#define SK_RASTER_PIPELINE_STAGES(M)                             \
+    M(swap_src_dst) M(constant_color) M(clamp_1)                 \
+    M(load_s_565)  M(load_d_565)  M(store_565)                   \
+    M(load_s_srgb) M(load_d_srgb) M(store_srgb)                  \
+    M(load_s_f16)  M(load_d_f16)  M(store_f16)                   \
+    M(scale_u8)                                                  \
+    M(lerp_u8) M(lerp_565) M(lerp_constant_float)                \
+    M(dst)                                                       \
+    M(dstatop) M(dstin) M(dstout) M(dstover)                     \
+    M(srcatop) M(srcin) M(srcout) M(srcover)                     \
+    M(clear) M(modulate) M(multiply) M(plus_) M(screen) M(xor_)  \
+    M(colorburn) M(colordodge) M(darken) M(difference)           \
+    M(exclusion) M(hardlight) M(lighten) M(overlay) M(softlight) \
+    M(luminance_to_alpha)
 
 class SkRasterPipeline {
 public:
diff --git a/src/effects/SkLumaColorFilter.cpp b/src/effects/SkLumaColorFilter.cpp
index 19b2c72956..e3c4f379ad 100644
--- a/src/effects/SkLumaColorFilter.cpp
+++ b/src/effects/SkLumaColorFilter.cpp
@@ -8,6 +8,7 @@
 #include "SkLumaColorFilter.h"
 
 #include "SkColorPriv.h"
+#include "SkRasterPipeline.h"
 #include "SkString.h"
 
 #if SK_SUPPORT_GPU
@@ -37,6 +38,11 @@ void SkLumaColorFilter::filterSpan(const SkPMColor src[], int count,
     }
 }
 
+bool SkLumaColorFilter::onAppendStages(SkRasterPipeline* p) const {
+    p->append(SkRasterPipeline::luminance_to_alpha);
+    return true;
+}
+
 sk_sp<SkColorFilter> SkLumaColorFilter::Make() {
     return sk_sp<SkColorFilter>(new SkLumaColorFilter);
 }
diff --git a/src/opts/SkRasterPipeline_opts.h b/src/opts/SkRasterPipeline_opts.h
index b22c9fc176..845fe40e41 100644
--- a/src/opts/SkRasterPipeline_opts.h
+++ b/src/opts/SkRasterPipeline_opts.h
@@ -8,6 +8,7 @@
 #ifndef SkRasterPipeline_opts_DEFINED
 #define SkRasterPipeline_opts_DEFINED
 
+#include "SkColorPriv.h"
 #include "SkHalf.h"
 #include "SkPM4f.h"
 #include "SkRasterPipeline.h"
@@ -444,6 +445,11 @@ RGB_XFERMODE(softlight) {
     return s*inv(da) + d*inv(sa) + (s2 <= sa).thenElse(darkSrc, liteSrc);  // 1 or (2 or 3)?
 }
 
+STAGE(luminance_to_alpha, true) {
+    a = SK_LUM_COEFF_R*r + SK_LUM_COEFF_G*g + SK_LUM_COEFF_B*b;
+    r = g = b = 0;
+}
+
 
 template <typename Fn>
 SI Fn enum_to_Fn(SkRasterPipeline::StockStage st) {