Make all blending up to GrOptDrawState be handled by the xp/xp factory.

include/gpu/GrXferProcessor.h

Index: include/gpu/GrXferProcessor.h
diff --git a/include/gpu/GrXferProcessor.h b/include/gpu/GrXferProcessor.h
index ee3764deda89a31d3970ebf00b4526e169bdbc53..c079838eaba015392acd41dbd9a14c8c58b73414 100644
--- a/include/gpu/GrXferProcessor.h
+++ b/include/gpu/GrXferProcessor.h
@@ -13,6 +13,8 @@
 #include "GrTypes.h"
 #include "SkXfermode.h"
 
+class GrProcOptInfo;
+
 /**
  * GrXferProcessor is responsible for implementing the xfer mode that blends the src color and dst
  * color. It does this by emitting fragment shader code and controlling the fixed-function blend
@@ -27,11 +29,87 @@
  * GrXPFactory once we have finalized the state of our draw.
  */
 class GrXferProcessor : public GrFragmentProcessor {
+public:
+    /**
+     * Optimizations for blending / coverage that an OptDrawState should apply to itself.
+     */
+    enum BlendOptFlags {

[bsalomon, 2014/12/04 21:54:24]

Howabout just OptFlags? It's already scoped by GrXFERProcessor.

[egdaniel, 2014/12/08 14:52:06]

Done.

+        /**
+         * No optimizations needed
+         */
+        kNone_BlendOpt                         = 0,
+        /**
+         * The draw can be skipped completely.
+         */
+        kSkipDraw_BlendOptFlag                 = 0x1,
+        /**
+         * Clear color stages, remove color vertex attribs, and use input color
+         */
+        kClearColorStages_BlendOptFlag         = 0x2,
+        /**
+         * Clear coverage stages, remove coverage vertex attribs, and use input coverage
+         */
+        kClearCoverageStages_BlendOptFlag      = 0x4,
+        /**
+         * Set CoverageDrawing_StateBit
+         */
+        kSetCoverageDrawing_BlendOptFlag       = 0x8,
+    };
+
+    GR_DECL_BITFIELD_OPS_FRIENDS(BlendOptFlags);
+
+    /**
+     * Determines which optimizations (as described by the BlendOptFlags above) can be performed by
+     * the draw with this xfer processor. If this function is called, the xfer processor may change
+     * its state to reflected the given blend optimizations. It will also set the output parameters,
+     * color and coverage, to specific values if it decides to remove all color or coverage stages.
+     * A caller who calls this function on a XP is required to honor the returned BlendOptFlags
+     * and color/coverage values for its draw.
+     */
+    // TODO: remove need for isCoverageDrawing once coverageDrawing is its own XP.
+    virtual BlendOptFlags getBlendOpts(const GrProcOptInfo& colorPOI,
+                                       const GrProcOptInfo& coveragePOI,
+                                       bool isCoverageDrawing,
+                                       bool colorWriteDisabled,
+                                       bool doesStencilWrite,
+                                       GrColor* color,
+                                       uint8_t* coverage) = 0;
+
+    struct BlendInfo {
+        GrBlendCoeff fSrcBlend;
+        GrBlendCoeff fDstBlend;
+        GrColor      fBlendConstant;
+    };
+
+    virtual void getBlendInfo(BlendInfo* blendInfo) const {

[bsalomon, 2014/12/04 21:54:24]

= 0?

[egdaniel, 2014/12/08 14:52:06]

Done.

+        SkASSERT(blendInfo);
+        blendInfo->fSrcBlend = kOne_GrBlendCoeff;
+        blendInfo->fDstBlend = kZero_GrBlendCoeff;
+        blendInfo->fBlendConstant = 0;
+    }
+
+    /** Will this prceossor read the destination pixel value? */
+    bool willReadDstColor() const { return fWillReadDstColor; }
+
+protected:
+    GrXferProcessor() : fWillReadDstColor(false) {}
+
+    /**
+     * If the prceossor subclass will read the destination pixel value then it must call this
+     * function from its constructor. Otherwise, when its generated backend-specific prceossor class
+     * attempts to generate code that reads the destination pixel it will fail.
+     */
+    void setWillReadDstColor() { fWillReadDstColor = true; }
+
 private:
 
+    bool         fWillReadDstColor;
+
     typedef GrFragmentProcessor INHERITED;
 };
 
+GR_MAKE_BITFIELD_OPS(GrXferProcessor::BlendOptFlags);
+
 /**
  * We install a GrXPFactory (XPF) early on in the pipeline before all the final draw information is
  * known (e.g. whether there is fractional pixel coverage, will coverage be 1 or 4 channel, is the
@@ -45,7 +123,8 @@ private:
  */
 class GrXPFactory : public SkRefCnt {
 public:
-    virtual const GrXferProcessor* createXferProcessor() const = 0;
+    virtual GrXferProcessor* createXferProcessor(const GrProcOptInfo& colorPOI,
+                                                 const GrProcOptInfo& coveragePOI) const = 0;
 
     /**
      * This function returns true if the GrXferProcessor generated from this factory will be able to
@@ -54,6 +133,39 @@ public:
      */
     virtual bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const = 0;
 
+    /**
+     * Depending on color blend mode requested it may or may not be possible to correctly blend with
+     * fractional pixel coverage generated by the fragment shader.
+     *
+     * This function considers the known color and coverage input into the xfer processor and
+     * certain state information (isCoverageDrawing and colorWriteDisabled) to determine whether
+     * coverage can be handled correctly.
+     */
+    // TODO: remove need for isCoverageDrawing once coverageDrawing is its own XP.

[bsalomon, 2014/12/04 21:54:24]

Do we need colorWriteDisabled?

Can we just not emit an XP in that case or something? Seems like we need some
thought on what it means to have an XP and color writes disabled.

[egdaniel, 2014/12/08 14:52:06]

Nothing changed as of now. But from our previous discussion later on we will ad
a colorWriteDisabled XP that will handle a lot of the logic here.

Added some additional todo comments on where to remove colorWriteDisabled logic.

+    virtual bool canApplyCoverage(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
+                                  bool isCoverageDrawing, bool colorWriteDisabled) const = 0;
+
+    /**
+     * This function returns true if the destination pixel values will be read for blending during
+     * draw.
+     */
+    // TODO: remove need for isCoverageDrawing once coverageDrawing is its own XP.
+    // TODO: remove need for colorWriteDisabled once only XP can read dst.
+    virtual bool willBlendWithDst(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
+                                  bool isCoverageDrawing, bool colorWriteDisabled) const = 0;
+
+    /**
+     * Determines whether multiplying the computed per-pixel color by the pixel's fractional
+     * coverage before the blend will give the correct final destination color. In general it
+     * will not as coverage is applied after blending.
+     */
+    // TODO: remove need for isCoverageDrawing once coverageDrawing is its own XP.
+    virtual bool canTweakAlphaForCoverage(bool isCoverageDrawing) const = 0;
+
+    virtual bool getOpaqueAndKnownColor(const GrProcOptInfo& colorPOI,
+                                        const GrProcOptInfo& coveragePOI, GrColor* solidColor,
+                                        uint32_t* solidColorKnownComponents) const = 0;
+
 private:
     typedef GrProgramElement INHERITED;
 };