Add XferProcessor factory in GrPaint and GrDrawState.

include/gpu/GrXferProcessor.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrXferProcessor_DEFINED
+#define GrXferProcessor_DEFINED
+
+#include "GrColor.h"
+#include "GrFragmentProcessor.h"
+#include "GrTypes.h"
+#include "SkXfermode.h"
+
+/**
+ * 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
+ * state. The inputs to its shader code are the final computed src color and fractional pixel
+ * coverage. The GrXferProcessor's shader code writes the fragment shader output color that coes

[bsalomon, 2014/12/02 15:36:32]

coes->goes

[egdaniel, 2014/12/02 18:09:51]

Done.

+ * into the fixed-function blend. When dual-source blending is available, it may also write a
+ * seconday fragment shader output color. When allowed by the backend API, the GrXferProcessor may
+ * read the destination color. The GrXferProcessor is responsible for setting the blend coefficients
+ * and blend constant color.
+ */
+class GrXferProcessor : public GrFragmentProcessor {

[bsalomon, 2014/12/02 15:36:32]

Somewhere we need an explanation (from a subclass author's perspective) of why
an XP and XPF. That is what there roles are and how they relate to each other.
Maybe something along the lines of the XPF is installed early before all the
other contributions to the shader are known (e.g. unknown whether we'll have to
consider fractional pixel coverage, whether the draw is opaque, ...). It exists
to install an XP that implements the desired blend once everything is finalized
and to answer queries about the capabilities of the XPs it can create. (this is
vague, needs specificity).

[egdaniel, 2014/12/02 18:09:51]

Added below before GrXPFactory.

+private:
+
+    typedef GrFragmentProcessor INHERITED;
+};
+
+class GrXPFactory : public GrProgramElement {
+public:
+    virtual const GrXferProcessor* createXferProcessor() const = 0;
+
+    /**
+     * This function returns true if the GrXferProcessor generated from this factory will be able to
+     * correctly blend when using RGB coverage. The knownColor and knownColorFlags represent the
+     * final computed color from the color stages.
+     */
+    virtual bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const = 0;
+
+private:
+    typedef GrProgramElement INHERITED;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+
+/**
+ * This creates a factory outside of the effect memory pool. The factory's destructor will be called
+ * at global destruction time. NAME will be the name of the created GrXPFactory.
+ */
+#define GR_CREATE_STATIC_XP_FACTORY(NAME, XPF_CLASS, ARGS)                                     \
+static SkAlignedSStorage<sizeof(XPF_CLASS)> g_##NAME##_Storage;                                \
+static GrXPFactory* NAME SkNEW_PLACEMENT_ARGS(g_##NAME##_Storage.get(), XPF_CLASS, ARGS);      \
+static SkAutoTDestroy<GrXPFactory> NAME##_ad(NAME);
+
+#endif
+