src/gpu/GrDrawTarget.h

/*
 * Copyright 2010 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */



#ifndef GrDrawTarget_DEFINED
#define GrDrawTarget_DEFINED

#include "GrClip.h"
#include "GrColor.h"
#include "GrDrawState.h"
#include "GrIndexBuffer.h"
#include "GrMatrix.h"
#include "GrRefCnt.h"
#include "GrSamplerState.h"
#include "GrStencil.h"
#include "GrTexture.h"

#include "SkXfermode.h"
#include "SkTLazy.h"

class GrTexture;
class GrClipIterator;
class GrVertexBuffer;
class GrIndexBuffer;

class GrDrawTarget : public GrRefCnt {
public:
    /**
     * Represents the draw target capabilities.
     */
    struct Caps {
        Caps() { memset(this, 0, sizeof(Caps)); }
        Caps(const Caps& c) { *this = c; }
        Caps& operator= (const Caps& c) {
            memcpy(this, &c, sizeof(Caps));
            return *this;
        }
        void print() const;
        bool f8BitPaletteSupport        : 1;
        bool fNPOTTextureTileSupport    : 1;
        bool fTwoSidedStencilSupport    : 1;
        bool fStencilWrapOpsSupport     : 1;
        bool fHWAALineSupport           : 1;
        bool fShaderDerivativeSupport   : 1;
        bool fGeometryShaderSupport     : 1;
        bool fFSAASupport               : 1;
        bool fDualSourceBlendingSupport : 1;
        bool fBufferLockSupport         : 1;
        int fMaxRenderTargetSize;
        int fMaxTextureSize;
    };

    // for convenience
    typedef GrDrawState::StageMask StageMask;

    ///////////////////////////////////////////////////////////////////////////

    GrDrawTarget();
    virtual ~GrDrawTarget();

    /**
     * Gets the capabilities of the draw target.
     */
    const Caps& getCaps() const { return fCaps; }

    /**
     * Sets the current clip to the region specified by clip. All draws will be
     * clipped against this clip if kClip_StateBit is enabled.
     *
     * Setting the clip may (or may not) zero out the client's stencil bits.
     *
     * @param description of the clipping region
     */
    void setClip(const GrClip& clip);

    /**
     * Gets the current clip.
     *
     * @return the clip.
     */
    const GrClip& getClip() const;

    /**
     * Sets the draw state object for the draw target. Note that this does not
     * make a copy. The GrDrawTarget will take a reference to passed object.
     * Passing NULL will cause the GrDrawTarget to use its own internal draw
     * state object rather than an externally provided one.
     */
    void setDrawState(GrDrawState*  drawState);

    /**
     * Read-only access to the GrDrawTarget's current draw state.
     */
    const GrDrawState& getDrawState() const { return *fDrawState; }

    /**
     * Read-write access to the GrDrawTarget's current draw state. Note that
     * this doesn't ref.
     */
    GrDrawState* drawState() { return fDrawState; }

    /**
     * Shortcut for drawState()->preConcatSamplerMatrices() on all enabled
     * stages
     *
     * @param matrix  the matrix to concat
     */
    void preConcatEnabledSamplerMatrices(const GrMatrix& matrix) {
        StageMask stageMask = this->enabledStages();
        this->drawState()->preConcatSamplerMatrices(stageMask, matrix);
    }

    /**
     * Color alpha and coverage are two inputs to the drawing pipeline. For some
     * blend modes it is safe to fold the coverage into constant or per-vertex
     * color alpha value. For other blend modes they must be handled separately.
     * Depending on features available in the underlying 3D API this may or may
     * not be possible.
     *
     * This function considers the current draw state and the draw target's
     * capabilities to determine whether coverage can be handled correctly. The
     * following assumptions are made:
     *    1. The caller intends to somehow specify coverage. This can be
     *       specified either by enabling a coverage stage on the GrDrawState or
     *       via the vertex layout.
     *    2. Other than enabling coverage stages, the current configuration of 
     *       the target's GrDrawState is as it will be at draw time.
     *    3. If a vertex source has not yet been specified then all stages with
     *       non-NULL textures will be referenced by the vertex layout.
     */
    bool canApplyCoverage() const;

    /**
     * Determines whether incorporating partial pixel coverage into the constant
     * color specified by setColor or per-vertex colors will give the right
     * blending result. If a vertex source has not yet been specified then
     * the function assumes that all stages with non-NULL textures will be
     * referenced by the vertex layout.
     */
    bool canTweakAlphaForCoverage() const;

    /**
     * Given the current draw state and hw support, will HW AA lines be used 
     * (if line primitive type is drawn)? If a vertex source has not yet been
     * specified then  the function assumes that all stages with non-NULL
     * textures will be referenced by the vertex layout.
     */
    bool willUseHWAALines() const;

    /**
     * The format of vertices is represented as a bitfield of flags.
     * Flags that indicate the layout of vertex data. Vertices always contain
     * positions and may also contain up to GrDrawState::kMaxTexCoords sets
     * of 2D texture * coordinates, per-vertex colors, and per-vertex coverage.
     * Each stage can 
     * use any of the texture coordinates as its input texture coordinates or it
     * may use the positions as texture coordinates.
     *
     * If no texture coordinates are specified for a stage then the stage is
     * disabled.
     *
     * Only one type of texture coord can be specified per stage. For
     * example StageTexCoordVertexLayoutBit(0, 2) and
     * StagePosAsTexCoordVertexLayoutBit(0) cannot both be specified.
     *
     * The order in memory is always (position, texture coord 0, ..., color, 
     * coverage) with any unused fields omitted. Note that this means that if 
     * only texture coordinates 1 is referenced then there is no texture
     * coordinates 0 and the order would be (position, texture coordinate 1
     * [, color][, coverage]).
     */

    /**
     * Generates a bit indicating that a texture stage uses texture coordinates
     *
     * @param stage       the stage that will use texture coordinates.
     * @param texCoordIdx the index of the texture coordinates to use
     *
     * @return the bit to add to a GrVertexLayout bitfield.
     */
    static int StageTexCoordVertexLayoutBit(int stage, int texCoordIdx) {
        GrAssert(stage < GrDrawState::kNumStages);
        GrAssert(texCoordIdx < GrDrawState::kMaxTexCoords);
        return 1 << (stage + (texCoordIdx * GrDrawState::kNumStages));
    }

private:
    static const int TEX_COORD_BIT_CNT = GrDrawState::kNumStages *
                                         GrDrawState::kMaxTexCoords;

public:
    /**
     * Generates a bit indicating that a texture stage uses the position
     * as its texture coordinate.
     *
     * @param stage       the stage that will use position as texture
     *                    coordinates.
     *
     * @return the bit to add to a GrVertexLayout bitfield.
     */
    static int StagePosAsTexCoordVertexLayoutBit(int stage) {
        GrAssert(stage < GrDrawState::kNumStages);
        return (1 << (TEX_COORD_BIT_CNT + stage));
    }

    /**
     * Modify the existing vertex layout. Realistically the only thing that 
     * can be added w/o recomputing the vertex layout is one of the 
     * StagePosAsTexCoordVertexLayoutBit flags
     */
    void addToVertexLayout(int flag) {
        GrAssert((1 << TEX_COORD_BIT_CNT) == flag ||
                 (1 << (TEX_COORD_BIT_CNT + 1)) == flag ||
                 (1 << (TEX_COORD_BIT_CNT + 2)) == flag ||
                 (1 << (TEX_COORD_BIT_CNT + 3)) == flag);
        fGeoSrcStateStack.back().fVertexLayout |= flag;
    }

private:
    static const int STAGE_BIT_CNT = TEX_COORD_BIT_CNT +
        GrDrawState::kNumStages;

public:

    /**
     * Additional Bits that can be specified in GrVertexLayout.
     */
    enum VertexLayoutBits {
        /* vertices have colors (GrColor) */
        kColor_VertexLayoutBit              = 1 << (STAGE_BIT_CNT + 0),
        /* vertices have coverage (GrColor)
         */
        kCoverage_VertexLayoutBit           = 1 << (STAGE_BIT_CNT + 1),
        /* Use text vertices. (Pos and tex coords may be a different type for
         * text [GrGpuTextVertex vs GrPoint].)
         */
        kTextFormat_VertexLayoutBit         = 1 << (STAGE_BIT_CNT + 2),

        /* Each vertex specificies an edge. Distance to the edge is used to
         * compute a coverage. See GrDrawState::setVertexEdgeType().
         */
        kEdge_VertexLayoutBit               = 1 << (STAGE_BIT_CNT + 3),
        // for below assert
        kDummyVertexLayoutBit,
        kHighVertexLayoutBit = kDummyVertexLayoutBit - 1
    };
    // make sure we haven't exceeded the number of bits in GrVertexLayout.
    GR_STATIC_ASSERT(kHighVertexLayoutBit < ((uint64_t)1 << 8*sizeof(GrVertexLayout)));

    /**
     * There are three types of "sources" of geometry (vertices and indices) for
     * draw calls made on the target. When performing an indexed draw, the
     * indices and vertices can use different source types. Once a source is
     * specified it can be used for multiple draws. However, the time at which
     * the geometry data is no longer editable depends on the source type.
     *
     * Sometimes it is necessary to perform a draw while upstack code has
     * already specified geometry that it isn't finished with. So there are push
     * and pop methods. This allows the client to push the sources, draw
     * something using alternate sources, and then pop to restore the original
     * sources.
     *
     * Aside from pushes and pops, a source remains valid until another source
     * is set or resetVertexSource / resetIndexSource is called. Drawing from
     * a reset source is an error.
     *
     * The three types of sources are:
     *
     * 1. A cpu array (set*SourceToArray). This is useful when the caller
     *    already provided vertex data in a format compatible with a
     *    GrVertexLayout. The data in the array is consumed at the time that
     *    set*SourceToArray is called and subsequent edits to the array will not
     *    be reflected in draws.
     *
     * 2. Reserve. This is most useful when the caller has data it must
     *    transform before drawing and is not long-lived. The caller requests
     *    that the draw target make room for some amount of vertex and/or index
     *    data. The target provides ptrs to hold the vertex and/or index data.
     *
     *    The data is writable up until the next drawIndexed, drawNonIndexed, 
     *    drawIndexedInstances, or pushGeometrySource. At this point the data is
     *    frozen and the ptrs are no longer valid.
     *
     *    Where the space is allocated and how it is uploaded to the GPU is
     *    subclass-dependent.
     *
     * 3. Vertex and Index Buffers. This is most useful for geometry that will
     *    is long-lived. When the data in the buffer is consumed depends on the
     *    GrDrawTarget subclass. For deferred subclasses the caller has to 
     *    guarantee that the data is still available in the buffers at playback.
     *    (TODO: Make this more automatic as we have done for read/write pixels)
     */

    /**
     * Reserves space for vertices and/or indices. Zero can be specifed as
     * either the vertex or index count if the caller desires to only reserve
     * space for only indices or only vertices. If zero is specifed for 
     * vertexCount then the vertex source will be unmodified and likewise for
     * indexCount.
     *
     * If the function returns true then the reserve suceeded and the vertices
     * and indices pointers will point to the space created.
     *
     * If the target cannot make space for the request then this function will
     * return false. If vertexCount was non-zero then upon failure the vertex
     * source is reset and likewise for indexCount.
     *
     * The pointers to the space allocated for vertices and indices remain valid
     * until a drawIndexed, drawNonIndexed, drawIndexedInstances, or push/
     * popGeomtrySource is called. At that point logically a snapshot of the
     * data is made and the pointers are invalid.
     *
     * @param vertexLayout the format of vertices (ignored if vertexCount == 0).
     * @param vertexCount  the number of vertices to reserve space for. Can be
     *                     0.
     * @param indexCount   the number of indices to reserve space for. Can be 0.
     * @param vertices     will point to reserved vertex space if vertexCount is
     *                     non-zero. Illegal to pass NULL if vertexCount > 0.    
     * @param indices      will point to reserved index space if indexCount is
     *                     non-zero. Illegal to pass NULL if indexCount > 0.
     */
     bool reserveVertexAndIndexSpace(GrVertexLayout vertexLayout,
                                     int vertexCount,
                                     int indexCount,
                                     void** vertices,
                                     void** indices);

    /**
     * Provides hints to caller about the number of vertices and indices
     * that can be allocated cheaply. This can be useful if caller is reserving
     * space but doesn't know exactly how much geometry is needed.
     *
     * Also may hint whether the draw target should be flushed first. This is
     * useful for deferred targets.
     *
     * @param vertexLayout layout of vertices caller would like to reserve
     * @param vertexCount  in: hint about how many vertices the caller would
     *                     like to allocate.
     *                     out: a hint about the number of vertices that can be
     *                     allocated cheaply. Negative means no hint.
     *                     Ignored if NULL.
     * @param indexCount   in: hint about how many indices the caller would
     *                     like to allocate.
     *                     out: a hint about the number of indices that can be
     *                     allocated cheaply. Negative means no hint.
     *                     Ignored if NULL.
     *
     * @return  true if target should be flushed based on the input values.
     */
    virtual bool geometryHints(GrVertexLayout vertexLayout,
                               int* vertexCount,
                               int* indexCount) const;

    /**
     * Sets source of vertex data for the next draw. Array must contain
     * the vertex data when this is called.
     *
     * @param array         cpu array containing vertex data.
     * @param size          size of the vertex data.
     * @param vertexCount   the number of vertices in the array.
     */
    void setVertexSourceToArray(GrVertexLayout vertexLayout,
                                const void* vertexArray,
                                int vertexCount);

    /**
     * Sets source of index data for the next indexed draw. Array must contain
     * the indices when this is called.
     *
     * @param array         cpu array containing index data.
     * @param indexCount    the number of indices in the array.
     */
    void setIndexSourceToArray(const void* indexArray, int indexCount);

    /**
     * Sets source of vertex data for the next draw. Data does not have to be
     * in the buffer until drawIndexed, drawNonIndexed, or drawIndexedInstances.
     *
     * @param buffer        vertex buffer containing vertex data. Must be
     *                      unlocked before draw call.
     * @param vertexLayout  layout of the vertex data in the buffer.
     */
    void setVertexSourceToBuffer(GrVertexLayout vertexLayout,
                                 const GrVertexBuffer* buffer);

    /**
     * Sets source of index data for the next indexed draw. Data does not have
     * to be in the buffer until drawIndexed.
     *
     * @param buffer index buffer containing indices. Must be unlocked
     *               before indexed draw call.
     */
    void setIndexSourceToBuffer(const GrIndexBuffer* buffer);
    
    /**
     * Resets vertex source. Drawing from reset vertices is illegal. Set vertex
     * source to reserved, array, or buffer before next draw. May be able to free
     * up temporary storage allocated by setVertexSourceToArray or
     * reserveVertexSpace.
     */
    void resetVertexSource();
    
    /**
     * Resets index source. Indexed Drawing from reset indices is illegal. Set
     * index source to reserved, array, or buffer before next indexed draw. May
     * be able to free up temporary storage allocated by setIndexSourceToArray
     * or reserveIndexSpace.
     */
    void resetIndexSource();
    
    /**
     * Query to find out if the vertex or index source is reserved.
     */
    bool hasReservedVerticesOrIndices() const {
        return kReserved_GeometrySrcType == this->getGeomSrc().fVertexSrc ||
        kReserved_GeometrySrcType == this->getGeomSrc().fIndexSrc;
    }

    /**
     * Pushes and resets the vertex/index sources. Any reserved vertex / index
     * data is finalized (i.e. cannot be updated after the matching pop but can
     * be drawn from). Must be balanced by a pop.
     */
    void pushGeometrySource();

    /**
     * Pops the vertex / index sources from the matching push.
     */
    void popGeometrySource();
    
    /**
     * Draws indexed geometry using the current state and current vertex / index
     * sources.
     *
     * @param type         The type of primitives to draw.
     * @param startVertex  the vertex in the vertex array/buffer corresponding
     *                     to index 0
     * @param startIndex   first index to read from index src.
     * @param vertexCount  one greater than the max index.
     * @param indexCount   the number of index elements to read. The index count
     *                     is effectively trimmed to the last completely
     *                     specified primitive.
     */
    void drawIndexed(GrPrimitiveType type,
                     int startVertex,
                     int startIndex,
                     int vertexCount,
                     int indexCount);

    /**
     * Draws non-indexed geometry using the current state and current vertex
     * sources.
     *
     * @param type         The type of primitives to draw.
     * @param startVertex  the vertex in the vertex array/buffer corresponding
     *                     to index 0
     * @param vertexCount  one greater than the max index.
     */
    void drawNonIndexed(GrPrimitiveType type,
                        int startVertex,
                        int vertexCount);

    /**
     * Helper function for drawing rects. This does not use the current index
     * and vertex sources. After returning, the vertex and index sources may
     * have changed. They should be reestablished before the next drawIndexed
     * or drawNonIndexed. This cannot be called between reserving and releasing
     * geometry. The GrDrawTarget subclass may be able to perform additional
     * optimizations if drawRect is used rather than drawIndexed or
     * drawNonIndexed.
     * @param rect      the rect to draw
     * @param matrix    optional matrix applied to rect (before viewMatrix)
     * @param stageMask bitmask indicating which stages are enabled.
     *                  Bit i indicates whether stage i is enabled.
     * @param srcRects  specifies rects for stages enabled by stageEnableMask.
     *                  if stageEnableMask bit i is 1, srcRects is not NULL,
     *                  and srcRects[i] is not NULL, then srcRects[i] will be
     *                  used as coordinates for stage i. Otherwise, if stage i
     *                  is enabled then rect is used as the coordinates.
     * @param srcMatrices   optional matrices applied to srcRects. If
     *                      srcRect[i] is non-NULL and srcMatrices[i] is
     *                      non-NULL then srcRect[i] will be transformed by
     *                      srcMatrix[i]. srcMatrices can be NULL when no
     *                      srcMatrices are desired.
     */
    virtual void drawRect(const GrRect& rect,
                          const GrMatrix* matrix,
                          StageMask stageMask,
                          const GrRect* srcRects[],
                          const GrMatrix* srcMatrices[]);

    /**
     * This call is used to draw multiple instances of some geometry with a
     * given number of vertices (V) and indices (I) per-instance. The indices in
     * the index source must have the form i[k+I] == i[k] + V. Also, all indices
     * i[kI] ... i[(k+1)I-1] must be elements of the range kV ... (k+1)V-1. As a
     * concrete example, the following index buffer for drawing a series of
     * quads each as two triangles each satisfies these conditions with V=4 and
     * I=6:
     *      (0,1,2,0,2,3, 4,5,6,4,6,7, 8,9,10,8,10,11, ...)
     *
     * The call assumes that the pattern of indices fills the entire index
     * source. The size of the index buffer limits the number of instances that
     * can be drawn by the GPU in a single draw. However, the caller may specify
     * any (positive) number for instanceCount and if necessary multiple GPU
     * draws will be issued. Morever, when drawIndexedInstances is called
     * multiple times it may be possible for GrDrawTarget to group them into a
     * single GPU draw.
     *
     * @param type          the type of primitives to draw
     * @param instanceCount the number of instances to draw. Each instance
     *                      consists of verticesPerInstance vertices indexed by
     *                      indicesPerInstance indices drawn as the primitive
     *                      type specified by type.
     * @param verticesPerInstance   The number of vertices in each instance (V
     *                              in the above description).
     * @param indicesPerInstance    The number of indices in each instance (I
     *                              in the above description).
     */
    virtual void drawIndexedInstances(GrPrimitiveType type,
                                      int instanceCount,
                                      int verticesPerInstance,
                                      int indicesPerInstance);

    /**
     * Helper for drawRect when the caller doesn't need separate src rects or
     * matrices.
     */
    void drawSimpleRect(const GrRect& rect,
                        const GrMatrix* matrix,
                        StageMask stageEnableBitfield) {
         drawRect(rect, matrix, stageEnableBitfield, NULL, NULL);
    }

    /**
     * Clear the render target. Ignores the clip and all other draw state
     * (blend mode, stages, etc). Clears the whole thing if rect is NULL,
     * otherwise just the rect.
     */
    virtual void clear(const GrIRect* rect, GrColor color) = 0;

    ////////////////////////////////////////////////////////////////////////////

    /**
     * See AutoStateRestore below.
     */
    enum ASRInit {
        kPreserve_ASRInit,
        kReset_ASRInit
    };

    /**
     * Saves off the current state and restores it in the destructor. It will
     * install a new GrDrawState object on the target (setDrawState) and restore
     * the previous one in the destructor. The caller should call drawState() to
     * get the new draw state after the ASR is installed.
     *
     * GrDrawState* state = target->drawState();
     * AutoStateRestore asr(target, GrDrawTarget::kReset_ASRInit).
     * state->setRenderTarget(rt); // state refers to the GrDrawState set on
     *                             // target before asr was initialized.
     *                             // Therefore, rt is set on the GrDrawState
     *                             // that will be restored after asr's
     *                             // destructor rather than target's current
     *                             // GrDrawState. 
     */
    class AutoStateRestore : ::GrNoncopyable {
    public:
        /**
         * Default ASR will have no effect unless set() is subsequently called.
         */
        AutoStateRestore();

        /**
         * Saves the state on target. The state will be restored when the ASR
         * is destroyed. If this constructor is used do not call set().
         *
         * @param init  Should the newly installed GrDrawState be a copy of the
         *              previous state or a default-initialized GrDrawState.
         */
        AutoStateRestore(GrDrawTarget* target, ASRInit init);

        ~AutoStateRestore();

        /**
         * Saves the state on target. The state will be restored when the ASR
         * is destroyed. This should only be called once per ASR object and only
         * when the default constructor was used. For nested saves use multiple
         * ASR objects.
         *
         * @param init  Should the newly installed GrDrawState be a copy of the
         *              previous state or a default-initialized GrDrawState.
         */
        void set(GrDrawTarget* target, ASRInit init);

    private:
        GrDrawTarget*        fDrawTarget;
        SkTLazy<GrDrawState> fTempState;
        GrDrawState*         fSavedState;
    };

    ////////////////////////////////////////////////////////////////////////////

    /** 
     * Sets the view matrix to I and preconcats all stage matrices enabled in
     * mask by the view inverse. Destructor undoes these changes.
     */
    class AutoDeviceCoordDraw : ::GrNoncopyable {
    public:
        AutoDeviceCoordDraw(GrDrawTarget* target, StageMask stageMask);
        ~AutoDeviceCoordDraw();
    private:
        GrDrawTarget*       fDrawTarget;
        GrMatrix            fViewMatrix;
        GrMatrix            fSamplerMatrices[GrDrawState::kNumStages];
        int                 fStageMask;
    };

    ////////////////////////////////////////////////////////////////////////////

    class AutoReleaseGeometry : ::GrNoncopyable {
    public:
        AutoReleaseGeometry(GrDrawTarget*  target,
                            GrVertexLayout vertexLayout,
                            int            vertexCount,
                            int            indexCount);
        AutoReleaseGeometry();
        ~AutoReleaseGeometry();
        bool set(GrDrawTarget*  target,
                 GrVertexLayout vertexLayout,
                 int            vertexCount,
                 int            indexCount);
        bool succeeded() const { return NULL != fTarget; }
        void* vertices() const { GrAssert(this->succeeded()); return fVertices; }
        void* indices() const { GrAssert(this->succeeded()); return fIndices; }
        GrPoint* positions() const {
            return static_cast<GrPoint*>(this->vertices());
        }

    private:
        void reset();
        
        GrDrawTarget* fTarget;
        void*         fVertices;
        void*         fIndices;
    };

    ////////////////////////////////////////////////////////////////////////////

    class AutoClipRestore : ::GrNoncopyable {
    public:
        AutoClipRestore(GrDrawTarget* target) {
            fTarget = target;
            fClip = fTarget->getClip();
        }

        ~AutoClipRestore() {
            fTarget->setClip(fClip);
        }
    private:
        GrDrawTarget* fTarget;
        GrClip        fClip;
    };
    
    ////////////////////////////////////////////////////////////////////////////
    
    class AutoGeometryPush : ::GrNoncopyable {
    public:
        AutoGeometryPush(GrDrawTarget* target) {
            GrAssert(NULL != target);
            fTarget = target;
            target->pushGeometrySource();
        }
        ~AutoGeometryPush() {
            fTarget->popGeometrySource();
        }
    private:
        GrDrawTarget* fTarget;
    };

    ////////////////////////////////////////////////////////////////////////////
    // Helpers for picking apart vertex layouts

    /**
     * Helper function to compute the size of a vertex from a vertex layout
     * @return size of a single vertex.
     */
    static size_t VertexSize(GrVertexLayout vertexLayout);

    /**
     * Helper function for determining the index of texture coordinates that
     * is input for a texture stage. Note that a stage may instead use positions
     * as texture coordinates, in which case the result of the function is
     * indistinguishable from the case when the stage is disabled.
     *
     * @param stage         the stage to query
     * @param vertexLayout  layout to query
     *
     * @return the texture coordinate index or -1 if the stage doesn't use
     *         separate (non-position) texture coordinates.
     */
    static int VertexTexCoordsForStage(int stage, GrVertexLayout vertexLayout);

    /**
     * Helper function to compute the offset of texture coordinates in a vertex
     * @return offset of texture coordinates in vertex layout or -1 if the
     *         layout has no texture coordinates. Will be 0 if positions are
     *         used as texture coordinates for the stage.
     */
    static int VertexStageCoordOffset(int stage, GrVertexLayout vertexLayout);

    /**
     * Helper function to compute the offset of the color in a vertex
     * @return offset of color in vertex layout or -1 if the
     *         layout has no color.
     */
    static int VertexColorOffset(GrVertexLayout vertexLayout);

    /**
     * Helper function to compute the offset of the coverage in a vertex
     * @return offset of coverage in vertex layout or -1 if the
     *         layout has no coverage.
     */
    static int VertexCoverageOffset(GrVertexLayout vertexLayout);

     /**
      * Helper function to compute the offset of the edge pts in a vertex
      * @return offset of edge in vertex layout or -1 if the
      *         layout has no edge.
      */
     static int VertexEdgeOffset(GrVertexLayout vertexLayout);

    /**
     * Helper function to determine if vertex layout contains explicit texture
     * coordinates of some index.
     *
     * @param coordIndex    the tex coord index to query
     * @param vertexLayout  layout to query
     *
     * @return true if vertex specifies texture coordinates for the index,
     *              false otherwise.
     */
    static bool VertexUsesTexCoordIdx(int coordIndex,
                                      GrVertexLayout vertexLayout);

    /**
     * Helper function to determine if vertex layout contains either explicit or
     * implicit texture coordinates for a stage.
     *
     * @param stage         the stage to query
     * @param vertexLayout  layout to query
     *
     * @return true if vertex specifies texture coordinates for the stage,
     *              false otherwise.
     */
    static bool VertexUsesStage(int stage, GrVertexLayout vertexLayout);

    /**
     * Helper function to compute the size of each vertex and the offsets of
     * texture coordinates and color. Determines tex coord offsets by tex coord
     * index rather than by stage. (Each stage can be mapped to any t.c. index
     * by StageTexCoordVertexLayoutBit.)
     *
     * @param vertexLayout          the layout to query
     * @param texCoordOffsetsByIdx  after return it is the offset of each
     *                              tex coord index in the vertex or -1 if
     *                              index isn't used. (optional)
     * @param colorOffset           after return it is the offset of the
     *                              color field in each vertex, or -1 if
     *                              there aren't per-vertex colors. (optional)
     * @param coverageOffset        after return it is the offset of the
     *                              coverage field in each vertex, or -1 if
     *                              there aren't per-vertex coeverages.
     *                              (optional)
     * @param edgeOffset            after return it is the offset of the
     *                              edge eq field in each vertex, or -1 if
     *                              there aren't per-vertex edge equations.
     *                              (optional)
     * @return size of a single vertex
     */
    static int VertexSizeAndOffsetsByIdx(GrVertexLayout vertexLayout,
                   int texCoordOffsetsByIdx[GrDrawState::kMaxTexCoords],
                   int *colorOffset,
                   int *coverageOffset,
                   int* edgeOffset);

    /**
     * Helper function to compute the size of each vertex and the offsets of
     * texture coordinates and color. Determines tex coord offsets by stage
     * rather than by index. (Each stage can be mapped to any t.c. index
     * by StageTexCoordVertexLayoutBit.) If a stage uses positions for
     * tex coords then that stage's offset will be 0 (positions are always at 0).
     *
     * @param vertexLayout              the layout to query
     * @param texCoordOffsetsByStage    after return it is the offset of each
     *                                  tex coord index in the vertex or -1 if
     *                                  index isn't used. (optional)
     * @param colorOffset               after return it is the offset of the
     *                                  color field in each vertex, or -1 if
     *                                  there aren't per-vertex colors.
     *                                  (optional)
     * @param coverageOffset            after return it is the offset of the
     *                                  coverage field in each vertex, or -1 if
     *                                  there aren't per-vertex coeverages.
     *                                  (optional)
     * @param edgeOffset                after return it is the offset of the
     *                                  edge eq field in each vertex, or -1 if
     *                                  there aren't per-vertex edge equations.
     *                                  (optional)
     * @return size of a single vertex
     */
    static int VertexSizeAndOffsetsByStage(GrVertexLayout vertexLayout,
                   int texCoordOffsetsByStage[GrDrawState::kNumStages],
                   int* colorOffset,
                   int* coverageOffset,
                   int* edgeOffset);

    /**
     * Accessing positions, texture coords, or colors, of a vertex within an
     * array is a hassle involving casts and simple math. These helpers exist
     * to keep GrDrawTarget clients' code a bit nicer looking.
     */

    /**
     * Gets a pointer to a GrPoint of a vertex's position or texture
     * coordinate.
     * @param vertices      the vetex array
     * @param vertexIndex   the index of the vertex in the array
     * @param vertexSize    the size of each vertex in the array
     * @param offset        the offset in bytes of the vertex component.
     *                      Defaults to zero (corresponding to vertex position)
     * @return pointer to the vertex component as a GrPoint
     */
    static GrPoint* GetVertexPoint(void* vertices,
                                   int vertexIndex,
                                   int vertexSize,
                                   int offset = 0) {
        intptr_t start = GrTCast<intptr_t>(vertices);
        return GrTCast<GrPoint*>(start + offset +
                                 vertexIndex * vertexSize);
    }
    static const GrPoint* GetVertexPoint(const void* vertices,
                                         int vertexIndex,
                                         int vertexSize,
                                         int offset = 0) {
        intptr_t start = GrTCast<intptr_t>(vertices);
        return GrTCast<const GrPoint*>(start + offset +
                                       vertexIndex * vertexSize);
    }

    /**
     * Gets a pointer to a GrColor inside a vertex within a vertex array.
     * @param vertices      the vetex array
     * @param vertexIndex   the index of the vertex in the array
     * @param vertexSize    the size of each vertex in the array
     * @param offset        the offset in bytes of the vertex color
     * @return pointer to the vertex component as a GrColor
     */
    static GrColor* GetVertexColor(void* vertices,
                                   int vertexIndex,
                                   int vertexSize,
                                   int offset) {
        intptr_t start = GrTCast<intptr_t>(vertices);
        return GrTCast<GrColor*>(start + offset +
                                 vertexIndex * vertexSize);
    }
    static const GrColor* GetVertexColor(const void* vertices,
                                         int vertexIndex,
                                         int vertexSize,
                                         int offset) {
        const intptr_t start = GrTCast<intptr_t>(vertices);
        return GrTCast<const GrColor*>(start + offset +
                                       vertexIndex * vertexSize);
    }

    static void VertexLayoutUnitTest();

protected:

    /**
     * Optimizations for blending / coverage to be applied based on the current
     * state.
     * Subclasses that actually draw (as opposed to those that just buffer for
     * playback) must implement the flags that replace the output color.
     */
    enum BlendOptFlags {
        /**
         * No optimization
         */
        kNone_BlendOpt = 0,
        /**
         * Don't draw at all
         */
        kSkipDraw_BlendOptFlag = 0x2,
        /**
         * Emit the src color, disable HW blending (replace dst with src)
         */
        kDisableBlend_BlendOptFlag = 0x4,
        /**
         * The coverage value does not have to be computed separately from
         * alpha, the the output color can be the modulation of the two.
         */
        kCoverageAsAlpha_BlendOptFlag = 0x1,
        /**
         * Instead of emitting a src color, emit coverage in the alpha channel
         * and r,g,b are "don't cares".
         */
        kEmitCoverage_BlendOptFlag = 0x10,
        /**
         * Emit transparent black instead of the src color, no need to compute
         * coverage.
         */
        kEmitTransBlack_BlendOptFlag = 0x8,
    };
    GR_DECL_BITFIELD_OPS_FRIENDS(BlendOptFlags);

    // Determines what optimizations can be applied based on the blend.
    // The coeffecients may have to be tweaked in order for the optimization
    // to work. srcCoeff and dstCoeff are optional params that receive the
    // tweaked coeffecients.
    // Normally the function looks at the current state to see if coverage
    // is enabled. By setting forceCoverage the caller can speculatively
    // determine the blend optimizations that would be used if there was
    // partial pixel coverage
    BlendOptFlags getBlendOpts(bool forceCoverage = false,
                               GrBlendCoeff* srcCoeff = NULL,
                               GrBlendCoeff* dstCoeff = NULL) const;

    // determine if src alpha is guaranteed to be one for all src pixels
    bool srcAlphaWillBeOne(GrVertexLayout vertexLayout) const;

    enum GeometrySrcType {
        kNone_GeometrySrcType,     //<! src has not been specified
        kReserved_GeometrySrcType, //<! src was set using reserve*Space
        kArray_GeometrySrcType,    //<! src was set using set*SourceToArray
        kBuffer_GeometrySrcType    //<! src was set using set*SourceToBuffer
    };
    
    struct GeometrySrcState {
        GeometrySrcType         fVertexSrc;
        union {
            // valid if src type is buffer
            const GrVertexBuffer*   fVertexBuffer;
            // valid if src type is reserved or array
            int                     fVertexCount;
        };
        
        GeometrySrcType         fIndexSrc;
        union {
            // valid if src type is buffer
            const GrIndexBuffer*    fIndexBuffer;
            // valid if src type is reserved or array
            int                     fIndexCount;
        };
        
        GrVertexLayout          fVertexLayout;
    };

    int indexCountInCurrentSource() const {
        const GeometrySrcState& src = this->getGeomSrc();
        switch (src.fIndexSrc) {
            case kNone_GeometrySrcType:
                return 0;
            case kReserved_GeometrySrcType:
            case kArray_GeometrySrcType:
                return src.fIndexCount;
            case kBuffer_GeometrySrcType:
                return src.fIndexBuffer->sizeInBytes() / sizeof(uint16_t);
            default:
                GrCrash("Unexpected Index Source.");
                return 0;
        }
    }
    // given a vertex layout and a draw state, will a stage be used?
    static bool StageWillBeUsed(int stage, GrVertexLayout layout, 
                                const GrDrawState& state) {
        return NULL != state.getTexture(stage) &&
               VertexUsesStage(stage, layout);
    }

    bool isStageEnabled(int stage) const {
        return StageWillBeUsed(stage, this->getVertexLayout(),
                               this->getDrawState());
    }

    StageMask enabledStages() const {
        StageMask mask = 0;
        for (int s = 0; s < GrDrawState::kNumStages; ++s) {
            mask |= this->isStageEnabled(s) ? 1 : 0;
        }
        return mask;
    }

    // A sublcass can optionally overload this function to be notified before
    // vertex and index space is reserved.
    virtual void willReserveVertexAndIndexSpace(GrVertexLayout vertexLayout,
                                                int vertexCount,
                                                int indexCount) {}
    

    // implemented by subclass to allocate space for reserved geom
    virtual bool onReserveVertexSpace(GrVertexLayout vertexLayout,
                                      int vertexCount,
                                      void** vertices) = 0;
    virtual bool onReserveIndexSpace(int indexCount, void** indices) = 0;
    // implemented by subclass to handle release of reserved geom space
    virtual void releaseReservedVertexSpace() = 0;
    virtual void releaseReservedIndexSpace() = 0;
    // subclass must consume array contents when set
    virtual void onSetVertexSourceToArray(const void* vertexArray,
                                          int vertexCount) = 0;
    virtual void onSetIndexSourceToArray(const void* indexArray,
                                         int indexCount) = 0;
    // subclass is notified that geom source will be set away from an array
    virtual void releaseVertexArray() = 0;
    virtual void releaseIndexArray() = 0;
    // subclass overrides to be notified just before geo src state
    // is pushed/popped.
    virtual void geometrySourceWillPush() = 0;
    virtual void geometrySourceWillPop(const GeometrySrcState& restoredState) = 0;
    // subclass called to perform drawing
    virtual void onDrawIndexed(GrPrimitiveType type,
                               int startVertex,
                               int startIndex,
                               int vertexCount,
                               int indexCount) = 0;
    virtual void onDrawNonIndexed(GrPrimitiveType type,
                                  int startVertex,
                                  int vertexCount) = 0;
    // subclass overrides to be notified when clip is set. Must call
    // INHERITED::clipwillBeSet
    virtual void clipWillBeSet(const GrClip& clip);

    // Helpers for drawRect, protected so subclasses that override drawRect
    // can use them.
    static GrVertexLayout GetRectVertexLayout(StageMask stageEnableBitfield,
                                              const GrRect* srcRects[]);

    static void SetRectVertices(const GrRect& rect,
                                const GrMatrix* matrix,
                                const GrRect* srcRects[],
                                const GrMatrix* srcMatrices[],
                                GrVertexLayout layout,
                                void* vertices);

    // accessors for derived classes
    const GeometrySrcState& getGeomSrc() const {
        return fGeoSrcStateStack.back();
    }
    // it is prefereable to call this rather than getGeomSrc()->fVertexLayout
    // because of the assert.
    GrVertexLayout getVertexLayout() const {
        // the vertex layout is only valid if a vertex source has been
        // specified.
        GrAssert(this->getGeomSrc().fVertexSrc != kNone_GeometrySrcType);
        return this->getGeomSrc().fVertexLayout;
    }

    GrClip fClip;

    GrDrawState* fDrawState;
    GrDrawState fDefaultDrawState;

    Caps fCaps;

    // subclasses must call this in their destructors to ensure all vertex
    // and index sources have been released (including those held by 
    // pushGeometrySource())
    void releaseGeometry();

private:
    // helpers for reserving vertex and index space.
    bool reserveVertexSpace(GrVertexLayout vertexLayout,
                            int vertexCount,
                            void** vertices);
    bool reserveIndexSpace(int indexCount, void** indices);
    
    // called by drawIndexed and drawNonIndexed. Use a negative indexCount to
    // indicate non-indexed drawing.
    bool checkDraw(GrPrimitiveType type, int startVertex,
                   int startIndex, int vertexCount,
                   int indexCount) const;
    // called when setting a new vert/idx source to unref prev vb/ib
    void releasePreviousVertexSource();
    void releasePreviousIndexSource();
    
    enum {
        kPreallocGeoSrcStateStackCnt = 4,
    };
    SkSTArray<kPreallocGeoSrcStateStackCnt, 
              GeometrySrcState, true>           fGeoSrcStateStack;
    
};

GR_MAKE_BITFIELD_OPS(GrDrawTarget::BlendOptFlags);

#endif