src/gpu/tessellate/GrTessellationPathRenderer.h

/*
 * Copyright 2019 Google LLC.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef GrTessellationPathRenderer_DEFINED
#define GrTessellationPathRenderer_DEFINED

#include "src/gpu/GrDynamicAtlas.h"
#include "src/gpu/GrOnFlushResourceProvider.h"
#include "src/gpu/GrPathRenderer.h"
#include <map>

// This is the tie-in point for path rendering via GrTessellatePathOp. This path renderer draws
// paths using a hybrid Red Book "stencil, then cover" method. Curves get linearized by GPU
// tessellation shaders. This path renderer doesn't apply analytic AA, so it requires a render
// target that supports either MSAA or mixed samples if AA is desired.
class GrTessellationPathRenderer : public GrPathRenderer, public GrOnFlushCallbackObject {
public:
    // Don't allow linearized segments to be off by more than 1/4th of a pixel from the true curve.
    constexpr static float kLinearizationIntolerance = 4;

    // This is the maximum resolve level supported by our internal indirect draw shaders. (Indirect
    // draws are an alternative to hardware tessellation, and we can use them when hardware support
    // is lacking.)
    //
    // At a given resolveLevel, a curve gets linearized into 2^resolveLevel line segments. So the
    // finest resolveLevel supported by our indirect draw shaders is 2^10 == 1024 line segments.
    //
    // 1024 line segments is enough resolution (with intolerance == 4) to guarantee we can render a
    // 123575px x 123575px path. (See GrWangsFormula::worst_case_cubic.)
    constexpr static int kMaxResolveLevel = 10;

    // We send these flags to the internal tessellation Ops to control how a path gets rendered.
    enum class OpFlags {
        kNone = 0,
        // Used when tessellation is not supported, or when a path will require more resolution than
        // the max number of segments supported by the hardware.
        kDisableHWTessellation = (1 << 0),
        kStencilOnly = (1 << 1),
        kWireframe = (1 << 2)
    };

    GrTessellationPathRenderer(const GrCaps&);
    const char* name() const final { return "GrTessellationPathRenderer"; }
    StencilSupport onGetStencilSupport(const GrStyledShape& shape) const override {
        // TODO: Single-pass (e.g., convex) paths can have full support.
        return kStencilOnly_StencilSupport;
    }
    CanDrawPath onCanDrawPath(const CanDrawPathArgs&) const override;
    bool onDrawPath(const DrawPathArgs&) override;
    void onStencilPath(const StencilPathArgs&) override;
    void preFlush(GrOnFlushResourceProvider*, const uint32_t* opsTaskIDs,
                  int numOpsTaskIDs) override;

private:
    void initAtlasFlags(const GrShaderCaps& shaderCaps);
    SkPath* getAtlasUberPath(SkPathFillType fillType, bool antialias) {
        int idx = (int)antialias << 1;
        idx |= (int)fillType & 1;
        return &fAtlasUberPaths[idx];
    }
    // Allocates space in fAtlas if the path is small and simple enough, and if there is room.
    bool tryAddPathToAtlas(const GrCaps&, const SkMatrix&, const SkPath&, const SkRect& devBounds,
                           GrAAType, SkIRect* devIBounds, SkIPoint16* locationInAtlas,
                           bool* transposedInAtlas);
    void renderAtlas(GrOnFlushResourceProvider*);

    GrDynamicAtlas fAtlas;
    OpFlags fStencilAtlasFlags;
    int fMaxAtlasPathWidth;
    SkPath fAtlasUberPaths[4];  // 2 fillTypes * 2 antialias modes.
};

GR_MAKE_BITFIELD_CLASS_OPS(GrTessellationPathRenderer::OpFlags);

#endif