2 * Copyright 2021 Google LLC
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
8 #ifndef skgpu_graphite_DrawOrder_DEFINED
9 #define skgpu_graphite_DrawOrder_DEFINED
11 #include "include/core/SkTypes.h"
13 namespace skgpu::graphite {
15 // Helper to encapsulate an unsigned number and enforce that it can only be used to create a
16 // monotonic sequence. The template argument 'Sequence' is used to define different sequences
17 // enforced by the compiler. For simplicity, and current needs within Graphite, it's assumed the
18 // entire sequence can be indexed by uint16_t.
19 template<typename Sequence>
20 class MonotonicValue {
22 static constexpr MonotonicValue First() { return 0; }
23 static constexpr MonotonicValue Last() { return 0xffff; }
25 MonotonicValue() = default;
26 MonotonicValue(const MonotonicValue& o) = default;
28 MonotonicValue& operator=(const MonotonicValue& o) = default;
30 bool operator< (MonotonicValue o) const { return fIndex < o.fIndex; }
31 bool operator<=(MonotonicValue o) const { return fIndex <= o.fIndex; }
33 bool operator> (MonotonicValue o) const { return fIndex > o.fIndex; }
34 bool operator>=(MonotonicValue o) const { return fIndex >= o.fIndex; }
36 bool operator==(MonotonicValue o) const { return fIndex == o.fIndex; }
37 bool operator!=(MonotonicValue o) const { return fIndex != o.fIndex; }
39 uint16_t bits() const { return fIndex; }
41 // Get the next value in the sequence after this one
42 MonotonicValue next() const { return fIndex + 1; }
45 constexpr MonotonicValue(uint16_t index) : fIndex(index) {}
51 * CompressedPaintersOrder is an ordinal number that allows draw commands to be re-ordered so long
52 * as when they are executed, the read/writes to the color|depth attachments respect the original
53 * painter's order. Logical draws with the same CompressedPaintersOrder can be assumed to be
54 * executed in any order, however that may have been determined (e.g. BoundsManager or relying on
55 * a depth test during rasterization).
57 struct CompressedPaintersOrderSequence {};
58 using CompressedPaintersOrder = MonotonicValue<CompressedPaintersOrderSequence>;
61 * Each DisjointStencilIndex specifies an implicit set of non-overlapping draws. Assuming that two
62 * draws have the same CompressedPaintersOrder and the same DisjointStencilIndex, their substeps
63 * for multi-pass rendering (stencil-then-cover, etc.) can be intermingled with each other and
64 * produce the same results as if each draw's substeps were executed in order before moving on to
67 * Ordering within a set can be entirely arbitrary (i.e. all stencil steps can go before all cover
68 * steps). Ordering between sets is also arbitrary since all draws share the same
69 * CompressedPaintersOrder, so long as one set is entirely drawn before the next.
71 * Two draws that have different CompressedPaintersOrders but the same DisjointStencilIndex are
72 * unrelated, they may or may not overlap. The painters order scopes the disjoint sets.
74 struct DisjointStencilIndexSequence {};
75 using DisjointStencilIndex = MonotonicValue<DisjointStencilIndexSequence>;
78 * Every draw has an associated depth value. The value is constant across the entire draw and is
79 * not related to any varying Z coordinate induced by a 4x4 transform. The painter's depth is stored
80 * in the depth attachment and the GREATER depth test is used to reject or accept pixels/samples
81 * relative to what has already been rendered into the depth attachment. This allows draws that do
82 * not depend on the previous color to be radically re-ordered relative to their original painter's
83 * order while producing correct results.
85 struct PaintersDepthSequence {};
86 using PaintersDepth = MonotonicValue<PaintersDepthSequence>;
89 * DrawOrder aggregates the three separate sequences that Graphite uses to re-order draws and their
90 * substeps as much as possible while preserving the painter's order semantics of the Skia API.
92 * To build the full DrawOrder for a draw, start with its assigned PaintersDepth (i.e. the original
93 * painter's order of the draw call). From there, the DrawOrder can be updated to reflect
94 * dependencies on previous draws, either from depth-only clip draws or because the draw is
95 * transparent and must blend with the previous color values. Lastly, once the
96 * CompressedPaintersOrder is finalized, the DrawOrder can be updated to reflect whether or not
97 * the draw will involve the stencil buffer--and if so, specify the disjoint stencil set it
100 * The original and effective order that draws are executed in is defined by the PaintersDepth.
101 * However, the actual execution order is defined by first the CompressedPaintersOrder and then
102 * the DisjointStencilIndex. This means that draws with much higher depths can be executed earlier
103 * if painter's order compression allows for it.
106 *FIXME Integrate this?
107 * This reduces to a vertex
108 * coloring problem on the intersection graph formed by the commands and how their bounds
109 * overlap, followed by ordering by pipeline description and uniform data. General vertex
110 * coloring is NP-complete so DrawPass uses a greedy algorithm where the order it "colors" the
111 * vertices is based on the ordering constraints for the color+depth buffer and optionally the
112 * stencil buffer (stored in fColorDepthIndex and fStencilIndex respectively). skgpu::Device
113 * determines the ordering on-the-fly by using BoundsManager to approximate intersections as
114 * draw commands are recorded. It is possible to issue draws to Skia that produce pathologic
115 * orderings using this method, but it strikes a reasonable balance between finding a near
116 * optimal ordering that respects painter's order and is very efficient to compute.
121 // The first PaintersDepth is reserved for clearing the depth attachment; any draw using this
122 // depth will always fail the depth test.
123 inline static constexpr PaintersDepth kClearDepth = PaintersDepth::First();
124 // The first CompressedPaintersOrder is reserved to indicate there is no previous draw that
125 // must come before a draw.
126 inline static constexpr
127 CompressedPaintersOrder kNoIntersection = CompressedPaintersOrder::First();
128 // The first DisjointStencilIndex is reserved to indicate an unassigned stencil set.
129 inline static constexpr DisjointStencilIndex kUnassigned = DisjointStencilIndex::First();
131 explicit DrawOrder(PaintersDepth originalOrder)
132 : fPaintOrder(kNoIntersection)
133 , fStencilIndex(kUnassigned)
134 , fDepth(originalOrder) {}
136 DrawOrder(PaintersDepth originalOrder, CompressedPaintersOrder compressedOrder)
137 : fPaintOrder(compressedOrder)
138 , fStencilIndex(kUnassigned)
139 , fDepth(originalOrder) {}
141 CompressedPaintersOrder paintOrder() const { return fPaintOrder; }
142 DisjointStencilIndex stencilIndex() const { return fStencilIndex; }
143 PaintersDepth depth() const { return fDepth; }
145 float depthAsFloat() const { return fDepth.bits() / (float) PaintersDepth::Last().bits(); }
147 DrawOrder& dependsOnPaintersOrder(CompressedPaintersOrder prevDraw) {
148 // A draw must be ordered after all previous draws that it depends on
149 CompressedPaintersOrder next = prevDraw.next();
150 if (fPaintOrder < next) {
156 DrawOrder& dependsOnStencil(DisjointStencilIndex disjointSet) {
157 // Stencil usage should only be set once
158 SkASSERT(fStencilIndex == kUnassigned);
159 fStencilIndex = disjointSet;
164 CompressedPaintersOrder fPaintOrder;
165 DisjointStencilIndex fStencilIndex;
166 PaintersDepth fDepth;
169 } // namespace skgpu::graphite
171 #endif // skgpu_graphite_DrawOrder_DEFINED