PerEdgeAAQuadRenderStep.cpp | Explore in Territory

/*
 * Copyright 2023 Google LLC
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/graphite/render/PerEdgeAAQuadRenderStep.h"

#include "src/base/SkVx.h"
#include "src/core/SkRRectPriv.h"
#include "src/gpu/graphite/DrawParams.h"
#include "src/gpu/graphite/DrawWriter.h"
#include "src/gpu/graphite/render/CommonDepthStencilSettings.h"

// This RenderStep is specialized to draw filled rectangles with per-edge AA.
//
// Each of these "primitives" is represented by a single instance. The instance attributes are
// flexible enough to describe per-edge AA quads without relying on uniforms to define its
// operation. The attributes encode shape as follows:
//
// float4 edgeFlags - per-edge AA defined by each component: aa != 0.
// float4 quadXs - these values provide the X coordinates of the quadrilateral in top-left CW order.
// float4 quadYs - these values provide the Y coordinates of the quadrilateral.
//
// From the other direction, per-edge AA quads produce instance values like:
//  - [aa(t,r,b,l) ? 255 : 0]   [xs(tl,tr,br,bl)]     [ys(tl,tr,br,bl)]
//
// From this encoding, data can be unpacked for each corner, which are equivalent under
// rotational symmetry. Per-edge quads are always mitered and fill the interior, but the
// vertices are placed such that the edge coverage ramps can collapse to 0 area on non-AA edges.
//
// The vertices that describe each corner are placed so that edges and miters calculate
// coverage by interpolating a varying and then clamping in the fragment shader. Triangles that
// cover the inner and outer curves calculate distance to the curve within the fragment shader.
//
// See https://docs.google.com/presentation/d/1MCPstNsSlDBhR8CrsJo0r-cZNbu-sEJEvU9W94GOJoY/edit?usp=sharing
// for diagrams and explanation of how the geometry is defined.
//
// PerEdgeAAQuadRenderStep uses the common technique of approximating distance to the level set by
// one expansion of the Taylor's series for the level set's equation. Given a level set function
// C(x,y), this amounts to calculating C(px,py)/|∇C(px,py)|. For the straight edges the level set
// is linear and calculated in the vertex shader and then interpolated exactly over the rectangle.
// This provides distances to all four exterior edges within the fragment shader and allows it to
// reconstruct a relative position per elliptical corner. Unfortunately this requires the fragment
// shader to calculate the length of the gradient for straight edges instead of interpolating
// exact device-space distance.
//
// Unlike AnalyticRRectRenderStep, for per-edge AA quads it's valid to have each pixel calculate a
// single corner's coverage that's controlled via the vertex shader. Any bias is a constant 1/2,
// so this is also added in the vertex shader.
//
// Analytic derivatives are used so that a single pipeline can be used regardless of HW derivative
// support or for geometry that would prove difficult for forward differencing. The device-space
// gradient for ellipses is calculated per-pixel by transforming a per-pixel local gradient vector
// with the Jacobian of the inverse local-to-device transform:
//
// (px,py) is the projected point of (u,v) transformed by a 3x3 matrix, M:
//                [x(u,v) / w(u,v)]       [x]   [m00 m01 m02] [u]
//      (px,py) = [y(u,v) / w(u,v)] where [y] = [m10 m11 m12]X[v] = M*(u,v,1)
//                                        [w]   [m20 m21 m22] [1]
//
// C(px,py) can be defined in terms of a local Cl(u,v) as C(px,py) = Cl(p^-1(px,py)), where p^-1 =
//
//               [x'(px,py) / w'(px,py)]       [x']   [m00' m01' * m02'] [px]
//      (u,v) =  [y'(px,py) / w'(px,py)] where [y'] = [m10' m11' * m12']X[py] = M^-1*(px,py,0,1)
//                                             [w']   [m20' m21' * m22'] [ 1]
//
// Note that if the 3x3 M was arrived by dropping the 3rd row and column from a 4x4 since we assume
// a local 3rd coordinate of 0, M^-1 is not equal to the 4x4 inverse with dropped rows and columns.
//
// Using the chain rule, then ∇C(px,py)
//   =  ∇Cl(u,v)X[1/w'(px,py)     0       -x'(px,py)/w'(px,py)^2] [m00' m01']
//               [    0       1/w'(px,py) -y'(px,py)/w'(px,py)^2]X[m10' m11']
//                                                                [m20' m21']
//
//   = 1/w'(px,py)*∇Cl(u,v)X[1 0 -x'(px,py)/w'(px,py)] [m00' m01']
//                          [0 1 -y'(px,py)/w'(px,py)]X[m10' m11']
//                                                     [m20' m21']
//
//   = w(u,v)*∇Cl(u,v)X[1 0 0 -u] [m00' m01']
//                     [0 1 0 -v]X[m10' m11']
//                                [m20' m21']
//
//   = w(u,v)*∇Cl(u,v)X[m00'-m20'u m01'-m21'u]
//                     [m10'-m20'v m11'-m21'v]
//
// The vertex shader calculates the rightmost 2x2 matrix and interpolates it across the shape since
// each component is linear in (u,v). ∇Cl(u,v) is evaluated per pixel in the fragment shader and
// depends on which corner and edge being evaluated. w(u,v) is the device-space W coordinate, so
// its reciprocal is provided in sk_FragCoord.w.
namespace skgpu::graphite {

AAFlags;

static bool is_clockwise(const EdgeAAQuad& quad) { … }

// Represents the per-vertex attributes used in each instance.
struct Vertex { … };

// Allowed values for the center weight instance value (selected at record time based on style
// and transform), and are defined such that when (insance-weight > vertex-weight) is true, the
// vertex should be snapped to the center instead of its regular calculation.
static constexpr int kCornerVertexCount = …; // sk_VertexID is divided by this in SkSL
static constexpr int kVertexCount = …;
static constexpr int kIndexCount = …;

static void write_index_buffer(VertexWriter writer) { … }

static void write_vertex_buffer(VertexWriter writer) { … }

PerEdgeAAQuadRenderStep::PerEdgeAAQuadRenderStep(StaticBufferManager* bufferManager)
        : … { … }

PerEdgeAAQuadRenderStep::~PerEdgeAAQuadRenderStep() { … }

std::string PerEdgeAAQuadRenderStep::vertexSkSL() const { … }

const char* PerEdgeAAQuadRenderStep::fragmentCoverageSkSL() const { … }

void PerEdgeAAQuadRenderStep::writeVertices(DrawWriter* writer,
                                           const DrawParams& params,
                                           skvx::ushort2 ssboIndices) const { … }

void PerEdgeAAQuadRenderStep::writeUniformsAndTextures(const DrawParams&,
                                                       PipelineDataGatherer*) const { … }

}  // namespace skgpu::graphite
chromium/third_party/skia/src/gpu/graphite/render/PerEdgeAAQuadRenderStep.cpp
