src/gpu/ccpr/GrCCCoverageProcessor.cpp

/*
 * Copyright 2017 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/ccpr/GrCCCoverageProcessor.h"

#include "src/core/SkMakeUnique.h"
#include "src/gpu/GrOpFlushState.h"
#include "src/gpu/GrOpsRenderPass.h"
#include "src/gpu/ccpr/GrCCConicShader.h"
#include "src/gpu/ccpr/GrCCCubicShader.h"
#include "src/gpu/ccpr/GrCCQuadraticShader.h"
#include "src/gpu/glsl/GrGLSLFragmentShaderBuilder.h"
#include "src/gpu/glsl/GrGLSLVertexGeoBuilder.h"
#include "src/gpu/glsl/GrGLSLVertexGeoBuilder.h"

class GrCCCoverageProcessor::TriangleShader : public GrCCCoverageProcessor::Shader {
    void onEmitVaryings(
            GrGLSLVaryingHandler* varyingHandler, GrGLSLVarying::Scope scope, SkString* code,
            const char* position, const char* coverage, const char* cornerCoverage,
            const char* /*wind*/) override {
        if (!cornerCoverage) {
            fCoverages.reset(kHalf_GrSLType, scope);
            varyingHandler->addVarying("coverage", &fCoverages);
            code->appendf("%s = %s;", OutName(fCoverages), coverage);
        } else {
            fCoverages.reset(kHalf3_GrSLType, scope);
            varyingHandler->addVarying("coverages", &fCoverages);
            code->appendf("%s = half3(%s, %s);", OutName(fCoverages), coverage, cornerCoverage);
        }
    }

    void emitFragmentCoverageCode(
            GrGLSLFPFragmentBuilder* f, const char* outputCoverage) const override {
        if (kHalf_GrSLType == fCoverages.type()) {
            f->codeAppendf("%s = %s;", outputCoverage, fCoverages.fsIn());
        } else {
            f->codeAppendf("%s = %s.z * %s.y + %s.x;",
                           outputCoverage, fCoverages.fsIn(), fCoverages.fsIn(), fCoverages.fsIn());
        }
    }

    void emitSampleMaskCode(GrGLSLFPFragmentBuilder*) const override { return; }

    GrGLSLVarying fCoverages;
};

void GrCCCoverageProcessor::Shader::CalcWind(const GrCCCoverageProcessor& proc,
                                             GrGLSLVertexGeoBuilder* s, const char* pts,
                                             const char* outputWind) {
    if (3 == proc.numInputPoints()) {
        s->codeAppendf("float2 a = %s[0] - %s[1], "
                              "b = %s[0] - %s[2];", pts, pts, pts, pts);
    } else {
        // All inputs are convex, so it's sufficient to just average the middle two input points.
        SkASSERT(4 == proc.numInputPoints());
        s->codeAppendf("float2 p12 = (%s[1] + %s[2]) * .5;", pts, pts);
        s->codeAppendf("float2 a = %s[0] - p12, "
                              "b = %s[0] - %s[3];", pts, pts, pts);
    }

    s->codeAppend ("float area_x2 = determinant(float2x2(a, b));");
    if (proc.isTriangles()) {
        // We cull extremely thin triangles by zeroing wind. When a triangle gets too thin it's
        // possible for FP round-off error to actually give us the wrong winding direction, causing
        // rendering artifacts. The criteria we choose is "height <~ 1/1024". So we drop a triangle
        // if the max effect it can have on any single pixel is <~ 1/1024, or 1/4 of a bit in 8888.
        s->codeAppend ("float2 bbox_size = max(abs(a), abs(b));");
        s->codeAppend ("float basewidth = max(bbox_size.x + bbox_size.y, 1);");
        s->codeAppendf("%s = (abs(area_x2 * 1024) > basewidth) ? sign(half(area_x2)) : 0;",
                       outputWind);
    } else {
        // We already converted nearly-flat curves to lines on the CPU, so no need to worry about
        // thin curve hulls at this point.
        s->codeAppendf("%s = sign(half(area_x2));", outputWind);
    }
}

void GrCCCoverageProcessor::Shader::CalcEdgeCoverageAtBloatVertex(GrGLSLVertexGeoBuilder* s,
                                                                  const char* leftPt,
                                                                  const char* rightPt,
                                                                  const char* rasterVertexDir,
                                                                  const char* outputCoverage) {
    // Here we find an edge's coverage at one corner of a conservative raster bloat box whose center
    // falls on the edge in question. (A bloat box is axis-aligned and the size of one pixel.) We
    // always set up coverage so it is -1 at the outermost corner, 0 at the innermost, and -.5 at
    // the center. Interpolated, these coverage values convert jagged conservative raster edges into
    // smooth antialiased edges.
    //
    // d1 == (P + sign(n) * bloat) dot n                   (Distance at the bloat box vertex whose
    //    == P dot n + (abs(n.x) + abs(n.y)) * bloatSize    coverage=-1, where the bloat box is
    //                                                      centered on P.)
    //
    // d0 == (P - sign(n) * bloat) dot n                   (Distance at the bloat box vertex whose
    //    == P dot n - (abs(n.x) + abs(n.y)) * bloatSize    coverage=0, where the bloat box is
    //                                                      centered on P.)
    //
    // d == (P + rasterVertexDir * bloatSize) dot n        (Distance at the bloat box vertex whose
    //   == P dot n + (rasterVertexDir dot n) * bloatSize   coverage we wish to calculate.)
    //
    // coverage == -(d - d0) / (d1 - d0)                   (coverage=-1 at d=d1; coverage=0 at d=d0)
    //
    //          == (rasterVertexDir dot n) / (abs(n.x) + abs(n.y)) * -.5 - .5
    //
    s->codeAppendf("float2 n = float2(%s.y - %s.y, %s.x - %s.x);",
                   rightPt, leftPt, leftPt, rightPt);
    s->codeAppend ("float nwidth = abs(n.x) + abs(n.y);");
    s->codeAppendf("float t = dot(%s, n);", rasterVertexDir);
    // The below conditional guarantees we get exactly 1 on the divide when nwidth=t (in case the
    // GPU divides by multiplying by the reciprocal?) It also guards against NaN when nwidth=0.
    s->codeAppendf("%s = half(abs(t) != nwidth ? t / nwidth : sign(t)) * -.5 - .5;",
                   outputCoverage);
}

void GrCCCoverageProcessor::Shader::CalcEdgeCoveragesAtBloatVertices(GrGLSLVertexGeoBuilder* s,
                                                                     const char* leftPt,
                                                                     const char* rightPt,
                                                                     const char* bloatDir1,
                                                                     const char* bloatDir2,
                                                                     const char* outputCoverages) {
    // See comments in CalcEdgeCoverageAtBloatVertex.
    s->codeAppendf("float2 n = float2(%s.y - %s.y, %s.x - %s.x);",
                   rightPt, leftPt, leftPt, rightPt);
    s->codeAppend ("float nwidth = abs(n.x) + abs(n.y);");
    s->codeAppendf("float2 t = n * float2x2(%s, %s);", bloatDir1, bloatDir2);
    s->codeAppendf("for (int i = 0; i < 2; ++i) {");
    s->codeAppendf(    "%s[i] = half(abs(t[i]) != nwidth ? t[i] / nwidth : sign(t[i])) * -.5 - .5;",
                       outputCoverages);
    s->codeAppendf("}");
}

void GrCCCoverageProcessor::Shader::CalcCornerAttenuation(GrGLSLVertexGeoBuilder* s,
                                                          const char* leftDir, const char* rightDir,
                                                          const char* outputAttenuation) {
    // obtuseness = cos(corner_angle)  if corner_angle > 90 degrees
    //                              0  if corner_angle <= 90 degrees
    //
    // NOTE: leftDir and rightDir are normalized and point in the same direction the path was
    // defined with, i.e., leftDir points into the corner and rightDir points away from the corner.
    s->codeAppendf("half obtuseness = max(half(dot(%s, %s)), 0);", leftDir, rightDir);

    // axis_alignedness = 1 - tan(angle_to_nearest_axis_from_corner_bisector)
    //                    (i.e.,  1  when the corner bisector is aligned with the x- or y-axis
    //                            0  when the corner bisector falls on a 45 degree angle
    //                         0..1  when the corner bisector falls somewhere in between
    s->codeAppendf("half2 abs_bisect_maybe_transpose = abs((0 == obtuseness) ? half2(%s - %s) : "
                                                                              "half2(%s + %s));",
                   leftDir, rightDir, leftDir, rightDir);
    s->codeAppend ("half axis_alignedness = "
                           "1 - min(abs_bisect_maybe_transpose.y, abs_bisect_maybe_transpose.x) / "
                               "max(abs_bisect_maybe_transpose.x, abs_bisect_maybe_transpose.y);");

    // ninety_degreesness = sin^2(corner_angle)
    // sin^2 just because... it's always positive and the results looked better than plain sine... ?
    s->codeAppendf("half ninety_degreesness = determinant(half2x2(%s, %s));", leftDir, rightDir);
    s->codeAppend ("ninety_degreesness = ninety_degreesness * ninety_degreesness;");

    // The below formula is not smart. It was just arrived at by considering the following
    // observations:
    //
    // 1. 90-degree, axis-aligned corners have full attenuation along the bisector.
    //    (i.e. coverage = 1 - distance_to_corner^2)
    //    (i.e. outputAttenuation = 0)
    //
    // 2. 180-degree corners always have zero attenuation.
    //    (i.e. coverage = 1 - distance_to_corner)
    //    (i.e. outputAttenuation = 1)
    //
    // 3. 90-degree corners whose bisector falls on a 45 degree angle also do not attenuate.
    //    (i.e. outputAttenuation = 1)
    s->codeAppendf("%s = max(obtuseness, axis_alignedness * ninety_degreesness);",
                   outputAttenuation);
}

GrGLSLPrimitiveProcessor* GrCCCoverageProcessor::createGLSLInstance(const GrShaderCaps&) const {
    std::unique_ptr<Shader> shader;
    switch (fPrimitiveType) {
        case PrimitiveType::kTriangles:
        case PrimitiveType::kWeightedTriangles:
            shader = skstd::make_unique<TriangleShader>();
            break;
        case PrimitiveType::kQuadratics:
            shader = skstd::make_unique<GrCCQuadraticShader>();
            break;
        case PrimitiveType::kCubics:
            shader = skstd::make_unique<GrCCCubicShader>();
            break;
        case PrimitiveType::kConics:
            shader = skstd::make_unique<GrCCConicShader>();
            break;
    }
    return this->onCreateGLSLInstance(std::move(shader));
}

void GrCCCoverageProcessor::draw(
        GrOpFlushState* flushState, const GrPipeline& pipeline, const SkIRect scissorRects[],
        const GrMesh meshes[], int meshCount, const SkRect& drawBounds) const {
    GrPipeline::DynamicStateArrays dynamicStateArrays;
    dynamicStateArrays.fScissorRects = scissorRects;
    GrOpsRenderPass* renderPass = flushState->opsRenderPass();
    renderPass->draw(*this, pipeline, nullptr, &dynamicStateArrays, meshes, meshCount, drawBounds);
}