src/gpu/ganesh/gradients/GrGradientShader.cpp

/*
 * Copyright 2018 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/ganesh/gradients/GrGradientShader.h"

#include "include/core/SkColorSpace.h"
#include "include/gpu/GrRecordingContext.h"
#include "src/base/SkMathPriv.h"
#include "src/core/SkColorSpacePriv.h"
#include "src/core/SkRuntimeEffectPriv.h"
#include "src/gpu/ganesh/GrCaps.h"
#include "src/gpu/ganesh/GrColor.h"
#include "src/gpu/ganesh/GrColorInfo.h"
#include "src/gpu/ganesh/GrColorSpaceXform.h"
#include "src/gpu/ganesh/GrRecordingContextPriv.h"
#include "src/gpu/ganesh/SkGr.h"
#include "src/gpu/ganesh/effects/GrMatrixEffect.h"
#include "src/gpu/ganesh/effects/GrSkSLFP.h"
#include "src/gpu/ganesh/effects/GrTextureEffect.h"
#include "src/gpu/ganesh/gradients/GrGradientBitmapCache.h"

using namespace skia_private;

using Vec4 = skvx::Vec<4, float>;

// Intervals smaller than this (that aren't hard stops) on low-precision-only devices force us to
// use the textured gradient
static const SkScalar kLowPrecisionIntervalLimit = 0.01f;

// Each cache entry costs 1K or 2K of RAM. Each bitmap will be 1x256 at either 32bpp or 64bpp.
static const int kMaxNumCachedGradientBitmaps = 32;
static const int kGradientTextureSize = 256;

// NOTE: signature takes raw pointers to the color/pos arrays and a count to make it easy for
// MakeColorizer to transparently take care of hard stops at the end points of the gradient.
// TODO(skia:13108): This code is totally wrong when using a non-destination interpolation space.
// We need to plumb the interpolation information to the gradient cache, and actually do the work
// to convert back to destination space before filling out the bitmap (otherwise we might have
// values that don't fit in 8888). This will also mean that the top-level colorizer factory will
// need to return whether or not the interpolated -> dst transform is still necessary.
static std::unique_ptr<GrFragmentProcessor> make_textured_colorizer(const SkPMColor4f* colors,
        const SkScalar* positions, int count, bool premul, const GrFPArgs& args) {
    static GrGradientBitmapCache gCache(kMaxNumCachedGradientBitmaps, kGradientTextureSize);

    // Use 8888 or F16, depending on the destination config.
    // TODO: Use 1010102 for opaque gradients, at least if destination is 1010102?
    SkColorType colorType = kRGBA_8888_SkColorType;
    if (GrColorTypeIsWiderThan(args.fDstColorInfo->colorType(), 8)) {
        auto f16Format = args.fContext->priv().caps()->getDefaultBackendFormat(
                GrColorType::kRGBA_F16, GrRenderable::kNo);
        if (f16Format.isValid()) {
            colorType = kRGBA_F16_SkColorType;
        }
    }
    SkAlphaType alphaType = premul ? kPremul_SkAlphaType : kUnpremul_SkAlphaType;

    SkBitmap bitmap;
    gCache.getGradient(colors, positions, count, colorType, alphaType, &bitmap);
    SkASSERT(1 == bitmap.height() && SkIsPow2(bitmap.width()));
    SkASSERT(bitmap.isImmutable());

    auto view = std::get<0>(GrMakeCachedBitmapProxyView(
            args.fContext, bitmap, /*label=*/"MakeTexturedColorizer", GrMipmapped::kNo));
    if (!view) {
        SkDebugf("Gradient won't draw. Could not create texture.");
        return nullptr;
    }

    auto m = SkMatrix::Scale(view.width(), 1.f);
    return GrTextureEffect::Make(std::move(view), alphaType, m, GrSamplerState::Filter::kLinear);
}


static std::unique_ptr<GrFragmentProcessor> make_single_interval_colorizer(const SkPMColor4f& start,
                                                                           const SkPMColor4f& end) {
    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForShader,
        "uniform half4 start;"
        "uniform half4 end;"
        "half4 main(float2 coord) {"
            // Clamping and/or wrapping was already handled by the parent shader so the output
            // color is a simple lerp.
            "return mix(start, end, half(coord.x));"
        "}"
    );
    return GrSkSLFP::Make(effect, "SingleIntervalColorizer", /*inputFP=*/nullptr,
                          GrSkSLFP::OptFlags::kNone,
                          "start", start,
                          "end", end);
}

static std::unique_ptr<GrFragmentProcessor> make_dual_interval_colorizer(const SkPMColor4f& c0,
                                                                         const SkPMColor4f& c1,
                                                                         const SkPMColor4f& c2,
                                                                         const SkPMColor4f& c3,
                                                                         float threshold) {
    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForShader,
        "uniform float4 scale[2];"
        "uniform float4 bias[2];"
        "uniform half threshold;"

        "half4 main(float2 coord) {"
            "half t = half(coord.x);"

            "float4 s, b;"
            "if (t < threshold) {"
                "s = scale[0];"
                "b = bias[0];"
            "} else {"
                "s = scale[1];"
                "b = bias[1];"
            "}"

            "return half4(t * s + b);"
        "}"
    );

    // Derive scale and biases from the 4 colors and threshold
    Vec4 vc0 = Vec4::Load(c0.vec());
    Vec4 vc1 = Vec4::Load(c1.vec());
    Vec4 vc2 = Vec4::Load(c2.vec());
    Vec4 vc3 = Vec4::Load(c3.vec());

    const Vec4 scale[2] = {(vc1 - vc0) / threshold,
                           (vc3 - vc2) / (1 - threshold)};
    const Vec4 bias[2]  = {vc0,
                           vc2 - threshold * scale[1]};
    return GrSkSLFP::Make(effect, "DualIntervalColorizer", /*inputFP=*/nullptr,
                          GrSkSLFP::OptFlags::kNone,
                          "scale", SkSpan(scale),
                          "bias", SkSpan(bias),
                          "threshold", threshold);
}

// The "unrolled" colorizer contains hand-written nested ifs which perform a binary search.
// This works on ES2 hardware that doesn't support non-constant array indexes.
// However, to keep code size under control, we are limited to a small number of stops.
static constexpr int kMaxUnrolledColorCount    = 16;
static constexpr int kMaxUnrolledIntervalCount = kMaxUnrolledColorCount / 2;

static std::unique_ptr<GrFragmentProcessor> make_unrolled_colorizer(int intervalCount,
                                                                    const SkPMColor4f* scale,
                                                                    const SkPMColor4f* bias,
                                                                    SkRect thresholds1_7,
                                                                    SkRect thresholds9_13) {
    SkASSERT(intervalCount >= 1 && intervalCount <= 8);

    static SkOnce                 once[kMaxUnrolledIntervalCount];
    static const SkRuntimeEffect* effects[kMaxUnrolledIntervalCount];

    once[intervalCount - 1]([intervalCount] {
        SkString sksl;

        // The 7 threshold positions that define the boundaries of the 8 intervals (excluding t = 0,
        // and t = 1) are packed into two half4s instead of having up to 7 separate scalar uniforms.
        // For low interval counts, the extra components are ignored in the shader, but the uniform
        // simplification is worth it. It is assumed thresholds are provided in increasing value,
        // mapped as:
        //  - thresholds1_7.x = boundary between (0,1) and (2,3) -> 1_2
        //  -              .y = boundary between (2,3) and (4,5) -> 3_4
        //  -              .z = boundary between (4,5) and (6,7) -> 5_6
        //  -              .w = boundary between (6,7) and (8,9) -> 7_8
        //  - thresholds9_13.x = boundary between (8,9) and (10,11) -> 9_10
        //  -               .y = boundary between (10,11) and (12,13) -> 11_12
        //  -               .z = boundary between (12,13) and (14,15) -> 13_14
        //  -               .w = unused
        sksl.append("uniform half4 thresholds1_7, thresholds9_13;");

        // With the current hardstop detection threshold of 0.00024, the maximum scale and bias
        // values will be on the order of 4k (since they divide by dt). That is well outside the
        // precision capabilities of half floats, which can lead to inaccurate gradient calculations
        sksl.appendf("uniform float4 scale[%d];", intervalCount);
        sksl.appendf("uniform float4 bias[%d];", intervalCount);

        // Explicit binary search for the proper interval that t falls within. The interval
        // count checks are constant expressions, which are then optimized to the minimal number
        // of branches for the specific interval count.
        sksl.appendf(
        "half4 main(float2 coord) {"
            "half t = half(coord.x);"
            "float4 s, b;"
            // thresholds1_7.w is mid point for intervals (0,7) and (8,15)
            "if (%d <= 4 || t < thresholds1_7.w) {"
                // thresholds1_7.y is mid point for intervals (0,3) and (4,7)
                "if (%d <= 2 || t < thresholds1_7.y) {"
                    // thresholds1_7.x is mid point for intervals (0,1) and (2,3)
                    "if (%d <= 1 || t < thresholds1_7.x) {"
                        "%s" // s = scale[0]; b = bias[0];
                    "} else {"
                        "%s" // s = scale[1]; b = bias[1];
                    "}"
                "} else {"
                    // thresholds1_7.z is mid point for intervals (4,5) and (6,7)
                    "if (%d <= 3 || t < thresholds1_7.z) {"
                        "%s" // s = scale[2]; b = bias[2];
                    "} else {"
                        "%s" // s = scale[3]; b = bias[3];
                    "}"
                "}"
            "} else {"
                // thresholds9_13.y is mid point for intervals (8,11) and (12,15)
                "if (%d <= 6 || t < thresholds9_13.y) {"
                    // thresholds9_13.x is mid point for intervals (8,9) and (10,11)
                    "if (%d <= 5 || t < thresholds9_13.x) {"
                        "%s"
                    "} else {"
                        "%s" // s = scale[5]; b = bias[5];
                    "}"
                "} else {"
                    // thresholds9_13.z is mid point for intervals (12,13) and (14,15)
                    "if (%d <= 7 || t < thresholds9_13.z) {"
                        "%s" // s = scale[6]; b = bias[6];
                    "} else {"
                        "%s" // s = scale[7]; b = bias[7];
                    "}"
                "}"
            "}"
            "return t * s + b;"
        "}"
        , intervalCount,
              intervalCount,
                intervalCount,
                  (intervalCount <= 0) ? "" : "s = scale[0]; b = bias[0];",
                  (intervalCount <= 1) ? "" : "s = scale[1]; b = bias[1];",
                intervalCount,
                  (intervalCount <= 2) ? "" : "s = scale[2]; b = bias[2];",
                  (intervalCount <= 3) ? "" : "s = scale[3]; b = bias[3];",
              intervalCount,
                intervalCount,
                  (intervalCount <= 4) ? "" : "s = scale[4]; b = bias[4];",
                  (intervalCount <= 5) ? "" : "s = scale[5]; b = bias[5];",
                intervalCount,
                  (intervalCount <= 6) ? "" : "s = scale[6]; b = bias[6];",
                  (intervalCount <= 7) ? "" : "s = scale[7]; b = bias[7];");

        auto result = SkRuntimeEffect::MakeForShader(std::move(sksl));
        SkASSERTF(result.effect, "%s", result.errorText.c_str());
        effects[intervalCount - 1] = result.effect.release();
    });

    return GrSkSLFP::Make(effects[intervalCount - 1], "UnrolledBinaryColorizer",
                          /*inputFP=*/nullptr, GrSkSLFP::OptFlags::kNone,
                          "thresholds1_7", thresholds1_7,
                          "thresholds9_13", thresholds9_13,
                          "scale", SkSpan(scale, intervalCount),
                          "bias", SkSpan(bias, intervalCount));
}

// The "looping" colorizer uses a real loop to binary-search the array of gradient stops.
static constexpr int kMaxLoopingColorCount    = 128;
static constexpr int kMaxLoopingIntervalCount = kMaxLoopingColorCount / 2;

static std::unique_ptr<GrFragmentProcessor> make_looping_colorizer(int intervalCount,
                                                                   const SkPMColor4f* scale,
                                                                   const SkPMColor4f* bias,
                                                                   const SkScalar* thresholds) {
    SkASSERT(intervalCount >= 1 && intervalCount <= kMaxLoopingIntervalCount);
    SkASSERT((intervalCount & 3) == 0);  // intervals are required to come in groups of four
    int intervalChunks = intervalCount / 4;
    int cacheIndex = (size_t)intervalChunks - 1;

    struct EffectCacheEntry {
        SkOnce once;
        const SkRuntimeEffect* effect;
    };

    static EffectCacheEntry effectCache[kMaxLoopingIntervalCount / 4];
    SkASSERT(cacheIndex >= 0 && cacheIndex < (int)std::size(effectCache));
    EffectCacheEntry* cacheEntry = &effectCache[cacheIndex];

    cacheEntry->once([intervalCount, intervalChunks, cacheEntry] {
        SkString sksl;

        // Binary search for the interval that `t` falls within. We can precalculate the number of
        // loop iterations we need, and we know `t` will always be in range, so we can just loop a
        // fixed number of times and can be guaranteed to have found the proper element.
        //
        // Threshold values are stored in half4s to keep them compact, so the last two rounds of
        // binary search are hand-unrolled to allow them to use swizzles.
        //
        // Note that this colorizer is also designed to handle the case of exactly 4 intervals (a
        // single chunk). In this case, the binary search for-loop will optimize away entirely, as
        // it can be proven to execute zero times. We also optimize away the calculation of `4 *
        // chunk` near the end via an if statement, as the result will always be in chunk 0.
        int loopCount = SkNextLog2(intervalChunks);
        sksl.appendf(
        "#version 300\n" // important space to separate token.
        "uniform half4 thresholds[%d];"
        "uniform float4 scale[%d];"
        "uniform float4 bias[%d];"

        "half4 main(float2 coord) {"
            "half t = half(coord.x);"

            // Choose a chunk from thresholds via binary search in a loop.
            "int low = 0;"
            "int high = %d;"
            "int chunk = %d;"
            "for (int loop = 0; loop < %d; ++loop) {"
                "if (t < thresholds[chunk].w) {"
                    "high = chunk;"
                "} else {"
                    "low = chunk + 1;"
                "}"
                "chunk = (low + high) / 2;"
            "}"

            // Choose the final position via explicit 4-way binary search.
            "int pos;"
            "if (t < thresholds[chunk].y) {"
                "pos = (t < thresholds[chunk].x) ? 0 : 1;"
            "} else {"
                "pos = (t < thresholds[chunk].z) ? 2 : 3;"
            "}"
            "if (%d > 0) {"
                "pos += 4 * chunk;"
            "}"
            "return t * scale[pos] + bias[pos];"
        "}"
        , /* thresholds: */ intervalChunks,
            /* scale: */ intervalCount,
            /* bias: */ intervalCount,
            /* high: */ intervalChunks - 1,
            /* chunk: */ (intervalChunks - 1) / 2,
            /* loopCount: */ loopCount,
            /* if (loopCount > 0): */ loopCount);

        auto result = SkRuntimeEffect::MakeForShader(std::move(sksl));
        SkASSERTF(result.effect, "%s", result.errorText.c_str());
        cacheEntry->effect = result.effect.release();
    });

    return GrSkSLFP::Make(cacheEntry->effect, "LoopingBinaryColorizer",
                          /*inputFP=*/nullptr, GrSkSLFP::OptFlags::kNone,
                          "thresholds", SkSpan((const SkV4*)thresholds, intervalChunks),
                          "scale", SkSpan(scale, intervalCount),
                          "bias", SkSpan(bias, intervalCount));
}

// Converts an input array of {colors, positions} into an array of {scales, biases, thresholds}.
// The length of the result array may differ from the input due to hard-stops or empty intervals.
int build_intervals(int inputLength,
                    const SkPMColor4f* inColors,
                    const SkScalar* inPositions,
                    int outputLength,
                    SkPMColor4f* outScales,
                    SkPMColor4f* outBiases,
                    SkScalar* outThresholds) {
    // Depending on how the positions resolve into hard stops or regular stops, the number of
    // intervals specified by the number of colors/positions can change. For instance, a plain
    // 3 color gradient is two intervals, but a 4 color gradient with a hard stop is also
    // two intervals. At the most extreme end, an 8 interval gradient made entirely of hard
    // stops has 16 colors.
    int intervalCount = 0;
    for (int i = 0; i < inputLength - 1; i++) {
        if (intervalCount >= outputLength) {
            // Already reached our output limit, and haven't run out of color stops. This gradient
            // cannot be represented without more intervals.
            return 0;
        }

        SkScalar t0 = inPositions[i];
        SkScalar t1 = inPositions[i + 1];
        SkScalar dt = t1 - t0;
        // If the interval is empty, skip to the next interval. This will automatically create
        // distinct hard stop intervals as needed. It also protects against malformed gradients
        // that have repeated hard stops at the very beginning that are effectively unreachable.
        if (SkScalarNearlyZero(dt)) {
            continue;
        }

        Vec4 c0 = Vec4::Load(inColors[i].vec());
        Vec4 c1 = Vec4::Load(inColors[i + 1].vec());
        Vec4 scale = (c1 - c0) / dt;
        Vec4 bias = c0 - t0 * scale;

        scale.store(outScales + intervalCount);
        bias.store(outBiases + intervalCount);
        outThresholds[intervalCount] = t1;
        intervalCount++;
    }
    return intervalCount;
}

static std::unique_ptr<GrFragmentProcessor> make_unrolled_binary_colorizer(
        const SkPMColor4f* colors, const SkScalar* positions, int count) {
    if (count > kMaxUnrolledColorCount) {
        // Definitely cannot represent this gradient configuration
        return nullptr;
    }

    SkPMColor4f scales[kMaxUnrolledIntervalCount];
    SkPMColor4f biases[kMaxUnrolledIntervalCount];
    SkScalar thresholds[kMaxUnrolledIntervalCount] = {};
    int intervalCount = build_intervals(count, colors, positions,
                                        kMaxUnrolledIntervalCount, scales, biases, thresholds);
    if (intervalCount <= 0) {
        return nullptr;
    }

    SkRect thresholds1_7  = {thresholds[0], thresholds[1], thresholds[2], thresholds[3]},
           thresholds9_13 = {thresholds[4], thresholds[5], thresholds[6], 0.0};

    return make_unrolled_colorizer(intervalCount, scales, biases, thresholds1_7, thresholds9_13);
}

static std::unique_ptr<GrFragmentProcessor> make_looping_binary_colorizer(const SkPMColor4f* colors,
                                                                          const SkScalar* positions,
                                                                          int count) {
    if (count > kMaxLoopingColorCount) {
        // Definitely cannot represent this gradient configuration
        return nullptr;
    }

    SkPMColor4f scales[kMaxLoopingIntervalCount];
    SkPMColor4f biases[kMaxLoopingIntervalCount];
    SkScalar thresholds[kMaxLoopingIntervalCount] = {};
    int intervalCount = build_intervals(count, colors, positions,
                                        kMaxLoopingIntervalCount, scales, biases, thresholds);
    if (intervalCount <= 0) {
        return nullptr;
    }

    // We round up the number of intervals to the next power of two. This reduces the number of
    // unique shaders and doesn't require any additional GPU processing power, but this does waste a
    // handful of uniforms.
    int roundedSize = std::max(4, SkNextPow2(intervalCount));
    SkASSERT(roundedSize <= kMaxLoopingIntervalCount);
    for (; intervalCount < roundedSize; ++intervalCount) {
        thresholds[intervalCount] = thresholds[intervalCount - 1];
        scales[intervalCount] = scales[intervalCount - 1];
        biases[intervalCount] = biases[intervalCount - 1];
    }

    return make_looping_colorizer(intervalCount, scales, biases, thresholds);
}

// Analyze the shader's color stops and positions and chooses an appropriate colorizer to represent
// the gradient.
static std::unique_ptr<GrFragmentProcessor> make_colorizer(const SkPMColor4f* colors,
                                                           const SkScalar* positions,
                                                           int count,
                                                           bool premul,
                                                           const GrFPArgs& args) {
    // If there are hard stops at the beginning or end, the first and/or last color should be
    // ignored by the colorizer since it should only be used in a clamped border color. By detecting
    // and removing these stops at the beginning, it makes optimizing the remaining color stops
    // simpler.

    // SkGradientBaseShader guarantees that pos[0] == 0 by adding a default value.
    bool bottomHardStop = SkScalarNearlyEqual(positions[0], positions[1]);
    // The same is true for pos[end] == 1
    bool topHardStop = SkScalarNearlyEqual(positions[count - 2], positions[count - 1]);

    if (bottomHardStop) {
        colors++;
        positions++;
        count--;
    }
    if (topHardStop) {
        count--;
    }

    // Two remaining colors means a single interval from 0 to 1
    // (but it may have originally been a 3 or 4 color gradient with 1-2 hard stops at the ends)
    if (count == 2) {
        return make_single_interval_colorizer(colors[0], colors[1]);
    }

    const GrShaderCaps* caps = args.fContext->priv().caps()->shaderCaps();
    auto intervalsExceedPrecisionLimit = [&]() -> bool {
        // The remaining analytic colorizers use scale*t+bias, and the scale/bias values can become
        // quite large when thresholds are close (but still outside the hardstop limit). If float
        // isn't 32-bit, output can be incorrect if the thresholds are too close together. However,
        // the analytic shaders are higher quality, so they can be used with lower precision
        // hardware when the thresholds are not ill-conditioned.
        if (!caps->fFloatIs32Bits) {
            // Could run into problems. Check if thresholds are close together (with a limit of .01,
            // so that scales will be less than 100, which leaves 4 decimals of precision on
            // 16-bit).
            for (int i = 0; i < count - 1; i++) {
                SkScalar dt = SkScalarAbs(positions[i] - positions[i + 1]);
                if (dt <= kLowPrecisionIntervalLimit && dt > SK_ScalarNearlyZero) {
                    return true;
                }
            }
        }
        return false;
    };

    auto makeDualIntervalColorizer = [&]() -> std::unique_ptr<GrFragmentProcessor> {
        // The dual-interval colorizer uses the same principles as the binary-search colorizer, but
        // is limited to exactly 2 intervals.
        if (count == 3) {
            // Must be a dual interval gradient, where the middle point is at 1 and the
            // two intervals share the middle color stop.
            return make_dual_interval_colorizer(colors[0], colors[1],
                                                colors[1], colors[2],
                                                positions[1]);
        }
        if (count == 4 && SkScalarNearlyEqual(positions[1], positions[2])) {
            // Two separate intervals that join at the same threshold position
            return make_dual_interval_colorizer(colors[0], colors[1],
                                                colors[2], colors[3],
                                                positions[1]);
        }
        // The gradient can't be represented in only two intervals.
        return nullptr;
    };

    int binaryColorizerLimit = caps->fNonconstantArrayIndexSupport ? kMaxLoopingColorCount
                                                                   : kMaxUnrolledColorCount;
    if ((count <= binaryColorizerLimit) && !intervalsExceedPrecisionLimit()) {
        // The dual-interval colorizer uses the same principles as the binary-search colorizer, but
        // is limited to exactly 2 intervals.
        std::unique_ptr<GrFragmentProcessor> colorizer = makeDualIntervalColorizer();
        if (colorizer) {
            return colorizer;
        }
        // Attempt to create an analytic colorizer that uses a binary-search loop.
        colorizer = caps->fNonconstantArrayIndexSupport
                            ? make_looping_binary_colorizer(colors, positions, count)
                            : make_unrolled_binary_colorizer(colors, positions, count);
        if (colorizer) {
            return colorizer;
        }
    }

    // Otherwise fall back to a rasterized gradient sampled by a texture, which can handle
    // arbitrary gradients. (This has limited sampling resolution, and always blurs hard-stops.)
    return make_textured_colorizer(colors, positions, count, premul, args);
}

// This top-level effect implements clamping on the layout coordinate and requires specifying the
// border colors that are used when outside the clamped boundary. Gradients with the
// SkTileMode::kClamp should use the colors at their first and last stop (after adding default stops
// for t=0,t=1) as the border color. This will automatically replicate the edge color, even when
// there is a hard stop.
//
// The SkTileMode::kDecal can be produced by specifying transparent black as the border colors,
// regardless of the gradient's stop colors.
static std::unique_ptr<GrFragmentProcessor> make_clamped_gradient(
        std::unique_ptr<GrFragmentProcessor> colorizer,
        std::unique_ptr<GrFragmentProcessor> gradLayout,
        SkPMColor4f leftBorderColor,
        SkPMColor4f rightBorderColor,
        bool colorsAreOpaque) {
    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForShader,
        "uniform shader colorizer;"
        "uniform shader gradLayout;"

        "uniform half4 leftBorderColor;"  // t < 0.0
        "uniform half4 rightBorderColor;" // t > 1.0

        "uniform int layoutPreservesOpacity;"  // specialized

        "half4 main(float2 coord) {"
            "half4 t = gradLayout.eval(coord);"
            "half4 outColor;"

            // If t.x is below 0, use the left border color without invoking the child processor.
            // If any t.x is above 1, use the right border color. Otherwise, t is in the [0, 1]
            // range assumed by the colorizer FP, so delegate to the child processor.
            "if (!bool(layoutPreservesOpacity) && t.y < 0) {"
                // layout has rejected this fragment (rely on sksl to remove this branch if the
                // layout FP preserves opacity is false)
                "outColor = half4(0);"
            "} else if (t.x < 0) {"
                "outColor = leftBorderColor;"
            "} else if (t.x > 1.0) {"
                "outColor = rightBorderColor;"
            "} else {"
                // Always sample from (x, 0), discarding y, since the layout FP can use y as a
                // side-channel.
                "outColor = colorizer.eval(t.x0);"
            "}"
            "return outColor;"
        "}"
    );

    // If the layout does not preserve opacity, remove the opaque optimization,
    // but otherwise respect the provided color opacity state (which should take
    // into account the opacity of the border colors).
    bool layoutPreservesOpacity = gradLayout->preservesOpaqueInput();
    GrSkSLFP::OptFlags optFlags = GrSkSLFP::OptFlags::kCompatibleWithCoverageAsAlpha;
    if (colorsAreOpaque && layoutPreservesOpacity) {
        optFlags |= GrSkSLFP::OptFlags::kPreservesOpaqueInput;
    }

    return GrSkSLFP::Make(effect, "ClampedGradient", /*inputFP=*/nullptr, optFlags,
                          "colorizer", GrSkSLFP::IgnoreOptFlags(std::move(colorizer)),
                          "gradLayout", GrSkSLFP::IgnoreOptFlags(std::move(gradLayout)),
                          "leftBorderColor", leftBorderColor,
                          "rightBorderColor", rightBorderColor,
                          "layoutPreservesOpacity",
                              GrSkSLFP::Specialize<int>(layoutPreservesOpacity));
}

static std::unique_ptr<GrFragmentProcessor> make_tiled_gradient(
        const GrFPArgs& args,
        std::unique_ptr<GrFragmentProcessor> colorizer,
        std::unique_ptr<GrFragmentProcessor> gradLayout,
        bool mirror,
        bool colorsAreOpaque) {
    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForShader,
        "uniform shader colorizer;"
        "uniform shader gradLayout;"

        "uniform int mirror;"                  // specialized
        "uniform int layoutPreservesOpacity;"  // specialized
        "uniform int useFloorAbsWorkaround;"   // specialized

        "half4 main(float2 coord) {"
            "half4 t = gradLayout.eval(coord);"

            "if (!bool(layoutPreservesOpacity) && t.y < 0) {"
                // layout has rejected this fragment (rely on sksl to remove this branch if the
                // layout FP preserves opacity is false)
                "return half4(0);"
            "} else {"
                "if (bool(mirror)) {"
                    "half t_1 = t.x - 1;"
                    "half tiled_t = t_1 - 2 * floor(t_1 * 0.5) - 1;"
                    "if (bool(useFloorAbsWorkaround)) {"
                        // At this point the expected value of tiled_t should between -1 and 1, so
                        // this clamp has no effect other than to break up the floor and abs calls
                        // and make sure the compiler doesn't merge them back together.
                        "tiled_t = clamp(tiled_t, -1, 1);"
                    "}"
                    "t.x = abs(tiled_t);"
                "} else {"
                    // Simple repeat mode
                    "t.x = fract(t.x);"
                "}"

                // Always sample from (x, 0), discarding y, since the layout FP can use y as a
                // side-channel.
                "half4 outColor = colorizer.eval(t.x0);"
                "return outColor;"
            "}"
        "}"
    );

    // If the layout does not preserve opacity, remove the opaque optimization,
    // but otherwise respect the provided color opacity state (which should take
    // into account the opacity of the border colors).
    bool layoutPreservesOpacity = gradLayout->preservesOpaqueInput();
    GrSkSLFP::OptFlags optFlags = GrSkSLFP::OptFlags::kCompatibleWithCoverageAsAlpha;
    if (colorsAreOpaque && layoutPreservesOpacity) {
        optFlags |= GrSkSLFP::OptFlags::kPreservesOpaqueInput;
    }
    const bool useFloorAbsWorkaround =
            args.fContext->priv().caps()->shaderCaps()->fMustDoOpBetweenFloorAndAbs;

    return GrSkSLFP::Make(effect, "TiledGradient", /*inputFP=*/nullptr, optFlags,
                          "colorizer", GrSkSLFP::IgnoreOptFlags(std::move(colorizer)),
                          "gradLayout", GrSkSLFP::IgnoreOptFlags(std::move(gradLayout)),
                          "mirror", GrSkSLFP::Specialize<int>(mirror),
                          "layoutPreservesOpacity",
                                GrSkSLFP::Specialize<int>(layoutPreservesOpacity),
                          "useFloorAbsWorkaround",
                                GrSkSLFP::Specialize<int>(useFloorAbsWorkaround));
}

static std::unique_ptr<GrFragmentProcessor> make_interpolated_to_dst(
        std::unique_ptr<GrFragmentProcessor> gradient,
        const SkGradientShader::Interpolation& interpolation,
        SkColorSpace* intermediateColorSpace,
        const GrColorInfo& dstInfo,
        bool allOpaque) {
    using ColorSpace = SkGradientShader::Interpolation::ColorSpace;

    // If these values change, you will need to edit sksl_shared
    static_assert(static_cast<int>(ColorSpace::kDestination) == 0);
    static_assert(static_cast<int>(ColorSpace::kSRGBLinear)  == 1);
    static_assert(static_cast<int>(ColorSpace::kLab)         == 2);
    static_assert(static_cast<int>(ColorSpace::kOKLab)       == 3);
    static_assert(static_cast<int>(ColorSpace::kLCH)         == 4);
    static_assert(static_cast<int>(ColorSpace::kOKLCH)       == 5);
    static_assert(static_cast<int>(ColorSpace::kSRGB)        == 6);
    static_assert(static_cast<int>(ColorSpace::kHSL)         == 7);
    static_assert(static_cast<int>(ColorSpace::kHWB)         == 8);

    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForColorFilter,
        "uniform int colorSpace;"    // specialized
        "uniform int do_unpremul;"   // specialized

        "half4 main(half4 color) {"
            "return $interpolated_to_rgb_unpremul(color, colorSpace, do_unpremul);"
        "}"
    );

    // Are we interpreting premul colors? We use this later to decide if we need to inject a final
    // premultiplication step.
    bool inputPremul = static_cast<bool>(interpolation.fInPremul);

    switch (interpolation.fColorSpace) {
        case ColorSpace::kLab:
        case ColorSpace::kOKLab:
        case ColorSpace::kLCH:
        case ColorSpace::kOKLCH:
        case ColorSpace::kHSL:
        case ColorSpace::kHWB:
            // In these exotic spaces, unpremul the colors if necessary (no need to do this if
            // they're all opaque), and then convert them to the intermediate SkColorSpace
            gradient = GrSkSLFP::Make(effect, "GradientCS", std::move(gradient),
                                      GrSkSLFP::OptFlags::kAll,
                                      "colorSpace", GrSkSLFP::Specialize<int>(
                                        static_cast<int>(interpolation.fColorSpace)),
                                      "do_unpremul", GrSkSLFP::Specialize<int>(
                                        inputPremul && !allOpaque));
            // We just forced the colors back to unpremul. Remember that for below
            inputPremul = false;
            break;
        default:
            break;
    }

    // Now transform from intermediate to destination color space. There are two tricky things here:
    // 1) Normally, we'd pass dstInfo to the transform effect. However, if someone is rendering to
    //    a non-color managed surface (nullptr dst color space), and they chose to interpolate in
    //    any of the exotic spaces, that transform would do nothing, and leave the colors in
    //    whatever intermediate space we chose. That could even be something like XYZ, which will
    //    produce nonsense. So, in this particular case, we break Skia's rules, and treat a null
    //    destination as sRGB.
    SkColorSpace* dstColorSpace = dstInfo.colorSpace() ? dstInfo.colorSpace() : sk_srgb_singleton();

    // 2) Alpha type: We already tweaked our idea of "inputPremul" above -- if we interpolated in a
    //    non-RGB space, then we had to unpremul the colors to get proper conversion back to RGB.
    //    Our final goal is to emit premul colors, but under certain conditions we don't need to do
    //    anything to achieve that: i.e. its interpolating already premul colors (inputPremul) or
    //    all the colors have a = 1, in which case premul is a no op. Note that this allOpaque check
    //    is more permissive than SkGradientBaseShader's isOpaque(), since we can optimize away the
    //    make-premul op for two point conical gradients (which report false for isOpaque).
    SkAlphaType intermediateAlphaType = inputPremul ? kPremul_SkAlphaType : kUnpremul_SkAlphaType;
    SkAlphaType dstAlphaType = kPremul_SkAlphaType;

    // If all the colors were opaque, then we don't need to do any premultiplication. We describe
    // all the colors as *unpremul*, though. That will eliminate any extra unpremul/premul pair
    // that would be injected if we have to do a color-space conversion here.
    if (allOpaque) {
        intermediateAlphaType = dstAlphaType = kUnpremul_SkAlphaType;
    }

    return GrColorSpaceXformEffect::Make(std::move(gradient),
                                         intermediateColorSpace, intermediateAlphaType,
                                         dstColorSpace, dstAlphaType);
}

namespace GrGradientShader {

/**
 * Produces an FP that muls its input coords by the inverse of the pending matrix and then
 * samples the passed FP with those coordinates. 'postInv' is an additional matrix to
 * post-apply to the inverted pending matrix. If the pending matrix is not invertible the
 * GrFPResult's bool will be false and the passed FP will be returned to the caller in the
 * GrFPResult.
 */
static GrFPResult apply_matrix(std::unique_ptr<GrFragmentProcessor> fp,
                               const SkShaders::MatrixRec& rec,
                               const SkMatrix& postInv) {
    auto [total, ok] = rec.applyForFragmentProcessor(postInv);
    if (!ok) {
        return {false, std::move(fp)};
    }
    // GrMatrixEffect returns 'fp' if total worked out to identity.
    return {true, GrMatrixEffect::Make(total, std::move(fp))};
}

// Combines the colorizer and layout with an appropriately configured top-level effect based on the
// gradient's tile mode
std::unique_ptr<GrFragmentProcessor> MakeGradientFP(const SkGradientBaseShader& shader,
                                                    const GrFPArgs& args,
                                                    const SkShaders::MatrixRec& mRec,
                                                    std::unique_ptr<GrFragmentProcessor> layout,
                                                    const SkMatrix* overrideMatrix) {
    // No shader is possible if a layout couldn't be created, e.g. a layout-specific Make() returned
    // null.
    if (layout == nullptr) {
        return nullptr;
    }

    // Some two-point conical gradients use a custom matrix here. Otherwise, use
    // SkGradientBaseShader's matrix;
    if (!overrideMatrix) {
        overrideMatrix = &shader.getGradientMatrix();
    }
    bool success;
    std::tie(success, layout) = apply_matrix(std::move(layout), mRec, *overrideMatrix);
    if (!success) {
        return nullptr;
    }

    // Convert all colors into destination space and into SkPMColor4fs, and handle
    // premul issues depending on the interpolation mode
    bool inputPremul = shader.interpolateInPremul();
    bool allOpaque = true;
    SkColor4fXformer xformedColors(&shader, args.fDstColorInfo->colorSpace());
    const SkPMColor4f* colors = xformedColors.fColors.begin();

    for (int i = 0; i < shader.fColorCount; i++) {
        if (allOpaque && !SkScalarNearlyEqual(colors[i].fA, 1.0)) {
            allOpaque = false;
        }
    }

    // SkGradientShader stores positions implicitly when they are evenly spaced, but the getPos()
    // implementation performs a branch for every position index. Since the shader conversion
    // requires lots of position tests, calculate all of the positions up front if needed.
    TArray<SkScalar, true> implicitPos;
    SkScalar* positions;
    if (shader.fPositions) {
        positions = shader.fPositions;
    } else {
        implicitPos.reserve_back(shader.fColorCount);
        SkScalar posScale = SK_Scalar1 / (shader.fColorCount - 1);
        for (int i = 0 ; i < shader.fColorCount; i++) {
            implicitPos.push_back(SkIntToScalar(i) * posScale);
        }
        positions = implicitPos.begin();
    }

    // All gradients are colorized the same way, regardless of layout
    std::unique_ptr<GrFragmentProcessor> colorizer = make_colorizer(
            colors, positions, shader.fColorCount, inputPremul, args);
    if (colorizer == nullptr) {
        return nullptr;
    }

    // All tile modes are supported (unless something was added to SkShader)
    std::unique_ptr<GrFragmentProcessor> gradient;
    switch(shader.getTileMode()) {
        case SkTileMode::kRepeat:
            gradient = make_tiled_gradient(args, std::move(colorizer), std::move(layout),
                                           /* mirror */ false, allOpaque);
            break;
        case SkTileMode::kMirror:
            gradient = make_tiled_gradient(args, std::move(colorizer), std::move(layout),
                                           /* mirror */ true, allOpaque);
            break;
        case SkTileMode::kClamp:
            // For the clamped mode, the border colors are the first and last colors, corresponding
            // to t=0 and t=1, because SkGradientBaseShader enforces that by adding color stops as
            // appropriate. If there is a hard stop, this grabs the expected outer colors for the
            // border.
            gradient = make_clamped_gradient(std::move(colorizer), std::move(layout),
                                             colors[0], colors[shader.fColorCount - 1],
                                             allOpaque);
            break;
        case SkTileMode::kDecal:
            // Even if the gradient colors are opaque, the decal borders are transparent so
            // disable that optimization
            gradient = make_clamped_gradient(std::move(colorizer), std::move(layout),
                                             SK_PMColor4fTRANSPARENT, SK_PMColor4fTRANSPARENT,
                                             /* colorsAreOpaque */ false);
            break;
    }

    // If interpolation space is different than destination, wrap the colorizer in a conversion.
    // This also handles any final premultiplication, etc.
    return make_interpolated_to_dst(std::move(gradient),
                                    shader.fInterpolation,
                                    xformedColors.fIntermediateColorSpace.get(),
                                    *args.fDstColorInfo,
                                    allOpaque);
}

std::unique_ptr<GrFragmentProcessor> MakeLinear(const SkLinearGradient& shader,
                                                const GrFPArgs& args,
                                                const SkShaders::MatrixRec& mRec) {
    // We add a tiny delta to t. When gradient stops are set up so that a hard stop in a vertically
    // or horizontally oriented gradient falls exactly at a column or row of pixel centers we can
    // get slightly different interpolated t values along the column/row. By adding the delta
    // we will consistently get the color to the "right" of the stop. Of course if the hard stop
    // falls at X.5 - delta then we still could get inconsistent results, but that is much less
    // likely. crbug.com/938592
    // If/when we add filtering of the gradient this can be removed.
    static const SkRuntimeEffect* effect = SkMakeRuntimeEffect(SkRuntimeEffect::MakeForShader,
        "half4 main(float2 coord) {"
            "return half4(half(coord.x) + 0.00001, 1, 0, 0);" // y = 1 for always valid
        "}"
    );
    // The linear gradient never rejects a pixel so it doesn't change opacity
    auto fp = GrSkSLFP::Make(effect, "LinearLayout", /*inputFP=*/nullptr,
                             GrSkSLFP::OptFlags::kPreservesOpaqueInput);
    return MakeGradientFP(shader, args, mRec, std::move(fp));
}

#if GR_TEST_UTILS
RandomParams::RandomParams(SkRandom* random) {
    // Set color count to min of 2 so that we don't trigger the const color optimization and make
    // a non-gradient processor.
    fColorCount = random->nextRangeU(2, kMaxRandomGradientColors);
    fUseColors4f = random->nextBool();

    // if one color, omit stops, otherwise randomly decide whether or not to
    if (fColorCount == 1 || (fColorCount >= 2 && random->nextBool())) {
        fStops = nullptr;
    } else {
        fStops = fStopStorage;
    }

    // if using SkColor4f, attach a random (possibly null) color space (with linear gamma)
    if (fUseColors4f) {
        fColorSpace = GrTest::TestColorSpace(random);
    }

    SkScalar stop = 0.f;
    for (int i = 0; i < fColorCount; ++i) {
        if (fUseColors4f) {
            fColors4f[i].fR = random->nextUScalar1();
            fColors4f[i].fG = random->nextUScalar1();
            fColors4f[i].fB = random->nextUScalar1();
            fColors4f[i].fA = random->nextUScalar1();
        } else {
            fColors[i] = random->nextU();
        }
        if (fStops) {
            fStops[i] = stop;
            stop = i < fColorCount - 1 ? stop + random->nextUScalar1() * (1.f - stop) : 1.f;
        }
    }
    fTileMode = static_cast<SkTileMode>(random->nextULessThan(kSkTileModeCount));
}
#endif

}  // namespace GrGradientShader