/*

 * Copyright 2015 Google Inc.

 *

 * Use of this source code is governed by a BSD-style license that can be

 * found in the LICENSE file.

 */

#include "src/text/gpu/DistanceFieldAdjustTable.h"

#include "include/core/SkTypes.h"

#include "include/private/base/SkDebug.h"

#include "include/private/base/SkTemplates.h"

#include "src/base/SkNoDestructor.h"

#include "src/core/SkScalerContext.h"

#include <cstddef>

#include <cstdint>

using namespace skia_private;

namespace sktext::gpu {

SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;)

SkScalar* build_distance_adjust_table(SkScalar paintGamma, SkScalar deviceGamma) {

    // This is used for an approximation of the mask gamma hack, used by raster and bitmap

    // text. The mask gamma hack is based off of guessing what the blend color is going to

    // be, and adjusting the mask so that when run through the linear blend will

    // produce the value closest to the desired result. However, in practice this means

    // that the 'adjusted' mask is just increasing or decreasing the coverage of

    // the mask depending on what it is thought it will blit against. For black (on

    // assumed white) this means that coverages are decreased (on a curve). For white (on

    // assumed black) this means that coverages are increased (on a a curve). At

    // middle (perceptual) gray (which could be blit against anything) the coverages

    // remain the same.

    //

    // The idea here is that instead of determining the initial (real) coverage and

    // then adjusting that coverage, we determine an adjusted coverage directly by

    // essentially manipulating the geometry (in this case, the distance to the glyph

    // edge). So for black (on assumed white) this thins a bit; for white (on

    // assumed black) this fake bolds the geometry a bit.

    //

    // The distance adjustment is calculated by determining the actual coverage value which

    // when fed into in the mask gamma table gives us an 'adjusted coverage' value of 0.5. This

    // actual coverage value (assuming it's between 0 and 1) corresponds to a distance from the

    // actual edge. So by subtracting this distance adjustment and computing without the

    // the coverage adjustment we should get 0.5 coverage at the same point.

    //

    // This has several implications:

    //     For non-gray lcd smoothed text, each subpixel essentially is using a

    //     slightly different geometry.

    //

    //     For black (on assumed white) this may not cover some pixels which were

    //     previously covered; however those pixels would have been only slightly

    //     covered and that slight coverage would have been decreased anyway. Also, some pixels

    //     which were previously fully covered may no longer be fully covered.

    //

    //     For white (on assumed black) this may cover some pixels which weren't

    //     previously covered at all.

    int width, height;

    size_t size;

    SkScalar contrast = SK_GAMMA_CONTRAST;

    size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma,

        &width, &height);

    SkASSERT(kExpectedDistanceAdjustTableSize == height);

    SkScalar* table = new SkScalar[height];

    AutoTArray<uint8_t> data((int)size);

    if (!SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get())) {

        // if no valid data is available simply do no adjustment

        for (int row = 0; row < height; ++row) {

            table[row] = 0;

        }

        return table;

    }

    // find the inverse points where we cross 0.5

    // binsearch might be better, but we only need to do this once on creation

    for (int row = 0; row < height; ++row) {

        uint8_t* rowPtr = data.get() + row*width;

        for (int col = 0; col < width - 1; ++col) {

            if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) {

                // compute point where a mask value will give us a result of 0.5

                float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]);

                float borderAlpha = (col + interp) / 255.f;

                // compute t value for that alpha

                // this is an approximate inverse for smoothstep()

                float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f;

                // compute distance which gives us that t value

                const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor

                float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor;

                table[row] = d;

                break;

            }

        }

    }

    return table;

}

sktext::gpu::build_distance_adjust_table(float, float)

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SkScalar* build_distance_adjust_table(SkScalar paintGamma, SkScalar deviceGamma) {

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    // This is used for an approximation of the mask gamma hack, used by raster and bitmap

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    // text. The mask gamma hack is based off of guessing what the blend color is going to

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    // be, and adjusting the mask so that when run through the linear blend will

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    // produce the value closest to the desired result. However, in practice this means

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    // that the 'adjusted' mask is just increasing or decreasing the coverage of

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    // the mask depending on what it is thought it will blit against. For black (on

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    // assumed white) this means that coverages are decreased (on a curve). For white (on

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    // assumed black) this means that coverages are increased (on a a curve). At

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    // middle (perceptual) gray (which could be blit against anything) the coverages

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    // remain the same.

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    //

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    // The idea here is that instead of determining the initial (real) coverage and

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    // then adjusting that coverage, we determine an adjusted coverage directly by

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    // essentially manipulating the geometry (in this case, the distance to the glyph

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    // edge). So for black (on assumed white) this thins a bit; for white (on

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    // assumed black) this fake bolds the geometry a bit.

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    //

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    // The distance adjustment is calculated by determining the actual coverage value which

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    // when fed into in the mask gamma table gives us an 'adjusted coverage' value of 0.5. This

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    // actual coverage value (assuming it's between 0 and 1) corresponds to a distance from the

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    // actual edge. So by subtracting this distance adjustment and computing without the

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    // the coverage adjustment we should get 0.5 coverage at the same point.

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    //

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    // This has several implications:

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    //     For non-gray lcd smoothed text, each subpixel essentially is using a

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    //     slightly different geometry.

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    //

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    //     For black (on assumed white) this may not cover some pixels which were

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    //     previously covered; however those pixels would have been only slightly

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    //     covered and that slight coverage would have been decreased anyway. Also, some pixels

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    //     which were previously fully covered may no longer be fully covered.

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    //

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    //     For white (on assumed black) this may cover some pixels which weren't

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    //     previously covered at all.

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    int width, height;

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    size_t size;

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    SkScalar contrast = SK_GAMMA_CONTRAST;

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    size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma,

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        &width, &height);

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    SkASSERT(kExpectedDistanceAdjustTableSize == height);

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    SkScalar* table = new SkScalar[height];

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    AutoTArray<uint8_t> data((int)size);

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    if (!SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get())) {

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        // if no valid data is available simply do no adjustment

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        for (int row = 0; row < height; ++row) {

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            table[row] = 0;

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        }

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        return table;

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    }

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    // find the inverse points where we cross 0.5

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    // binsearch might be better, but we only need to do this once on creation

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    for (int row = 0; row < height; ++row) {

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        uint8_t* rowPtr = data.get() + row*width;

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        for (int col = 0; col < width - 1; ++col) {

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            if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) {

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                // compute point where a mask value will give us a result of 0.5

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                float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]);

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                float borderAlpha = (col + interp) / 255.f;

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                // compute t value for that alpha

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                // this is an approximate inverse for smoothstep()

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                float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f;

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                // compute distance which gives us that t value

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                const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor

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                float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor;

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                table[row] = d;

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                break;

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            }

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        }

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    }

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    return table;

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}

Unexecuted instantiation: sktext::gpu::build_distance_adjust_table(float, float)

const DistanceFieldAdjustTable* DistanceFieldAdjustTable::Get() {

    static const SkNoDestructor<DistanceFieldAdjustTable> dfat;

    return dfat.get();

}

DistanceFieldAdjustTable::DistanceFieldAdjustTable() {

    fTable = build_distance_adjust_table(SK_GAMMA_EXPONENT, SK_GAMMA_EXPONENT);

    fGammaCorrectTable = build_distance_adjust_table(SK_Scalar1, SK_Scalar1);

}

}  // namespace sktext::gpu