src/core/SkLinearBitmapPipeline_core.h

/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef SkLinearBitmapPipeline_core_DEFINED
#define SkLinearBitmapPipeline_core_DEFINED

#include <algorithm>
#include <cmath>
#include "SkNx.h"

// New bilerp strategy:
// Pass through on bilerpList4 and bilerpListFew (analogs to pointList), introduce bilerpEdge
// which takes 4 points. If the sample spans an edge, then break it into a bilerpEdge. Bilerp
// span then becomes a normal span except in special cases where an extra Y is given. The bilerp
// need to stay single point calculations until the tile layer.
// TODO:
//  - edge span predicate.
//  - introduce new point API
//  - Add tile for new api.

namespace {
struct X {
    explicit X(SkScalar val) : fVal{val} { }
    explicit X(SkPoint pt)   : fVal{pt.fX} { }
    explicit X(SkSize s)     : fVal{s.fWidth} { }
    explicit X(SkISize s)    : fVal((SkScalar)s.fWidth) { }
    operator SkScalar () const {return fVal;}
private:
    SkScalar fVal;
};

struct Y {
    explicit Y(SkScalar val) : fVal{val} { }
    explicit Y(SkPoint pt)   : fVal{pt.fY} { }
    explicit Y(SkSize s)     : fVal{s.fHeight} { }
    explicit Y(SkISize s)    : fVal((SkScalar)s.fHeight) { }
    operator SkScalar () const {return fVal;}
private:
    SkScalar fVal;
};

// The Span class enables efficient processing horizontal spans of pixels.
// * start - the point where to start the span.
// * length - the number of pixels to traverse in source space.
// * count - the number of pixels to produce in destination space.
// Both start and length are mapped through the inversion matrix to produce values in source
// space. After the matrix operation, the tilers may break the spans up into smaller spans.
// The tilers can produce spans that seem nonsensical.
// * The clamp tiler can create spans with length of 0. This indicates to copy an edge pixel out
//   to the edge of the destination scan.
// * The mirror tiler can produce spans with negative length. This indicates that the source
//   should be traversed in the opposite direction to the destination pixels.
class Span {
public:
    Span(SkPoint start, SkScalar length, int count)
        : fStart(start)
        , fLength(length)
        , fCount{count} {
        SkASSERT(std::isfinite(length));
    }

    operator std::tuple<SkPoint&, SkScalar&, int&>() {
        return std::tie(fStart, fLength, fCount);
    }

    bool isEmpty() const { return 0 == fCount; }
    void clear() { fCount = 0; }
    int count() const { return fCount; }
    SkScalar length() const { return fLength; }
    SkScalar startX() const { return X(fStart); }
    SkScalar endX() const { return this->startX() + this->length(); }
    SkScalar startY() const { return Y(fStart); }
    Span emptySpan() { return Span{{0.0, 0.0}, 0.0f, 0}; }

    bool completelyWithin(SkScalar xMin, SkScalar xMax) const {
        SkScalar sMin, sMax;
        std::tie(sMin, sMax) = std::minmax(startX(), endX());
        return xMin <= sMin && sMax < xMax;
    }

    void offset(SkScalar offsetX) {
        fStart.offset(offsetX, 0.0f);
    }

    Span breakAt(SkScalar breakX, SkScalar dx) {
        SkASSERT(std::isfinite(breakX));
        SkASSERT(std::isfinite(dx));
        SkASSERT(dx != 0.0f);

        if (this->isEmpty()) {
            return this->emptySpan();
        }

        int dxSteps = SkScalarFloorToInt((breakX - this->startX()) / dx);

        if (dxSteps < 0) {
            // The span is wholly after breakX.
            return this->emptySpan();
        } else if (dxSteps >= fCount) {
            // The span is wholly before breakX.
            Span answer = *this;
            this->clear();
            return answer;
        }

        // Calculate the values for the span to cleave off.
        SkScalar newLength = dxSteps * dx;

        // If the last (or first if count = 1) sample lands directly on the boundary. Include it
        // when dx < 0 and exclude it when dx > 0.
        // Reasoning:
        //  dx > 0: The sample point on the boundary is part of the next span because the entire
        // pixel is after the boundary.
        //  dx < 0: The sample point on the boundary is part of the current span because the
        // entire pixel is before the boundary.
        if (this->startX() + newLength == breakX && dx > 0) {
            if (dxSteps > 0) {
                dxSteps -= 1;
                newLength -= dx;
            } else {
                return this->emptySpan();
            }
        }

        // Calculate new span parameters
        SkPoint newStart = fStart;
        int newCount = dxSteps + 1;
        SkASSERT(newCount > 0);

        // Update this span to reflect the break.
        SkScalar lengthToStart = newLength + dx;
        fLength -= lengthToStart;
        fCount -= newCount;
        fStart = {this->startX() + lengthToStart, Y(fStart)};

        return Span{newStart, newLength, newCount};
    }

    void clampToSinglePixel(SkPoint pixel) {
        fStart = pixel;
        fLength = 0.0f;
    }

private:
    SkPoint  fStart;
    SkScalar fLength;
    int      fCount;
};

template<typename Stage>
void span_fallback(Span span, Stage* stage) {
    SkPoint start;
    SkScalar length;
    int count;
    std::tie(start, length, count) = span;
    Sk4f startXs{X(start)};
    Sk4f ys{Y(start)};
    Sk4f mults = {0.0f, 1.0f, 2.0f, 3.0f};

    // Initializing this is not needed, but some compilers can't figure this out.
    Sk4s dXs{0.0f};
    if (count > 1) {
        SkScalar dx = length / (count - 1);
        dXs = Sk4f{dx};
    }

    // Instead of using xs = xs + dx every round, this uses xs = i * dx + X(start). This
    // eliminates the rounding error for the sum.
    Sk4f xs = startXs + mults * dXs;
    while (count >= 4) {
        stage->pointList4(xs, ys);

        mults += Sk4f{4.0f};
        xs = mults * dXs + startXs;
        count -= 4;
    }

    if (count > 0) {
        stage->pointListFew(count, xs, ys);
    }
}

inline Sk4f SK_VECTORCALL check_pixel(const Sk4f& pixel) {
    SkASSERTF(0.0f <= pixel[0] && pixel[0] <= 1.0f, "pixel[0]: %f", pixel[0]);
    SkASSERTF(0.0f <= pixel[1] && pixel[1] <= 1.0f, "pixel[1]: %f", pixel[1]);
    SkASSERTF(0.0f <= pixel[2] && pixel[2] <= 1.0f, "pixel[2]: %f", pixel[2]);
    SkASSERTF(0.0f <= pixel[3] && pixel[3] <= 1.0f, "pixel[3]: %f", pixel[3]);
    return pixel;
}

}  // namespace

class SkLinearBitmapPipeline::PointProcessorInterface {
public:
    virtual ~PointProcessorInterface() { }
    // Take the first n (where 0 < n && n < 4) items from xs and ys and sample those points. For
    // nearest neighbor, that means just taking the floor xs and ys. For bilerp, this means
    // to expand the bilerp filter around the point and sample using that filter.
    virtual void SK_VECTORCALL pointListFew(int n, Sk4s xs, Sk4s ys) = 0;
    // Same as pointListFew, but n = 4.
    virtual void SK_VECTORCALL pointList4(Sk4s xs, Sk4s ys) = 0;
    // A span is a compact form of sample points that are obtained by mapping points from
    // destination space to source space. This is used for horizontal lines only, and is mainly
    // used to take advantage of memory coherence for horizontal spans.
    virtual void pointSpan(Span span) = 0;
};

class SkLinearBitmapPipeline::SampleProcessorInterface
    : public SkLinearBitmapPipeline::PointProcessorInterface {
public:
    // Used for nearest neighbor when scale factor is 1.0. The span can just be repeated with no
    // edge pixel alignment problems. This is for handling a very common case.
    virtual void repeatSpan(Span span, int32_t repeatCount) = 0;
};

class SkLinearBitmapPipeline::DestinationInterface {
public:
    virtual ~DestinationInterface() { }
    // Count is normally not needed, but in these early stages of development it is useful to
    // check bounds.
    // TODO(herb): 4/6/2016 - remove count when code is stable.
    virtual void setDestination(void* dst, int count) = 0;
};

class SkLinearBitmapPipeline::BlendProcessorInterface
    : public SkLinearBitmapPipeline::DestinationInterface {
public:
    virtual void SK_VECTORCALL blendPixel(Sk4f pixel0) = 0;
    virtual void SK_VECTORCALL blend4Pixels(Sk4f p0, Sk4f p1, Sk4f p2, Sk4f p3) = 0;
};

class SkLinearBitmapPipeline::PixelAccessorInterface {
public:
    virtual ~PixelAccessorInterface() { }
    virtual void SK_VECTORCALL getFewPixels(
        int n, Sk4i xs, Sk4i ys, Sk4f* px0, Sk4f* px1, Sk4f* px2) const = 0;

    virtual void SK_VECTORCALL get4Pixels(
        Sk4i xs, Sk4i ys, Sk4f* px0, Sk4f* px1, Sk4f* px2, Sk4f* px3) const = 0;

    virtual void get4Pixels(
        const void* src, int index, Sk4f* px0, Sk4f* px1, Sk4f* px2, Sk4f* px3) const = 0;

    virtual Sk4f getPixelFromRow(const void* row, int index) const = 0;

    virtual Sk4f getPixelAt(int index) const = 0;

    virtual const void* row(int y) const = 0;
};

#endif // SkLinearBitmapPipeline_core_DEFINED