tor-browser

SkConvolver.cpp (26681B)

---

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
// Copyright (c) 2011-2016 Google Inc.
// Use of this source code is governed by a BSD-style license that can be
// found in the gfx/skia/LICENSE file.

#include "SkConvolver.h"

#ifdef USE_SSE2
#  include "mozilla/SSE.h"
#endif

#ifdef USE_NEON
#  include "mozilla/arm.h"
#endif

namespace skia {

using mozilla::gfx::BytesPerPixel;
using mozilla::gfx::IsOpaque;
using mozilla::gfx::SurfaceFormat;

// Converts the argument to an 8-bit unsigned value by clamping to the range
// 0-255.
static inline unsigned char ClampTo8(int a) {
 if (static_cast<unsigned>(a) < 256) {
   return a;  // Avoid the extra check in the common case.
 }
 if (a < 0) {
   return 0;
 }
 return 255;
}

// Convolves horizontally along a single row. The row data is given in
// |srcData| and continues for the numValues() of the filter.
template <bool hasAlpha>
void ConvolveHorizontally(const unsigned char* srcData,
                         const SkConvolutionFilter1D& filter,
                         unsigned char* outRow) {
 // Loop over each pixel on this row in the output image.
 int numValues = filter.numValues();
 for (int outX = 0; outX < numValues; outX++) {
   // Get the filter that determines the current output pixel.
   int filterOffset, filterLength;
   const SkConvolutionFilter1D::ConvolutionFixed* filterValues =
       filter.FilterForValue(outX, &filterOffset, &filterLength);

   // Compute the first pixel in this row that the filter affects. It will
   // touch |filterLength| pixels (4 bytes each) after this.
   const unsigned char* rowToFilter = &srcData[filterOffset * 4];

   // Apply the filter to the row to get the destination pixel in |accum|.
   int accum[4] = {0};
   for (int filterX = 0; filterX < filterLength; filterX++) {
     SkConvolutionFilter1D::ConvolutionFixed curFilter = filterValues[filterX];
     accum[0] += curFilter * rowToFilter[filterX * 4 + 0];
     accum[1] += curFilter * rowToFilter[filterX * 4 + 1];
     accum[2] += curFilter * rowToFilter[filterX * 4 + 2];
     if (hasAlpha) {
       accum[3] += curFilter * rowToFilter[filterX * 4 + 3];
     }
   }

   // Bring this value back in range. All of the filter scaling factors
   // are in fixed point with kShiftBits bits of fractional part.
   accum[0] >>= SkConvolutionFilter1D::kShiftBits;
   accum[1] >>= SkConvolutionFilter1D::kShiftBits;
   accum[2] >>= SkConvolutionFilter1D::kShiftBits;

   if (hasAlpha) {
     accum[3] >>= SkConvolutionFilter1D::kShiftBits;
   }

   // Store the new pixel.
   outRow[outX * 4 + 0] = ClampTo8(accum[0]);
   outRow[outX * 4 + 1] = ClampTo8(accum[1]);
   outRow[outX * 4 + 2] = ClampTo8(accum[2]);
   if (hasAlpha) {
     outRow[outX * 4 + 3] = ClampTo8(accum[3]);
   }
 }
}

// Does vertical convolution to produce one output row. The filter values and
// length are given in the first two parameters. These are applied to each
// of the rows pointed to in the |sourceDataRows| array, with each row
// being |pixelWidth| wide.
//
// The output must have room for |pixelWidth * 4| bytes.
template <bool hasAlpha>
void ConvolveVertically(
   const SkConvolutionFilter1D::ConvolutionFixed* filterValues,
   int filterLength, unsigned char* const* sourceDataRows, int pixelWidth,
   unsigned char* outRow) {
 // We go through each column in the output and do a vertical convolution,
 // generating one output pixel each time.
 for (int outX = 0; outX < pixelWidth; outX++) {
   // Compute the number of bytes over in each row that the current column
   // we're convolving starts at. The pixel will cover the next 4 bytes.
   int byteOffset = outX * 4;

   // Apply the filter to one column of pixels.
   int accum[4] = {0};
   for (int filterY = 0; filterY < filterLength; filterY++) {
     SkConvolutionFilter1D::ConvolutionFixed curFilter = filterValues[filterY];
     accum[0] += curFilter * sourceDataRows[filterY][byteOffset + 0];
     accum[1] += curFilter * sourceDataRows[filterY][byteOffset + 1];
     accum[2] += curFilter * sourceDataRows[filterY][byteOffset + 2];
     if (hasAlpha) {
       accum[3] += curFilter * sourceDataRows[filterY][byteOffset + 3];
     }
   }

   // Bring this value back in range. All of the filter scaling factors
   // are in fixed point with kShiftBits bits of precision.
   accum[0] >>= SkConvolutionFilter1D::kShiftBits;
   accum[1] >>= SkConvolutionFilter1D::kShiftBits;
   accum[2] >>= SkConvolutionFilter1D::kShiftBits;
   if (hasAlpha) {
     accum[3] >>= SkConvolutionFilter1D::kShiftBits;
   }

   // Store the new pixel.
   outRow[byteOffset + 0] = ClampTo8(accum[0]);
   outRow[byteOffset + 1] = ClampTo8(accum[1]);
   outRow[byteOffset + 2] = ClampTo8(accum[2]);

   if (hasAlpha) {
     unsigned char alpha = ClampTo8(accum[3]);

     // Make sure the alpha channel doesn't come out smaller than any of the
     // color channels. We use premultipled alpha channels, so this should
     // never happen, but rounding errors will cause this from time to time.
     // These "impossible" colors will cause overflows (and hence random pixel
     // values) when the resulting bitmap is drawn to the screen.
     //
     // We only need to do this when generating the final output row (here).
     int maxColorChannel =
         std::max(outRow[byteOffset + 0],
                  std::max(outRow[byteOffset + 1], outRow[byteOffset + 2]));
     if (alpha < maxColorChannel) {
       outRow[byteOffset + 3] = maxColorChannel;
     } else {
       outRow[byteOffset + 3] = alpha;
     }
   } else {
     // No alpha channel, the image is opaque.
     outRow[byteOffset + 3] = 0xff;
   }
 }
}

// Convolves horizontally along a single row. The row data is given in
// |srcData| and continues for the numValues() of the filter.
void ConvolveHorizontallyA8(const unsigned char* srcData,
                           const SkConvolutionFilter1D& filter,
                           unsigned char* outRow) {
 // Loop over each pixel on this row in the output image.
 int numValues = filter.numValues();
 for (int outX = 0; outX < numValues; outX++) {
   // Get the filter that determines the current output pixel.
   int filterOffset, filterLength;
   const SkConvolutionFilter1D::ConvolutionFixed* filterValues =
       filter.FilterForValue(outX, &filterOffset, &filterLength);

   // Compute the first pixel in this row that the filter affects. It will
   // touch |filterLength| pixels (4 bytes each) after this.
   const unsigned char* rowToFilter = &srcData[filterOffset];

   // Apply the filter to the row to get the destination pixel in |accum|.
   int accum = 0;
   for (int filterX = 0; filterX < filterLength; filterX++) {
     SkConvolutionFilter1D::ConvolutionFixed curFilter = filterValues[filterX];
     accum += curFilter * rowToFilter[filterX];
   }

   // Bring this value back in range. All of the filter scaling factors
   // are in fixed point with kShiftBits bits of fractional part.
   accum >>= SkConvolutionFilter1D::kShiftBits;

   // Store the new pixel.
   outRow[outX] = ClampTo8(accum);
 }
}

// Does vertical convolution to produce one output row. The filter values and
// length are given in the first two parameters. These are applied to each
// of the rows pointed to in the |sourceDataRows| array, with each row
// being |pixelWidth| wide.
//
// The output must have room for |pixelWidth| bytes.
void ConvolveVerticallyA8(
   const SkConvolutionFilter1D::ConvolutionFixed* filterValues,
   int filterLength, unsigned char* const* sourceDataRows, int pixelWidth,
   unsigned char* outRow) {
 // We go through each column in the output and do a vertical convolution,
 // generating one output pixel each time.
 for (int outX = 0; outX < pixelWidth; outX++) {
   // Apply the filter to one column of pixels.
   int accum = 0;
   for (int filterY = 0; filterY < filterLength; filterY++) {
     SkConvolutionFilter1D::ConvolutionFixed curFilter = filterValues[filterY];
     accum += curFilter * sourceDataRows[filterY][outX];
   }

   // Bring this value back in range. All of the filter scaling factors
   // are in fixed point with kShiftBits bits of precision.
   accum >>= SkConvolutionFilter1D::kShiftBits;

   // Store the new pixel.
   outRow[outX] = ClampTo8(accum);
 }
}

#ifdef USE_SSE2
void convolve_vertically_avx2(const int16_t* filter, int filterLen,
                             uint8_t* const* srcRows, int width, uint8_t* out,
                             bool hasAlpha);
void convolve_horizontally_sse2(const unsigned char* srcData,
                               const SkConvolutionFilter1D& filter,
                               unsigned char* outRow, bool hasAlpha);
void convolve_vertically_sse2(const int16_t* filter, int filterLen,
                             uint8_t* const* srcRows, int width, uint8_t* out,
                             bool hasAlpha);
#elif defined(USE_NEON)
void convolve_horizontally_neon(const unsigned char* srcData,
                               const SkConvolutionFilter1D& filter,
                               unsigned char* outRow, bool hasAlpha);
void convolve_vertically_neon(const int16_t* filter, int filterLen,
                             uint8_t* const* srcRows, int width, uint8_t* out,
                             bool hasAlpha);
#endif

void convolve_horizontally(const unsigned char* srcData,
                          const SkConvolutionFilter1D& filter,
                          unsigned char* outRow, SurfaceFormat format) {
 if (format == SurfaceFormat::A8) {
   ConvolveHorizontallyA8(srcData, filter, outRow);
   return;
 }

 bool hasAlpha = !IsOpaque(format);
#ifdef USE_SSE2
 if (mozilla::supports_sse2()) {
   convolve_horizontally_sse2(srcData, filter, outRow, hasAlpha);
   return;
 }
#elif defined(USE_NEON)
 if (mozilla::supports_neon()) {
   convolve_horizontally_neon(srcData, filter, outRow, hasAlpha);
   return;
 }
#endif
 if (hasAlpha) {
   ConvolveHorizontally<true>(srcData, filter, outRow);
 } else {
   ConvolveHorizontally<false>(srcData, filter, outRow);
 }
}

void convolve_vertically(
   const SkConvolutionFilter1D::ConvolutionFixed* filterValues,
   int filterLength, unsigned char* const* sourceDataRows, int pixelWidth,
   unsigned char* outRow, SurfaceFormat format) {
 if (format == SurfaceFormat::A8) {
   ConvolveVerticallyA8(filterValues, filterLength, sourceDataRows, pixelWidth,
                        outRow);
   return;
 }

 bool hasAlpha = !IsOpaque(format);
#ifdef USE_SSE2
 if (mozilla::supports_avx2()) {
   convolve_vertically_avx2(filterValues, filterLength, sourceDataRows,
                            pixelWidth, outRow, hasAlpha);
   return;
 }
 if (mozilla::supports_sse2()) {
   convolve_vertically_sse2(filterValues, filterLength, sourceDataRows,
                            pixelWidth, outRow, hasAlpha);
   return;
 }
#elif defined(USE_NEON)
 if (mozilla::supports_neon()) {
   convolve_vertically_neon(filterValues, filterLength, sourceDataRows,
                            pixelWidth, outRow, hasAlpha);
   return;
 }
#endif
 if (hasAlpha) {
   ConvolveVertically<true>(filterValues, filterLength, sourceDataRows,
                            pixelWidth, outRow);
 } else {
   ConvolveVertically<false>(filterValues, filterLength, sourceDataRows,
                             pixelWidth, outRow);
 }
}

// Stores a list of rows in a circular buffer. The usage is you write into it
// by calling AdvanceRow. It will keep track of which row in the buffer it
// should use next, and the total number of rows added.
class CircularRowBuffer {
public:
 // The number of pixels in each row is given in |sourceRowPixelWidth|.
 // The maximum number of rows needed in the buffer is |maxYFilterSize|
 // (we only need to store enough rows for the biggest filter).
 //
 // We use the |firstInputRow| to compute the coordinates of all of the
 // following rows returned by Advance().
 CircularRowBuffer(int destRowPixelWidth, int maxYFilterSize,
                   int firstInputRow)
     : fRowByteWidth(destRowPixelWidth * 4),
       fNumRows(maxYFilterSize),
       fNextRow(0),
       fNextRowCoordinate(firstInputRow) {}

 bool AllocBuffer() {
   return fBuffer.resize(fRowByteWidth * fNumRows) &&
          fRowAddresses.resize(fNumRows);
 }

 // Moves to the next row in the buffer, returning a pointer to the beginning
 // of it.
 unsigned char* advanceRow() {
   unsigned char* row = &fBuffer[fNextRow * fRowByteWidth];
   fNextRowCoordinate++;

   // Set the pointer to the next row to use, wrapping around if necessary.
   fNextRow++;
   if (fNextRow == fNumRows) {
     fNextRow = 0;
   }
   return row;
 }

 // Returns a pointer to an "unrolled" array of rows. These rows will start
 // at the y coordinate placed into |*firstRowIndex| and will continue in
 // order for the maximum number of rows in this circular buffer.
 //
 // The |firstRowIndex_| may be negative. This means the circular buffer
 // starts before the top of the image (it hasn't been filled yet).
 unsigned char* const* GetRowAddresses(int* firstRowIndex) {
   // Example for a 4-element circular buffer holding coords 6-9.
   //   Row 0   Coord 8
   //   Row 1   Coord 9
   //   Row 2   Coord 6  <- fNextRow = 2, fNextRowCoordinate = 10.
   //   Row 3   Coord 7
   //
   // The "next" row is also the first (lowest) coordinate. This computation
   // may yield a negative value, but that's OK, the math will work out
   // since the user of this buffer will compute the offset relative
   // to the firstRowIndex and the negative rows will never be used.
   *firstRowIndex = fNextRowCoordinate - fNumRows;

   int curRow = fNextRow;
   for (int i = 0; i < fNumRows; i++) {
     fRowAddresses[i] = &fBuffer[curRow * fRowByteWidth];

     // Advance to the next row, wrapping if necessary.
     curRow++;
     if (curRow == fNumRows) {
       curRow = 0;
     }
   }
   return &fRowAddresses[0];
 }

private:
 // The buffer storing the rows. They are packed, each one fRowByteWidth.
 mozilla::Vector<unsigned char> fBuffer;

 // Number of bytes per row in the |buffer|.
 int fRowByteWidth;

 // The number of rows available in the buffer.
 int fNumRows;

 // The next row index we should write into. This wraps around as the
 // circular buffer is used.
 int fNextRow;

 // The y coordinate of the |fNextRow|. This is incremented each time a
 // new row is appended and does not wrap.
 int fNextRowCoordinate;

 // Buffer used by GetRowAddresses().
 mozilla::Vector<unsigned char*> fRowAddresses;
};

SkConvolutionFilter1D::SkConvolutionFilter1D() : fMaxFilter(0) {}

SkConvolutionFilter1D::~SkConvolutionFilter1D() = default;

bool SkConvolutionFilter1D::AddFilter(int filterOffset,
                                     const ConvolutionFixed* filterValues,
                                     int filterLength) {
 // It is common for leading/trailing filter values to be zeros. In such
 // cases it is beneficial to only store the central factors.
 // For a scaling to 1/4th in each dimension using a Lanczos-2 filter on
 // a 1080p image this optimization gives a ~10% speed improvement.
 int filterSize = filterLength;
 int firstNonZero = 0;
 while (firstNonZero < filterLength && filterValues[firstNonZero] == 0) {
   firstNonZero++;
 }

 if (firstNonZero < filterLength) {
   // Here we have at least one non-zero factor.
   int lastNonZero = filterLength - 1;
   while (lastNonZero >= 0 && filterValues[lastNonZero] == 0) {
     lastNonZero--;
   }

   filterOffset += firstNonZero;
   filterLength = lastNonZero + 1 - firstNonZero;
   MOZ_ASSERT(filterLength > 0);

   if (!fFilterValues.append(&filterValues[firstNonZero], filterLength)) {
     return false;
   }
 } else {
   // Here all the factors were zeroes.
   filterLength = 0;
 }

 FilterInstance instance = {
     // We pushed filterLength elements onto fFilterValues
     int(fFilterValues.length()) - filterLength, filterOffset, filterLength,
     filterSize};
 if (!fFilters.append(instance)) {
   if (filterLength > 0) {
     fFilterValues.shrinkBy(filterLength);
   }
   return false;
 }

 fMaxFilter = std::max(fMaxFilter, filterLength);
 return true;
}

bool SkConvolutionFilter1D::ComputeFilterValues(
   const SkBitmapFilter& aBitmapFilter, int32_t aSrcSize, int32_t aDstSize) {
 // When we're doing a magnification, the scale will be larger than one. This
 // means the destination pixels are much smaller than the source pixels, and
 // that the range covered by the filter won't necessarily cover any source
 // pixel boundaries. Therefore, we use these clamped values (max of 1) for
 // some computations.
 float scale = float(aDstSize) / float(aSrcSize);
 float clampedScale = std::min(1.0f, scale);
 // This is how many source pixels from the center we need to count
 // to support the filtering function.
 float srcSupport = aBitmapFilter.width() / clampedScale;
 float invScale = 1.0f / scale;

 mozilla::Vector<float, 64> filterValues;
 mozilla::Vector<ConvolutionFixed, 64> fixedFilterValues;

 // Loop over all pixels in the output range. We will generate one set of
 // filter values for each one. Those values will tell us how to blend the
 // source pixels to compute the destination pixel.

 // This value is computed based on how SkTDArray::resizeStorageToAtLeast works
 // in order to ensure that it does not overflow or assert. That functions
 // computes
 //   n+4 + (n+4)/4
 // and we want to to fit in a 32 bit signed int. Equating that to 2^31-1 and
 // solving n gives n = (2^31-6)*4/5 = 1717986913.6
 const int32_t maxToPassToReserveAdditional = 1717986913;

 int32_t filterValueCount = int32_t(ceilf(aDstSize * srcSupport * 2));
 if (aDstSize > maxToPassToReserveAdditional || filterValueCount < 0 ||
     filterValueCount > maxToPassToReserveAdditional ||
     !reserveAdditional(aDstSize, filterValueCount)) {
   return false;
 }
 size_t oldFiltersLength = fFilters.length();
 size_t oldFilterValuesLength = fFilterValues.length();
 int oldMaxFilter = fMaxFilter;
 for (int32_t destI = 0; destI < aDstSize; destI++) {
   // This is the pixel in the source directly under the pixel in the dest.
   // Note that we base computations on the "center" of the pixels. To see
   // why, observe that the destination pixel at coordinates (0, 0) in a 5.0x
   // downscale should "cover" the pixels around the pixel with *its center*
   // at coordinates (2.5, 2.5) in the source, not those around (0, 0).
   // Hence we need to scale coordinates (0.5, 0.5), not (0, 0).
   float srcPixel = (static_cast<float>(destI) + 0.5f) * invScale;

   // Compute the (inclusive) range of source pixels the filter covers.
   float srcBegin = std::max(0.0f, floorf(srcPixel - srcSupport));
   float srcEnd = std::min(aSrcSize - 1.0f, ceilf(srcPixel + srcSupport));

   // Compute the unnormalized filter value at each location of the source
   // it covers.

   // Sum of the filter values for normalizing.
   // Distance from the center of the filter, this is the filter coordinate
   // in source space. We also need to consider the center of the pixel
   // when comparing distance against 'srcPixel'. In the 5x downscale
   // example used above the distance from the center of the filter to
   // the pixel with coordinates (2, 2) should be 0, because its center
   // is at (2.5, 2.5).
   int32_t filterCount = int32_t(srcEnd - srcBegin) + 1;
   if (filterCount <= 0 || !filterValues.resize(filterCount) ||
       !fixedFilterValues.resize(filterCount)) {
     return false;
   }

   float destFilterDist = (srcBegin + 0.5f - srcPixel) * clampedScale;
   float filterSum = 0.0f;
   for (int32_t index = 0; index < filterCount; index++) {
     float filterValue = aBitmapFilter.evaluate(destFilterDist);
     filterValues[index] = filterValue;
     filterSum += filterValue;
     destFilterDist += clampedScale;
   }

   // The filter must be normalized so that we don't affect the brightness of
   // the image. Convert to normalized fixed point.
   ConvolutionFixed fixedSum = 0;
   float invFilterSum = 1.0f / filterSum;
   for (int32_t fixedI = 0; fixedI < filterCount; fixedI++) {
     ConvolutionFixed curFixed = ToFixed(filterValues[fixedI] * invFilterSum);
     fixedSum += curFixed;
     fixedFilterValues[fixedI] = curFixed;
   }

   // The conversion to fixed point will leave some rounding errors, which
   // we add back in to avoid affecting the brightness of the image. We
   // arbitrarily add this to the center of the filter array (this won't always
   // be the center of the filter function since it could get clipped on the
   // edges, but it doesn't matter enough to worry about that case).
   ConvolutionFixed leftovers = ToFixed(1) - fixedSum;
   fixedFilterValues[filterCount / 2] += leftovers;

   if (!AddFilter(int32_t(srcBegin), fixedFilterValues.begin(), filterCount)) {
     fFilters.shrinkTo(oldFiltersLength);
     fFilterValues.shrinkTo(oldFilterValuesLength);
     fMaxFilter = oldMaxFilter;
     return false;
   }
 }

 return maxFilter() > 0 && numValues() == aDstSize;
}

// Does a two-dimensional convolution on the given source image.
//
// It is assumed the source pixel offsets referenced in the input filters
// reference only valid pixels, so the source image size is not required. Each
// row of the source image starts |sourceByteRowStride| after the previous
// one (this allows you to have rows with some padding at the end).
//
// The result will be put into the given output buffer. The destination image
// size will be xfilter.numValues() * yfilter.numValues() pixels. It will be
// in rows of exactly xfilter.numValues() * 4 bytes.
//
// |sourceHasAlpha| is a hint that allows us to avoid doing computations on
// the alpha channel if the image is opaque. If you don't know, set this to
// true and it will work properly, but setting this to false will be a few
// percent faster if you know the image is opaque.
//
// The layout in memory is assumed to be 4-bytes per pixel in B-G-R-A order
// (this is ARGB when loaded into 32-bit words on a little-endian machine).
/**
*  Returns false if it was unable to perform the convolution/rescale. in which
* case the output buffer is assumed to be undefined.
*/
bool BGRAConvolve2D(const unsigned char* sourceData, int sourceByteRowStride,
                   SurfaceFormat format, const SkConvolutionFilter1D& filterX,
                   const SkConvolutionFilter1D& filterY,
                   int outputByteRowStride, unsigned char* output) {
 int maxYFilterSize = filterY.maxFilter();

 // The next row in the input that we will generate a horizontally
 // convolved row for. If the filter doesn't start at the beginning of the
 // image (this is the case when we are only resizing a subset), then we
 // don't want to generate any output rows before that. Compute the starting
 // row for convolution as the first pixel for the first vertical filter.
 int filterOffset = 0, filterLength = 0;
 const SkConvolutionFilter1D::ConvolutionFixed* filterValues =
     filterY.FilterForValue(0, &filterOffset, &filterLength);
 int nextXRow = filterOffset;

 // We loop over each row in the input doing a horizontal convolution. This
 // will result in a horizontally convolved image. We write the results into
 // a circular buffer of convolved rows and do vertical convolution as rows
 // are available. This prevents us from having to store the entire
 // intermediate image and helps cache coherency.
 // We will need four extra rows to allow horizontal convolution could be done
 // simultaneously. We also pad each row in row buffer to be aligned-up to
 // 32 bytes.
 // TODO(jiesun): We do not use aligned load from row buffer in vertical
 // convolution pass yet. Somehow Windows does not like it.
 int rowBufferWidth = (filterX.numValues() + 31) & ~0x1F;
 int rowBufferHeight = maxYFilterSize;

 // check for too-big allocation requests : crbug.com/528628
 {
   int64_t size = int64_t(rowBufferWidth) * int64_t(rowBufferHeight);
   // need some limit, to avoid over-committing success from malloc, but then
   // crashing when we try to actually use the memory.
   // 100meg seems big enough to allow "normal" zoom factors and image sizes
   // through while avoiding the crash seen by the bug (crbug.com/528628)
   if (size > 100 * 1024 * 1024) {
     //            printf_stderr("BGRAConvolve2D: tmp allocation [%lld] too
     //            big\n", size);
     return false;
   }
 }

 CircularRowBuffer rowBuffer(rowBufferWidth, rowBufferHeight, filterOffset);
 if (!rowBuffer.AllocBuffer()) {
   return false;
 }

 // Loop over every possible output row, processing just enough horizontal
 // convolutions to run each subsequent vertical convolution.
 MOZ_ASSERT(outputByteRowStride >=
            filterX.numValues() * BytesPerPixel(format));
 int numOutputRows = filterY.numValues();

 // We need to check which is the last line to convolve before we advance 4
 // lines in one iteration.
 int lastFilterOffset, lastFilterLength;
 filterY.FilterForValue(numOutputRows - 1, &lastFilterOffset,
                        &lastFilterLength);

 for (int outY = 0; outY < numOutputRows; outY++) {
   filterValues = filterY.FilterForValue(outY, &filterOffset, &filterLength);

   // Generate output rows until we have enough to run the current filter.
   while (nextXRow < filterOffset + filterLength) {
     convolve_horizontally(
         &sourceData[(uint64_t)nextXRow * sourceByteRowStride], filterX,
         rowBuffer.advanceRow(), format);
     nextXRow++;
   }

   // Compute where in the output image this row of final data will go.
   unsigned char* curOutputRow = &output[(uint64_t)outY * outputByteRowStride];

   // Get the list of rows that the circular buffer has, in order.
   int firstRowInCircularBuffer;
   unsigned char* const* rowsToConvolve =
       rowBuffer.GetRowAddresses(&firstRowInCircularBuffer);

   // Now compute the start of the subset of those rows that the filter needs.
   unsigned char* const* firstRowForFilter =
       &rowsToConvolve[filterOffset - firstRowInCircularBuffer];

   convolve_vertically(filterValues, filterLength, firstRowForFilter,
                       filterX.numValues(), curOutputRow, format);
 }
 return true;
}

}  // namespace skia