Production quality fast image up/downsampler

src/core/SkConvolver.cpp

Index: src/core/SkConvolver.cpp
diff --git a/src/core/SkConvolver.cpp b/src/core/SkConvolver.cpp
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+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "SkConvolver.h"
+#include "SkSize.h"
+#include "SkTypes.h"
+
+namespace {
+
+    // Converts the argument to an 8-bit unsigned value by clamping to the range
+    // 0-255.
+    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;
+    }
+
+    // Takes the value produced by accumulating element-wise product of image with
+    // a kernel and brings it back into range.
+    // All of the filter scaling factors are in fixed point with kShiftBits bits of
+    // fractional part.
+    inline unsigned char BringBackTo8(int a, bool takeAbsolute) {
+        a >>= SkConvolutionFilter1D::kShiftBits;
+        if (takeAbsolute)

[reed1, 2013/07/19 17:47:23]

nit: if-blocks should always have {}

+            a = abs(a);
+        return ClampTo8(a);
+    }
+
+    // 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 |source_row_pixel_width|.
+          // The maximum number of rows needed in the buffer is |max_y_filter_size|
+          // (we only need to store enough rows for the biggest filter).
+          //
+          // We use the |first_input_row| 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) {
+            fBuffer.reset(fRowByteWidth * maxYFilterSize);
+            fRowAddresses.reset(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 |*first_row_index| and will continue in
+        // order for the maximum number of rows in this circular buffer.
+        //
+        // The |first_row_index_| 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 cur_row = fNextRow;
+            for (int i = 0; i < fNumRows; i++) {
+                fRowAddresses[i] = &fBuffer[cur_row * fRowByteWidth];
+
+                // Advance to the next row, wrapping if necessary.
+                cur_row++;
+                if (cur_row == fNumRows)
+                    cur_row = 0;
+            }
+            return &fRowAddresses[0];
+        }
+
+    private:
+        // The buffer storing the rows. They are packed, each one fRowByteWidth.
+        SkTArray<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().
+        SkTArray<unsigned char*> fRowAddresses;
+    };
+
+// Convolves horizontally along a single row. The row data is given in
+// |src_data| and continues for the numValues() of the filter.
+template<bool has_alpha>
+    void ConvolveHorizontally(const unsigned char* src_data,
+        const SkConvolutionFilter1D& filter,
+    unsigned char* out_row) {
+  // Loop over each pixel on this row in the output image.
+        int numValues = filter.numValues();
+        for (int out_x = 0; out_x < numValues; out_x++) {
+    // Get the filter that determines the current output pixel.
+            int filterOffset, filterLength;
+            const SkConvolutionFilter1D::Fixed* filterValues =
+                filter.FilterForValue(out_x, &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* row_to_filter = &src_data[filterOffset * 4];
+
+    // Apply the filter to the row to get the destination pixel in |accum|.
+            int accum[4] = {0};
+            for (int filter_x = 0; filter_x < filterLength; filter_x++) {
+                SkConvolutionFilter1D::Fixed cur_filter = filterValues[filter_x];
+                accum[0] += cur_filter * row_to_filter[filter_x * 4 + 0];
+                accum[1] += cur_filter * row_to_filter[filter_x * 4 + 1];
+                accum[2] += cur_filter * row_to_filter[filter_x * 4 + 2];
+                if (has_alpha)
+                    accum[3] += cur_filter * row_to_filter[filter_x * 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 (has_alpha)
+                accum[3] >>= SkConvolutionFilter1D::kShiftBits;
+
+    // Store the new pixel.
+            out_row[out_x * 4 + 0] = ClampTo8(accum[0]);
+            out_row[out_x * 4 + 1] = ClampTo8(accum[1]);
+            out_row[out_x * 4 + 2] = ClampTo8(accum[2]);
+            if (has_alpha)
+                out_row[out_x * 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 |source_data_rows| array, with each row
+// being |pixel_width| wide.
+//
+// The output must have room for |pixel_width * 4| bytes.
+template<bool has_alpha>
+    void ConvolveVertically(const SkConvolutionFilter1D::Fixed* filterValues,
+        int filterLength,
+        unsigned char* const* source_data_rows,
+        int pixel_width,
+    unsigned char* out_row) {
+  // We go through each column in the output and do a vertical convolution,
+  // generating one output pixel each time.
+        for (int out_x = 0; out_x < pixel_width; out_x++) {
+    // 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 byte_offset = out_x * 4;
+
+    // Apply the filter to one column of pixels.
+            int accum[4] = {0};
+            for (int filter_y = 0; filter_y < filterLength; filter_y++) {
+                SkConvolutionFilter1D::Fixed cur_filter = filterValues[filter_y];
+                accum[0] += cur_filter * source_data_rows[filter_y][byte_offset + 0];
+                accum[1] += cur_filter * source_data_rows[filter_y][byte_offset + 1];
+                accum[2] += cur_filter * source_data_rows[filter_y][byte_offset + 2];
+                if (has_alpha)
+                    accum[3] += cur_filter * source_data_rows[filter_y][byte_offset + 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 (has_alpha)
+                accum[3] >>= SkConvolutionFilter1D::kShiftBits;
+
+    // Store the new pixel.
+            out_row[byte_offset + 0] = ClampTo8(accum[0]);
+            out_row[byte_offset + 1] = ClampTo8(accum[1]);
+            out_row[byte_offset + 2] = ClampTo8(accum[2]);
+            if (has_alpha) {
+                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 max_color_channel = SkTMax(out_row[byte_offset + 0],
+                    SkTMax(out_row[byte_offset + 1], out_row[byte_offset + 2]));
+                if (alpha < max_color_channel)
+                    out_row[byte_offset + 3] = max_color_channel;
+                else
+                    out_row[byte_offset + 3] = alpha;
+            } else {
+      // No alpha channel, the image is opaque.
+                out_row[byte_offset + 3] = 0xff;
+            }
+        }
+    }
+
+    void ConvolveVertically(const SkConvolutionFilter1D::Fixed* filterValues,
+        int filterLength,
+        unsigned char* const* source_data_rows,
+        int pixel_width,
+        unsigned char* out_row,
+    bool source_has_alpha) {
+        if (source_has_alpha) {
+            ConvolveVertically<true>(filterValues, filterLength,
+                source_data_rows,
+                pixel_width,
+                out_row);
+        } else {
+            ConvolveVertically<false>(filterValues, filterLength,
+                source_data_rows,
+                pixel_width,
+                out_row);
+        }
+    }
+
+}  // namespace
+
+// SkConvolutionFilter1D ---------------------------------------------------------
+
+SkConvolutionFilter1D::SkConvolutionFilter1D()
+: fMaxFilter(0) {
+}
+
+SkConvolutionFilter1D::~SkConvolutionFilter1D() {
+}
+
+void SkConvolutionFilter1D::AddFilter(int filterOffset,
+    const float* filterValues,
+int filterLength) {
+    SkASSERT(filterLength > 0);
+
+    SkTArray<Fixed> fixed_values;
+    fixed_values.reset(filterLength);
+
+    for (int i = 0; i < filterLength; ++i)
+        fixed_values.push_back(FloatToFixed(filterValues[i]));
+
+    AddFilter(filterOffset, &fixed_values[0], filterLength);
+}
+
+void SkConvolutionFilter1D::AddFilter(int filterOffset,
+                                      const Fixed* 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 filter_size = filterLength;
+    int first_non_zero = 0;
+    while (first_non_zero < filterLength && filterValues[first_non_zero] == 0)
+        first_non_zero++;
+
+    if (first_non_zero < filterLength) {
+        // Here we have at least one non-zero factor.
+        int last_non_zero = filterLength - 1;
+        while (last_non_zero >= 0 && filterValues[last_non_zero] == 0)
+            last_non_zero--;
+
+        filterOffset += first_non_zero;
+        filterLength = last_non_zero + 1 - first_non_zero;
+        SkASSERT(filterLength > 0);
+
+        for (int i = first_non_zero; i <= last_non_zero; i++)
+            fFilterValues.push_back(filterValues[i]);
+    } else {
+        // Here all the factors were zeroes.
+        filterLength = 0;
+    }
+
+    FilterInstance instance;
+
+    // We pushed filterLength elements onto fFilterValues
+    instance.fDataLocation = (static_cast<int>(fFilterValues.count()) -
+        filterLength);
+    instance.fOffset = filterOffset;
+    instance.fTrimmedLength = filterLength;
+    instance.fLength = filter_size;
+    fFilters.push_back(instance);
+
+    fMaxFilter = SkTMax(fMaxFilter, filterLength);
+}
+
+const SkConvolutionFilter1D::Fixed* SkConvolutionFilter1D::GetSingleFilter(
+                                        int* specified_filterLength,
+                                        int* filterOffset,
+                                        int* filterLength) const {
+    const FilterInstance& filter = fFilters[0];
+    *filterOffset = filter.fOffset;
+    *filterLength = filter.fTrimmedLength;
+    *specified_filterLength = filter.fLength;
+    if (filter.fTrimmedLength == 0) {
+        return NULL;
+    }
+
+    return &fFilterValues[filter.fDataLocation];
+}
+
+void BGRAConvolve2D(const unsigned char* sourceData,
+                    int sourceByteRowStride,
+                    bool sourceHasAlpha,
+                    const SkConvolutionFilter1D& filterX,
+                    const SkConvolutionFilter1D& filterY,
+                    int outputByteRowStride,
+                    unsigned char* output,
+                    SkConvolutionProcs *convolveProcs,
+                    bool useSimdIfPossible) {
+
+    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, filterLength;
+    const SkConvolutionFilter1D::Fixed* 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
+    // 16 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() + 15) & ~0xF;
+    int rowBufferHeight = maxYFilterSize +
+                          (convolveProcs->fConvolve4RowsHorizontally ? 4 : 0);
+    CircularRowBuffer rowBuffer(rowBufferWidth,
+                                rowBufferHeight,
+                                filterOffset);
+
+    // Loop over every possible output row, processing just enough horizontal
+    // convolutions to run each subsequent vertical convolution.
+    SkASSERT(outputByteRowStride >= filterX.numValues() * 4);
+    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;
+
+    // SSE2 can access up to 3 extra pixels past the end of the
+    // buffer. At the bottom of the image, we have to be careful
+    // not to access data past the end of the buffer. Normally
+    // we fall back to the C++ implementation for the last row.
+    // If the last row is less than 3 pixels wide, we may have to fall
+    // back to the C++ version for more rows. Compute how many
+    // rows we need to avoid the SSE implementation for here.
+    filterX.FilterForValue(filterX.numValues() - 1, &lastFilterOffset,
+        &lastFilterLength);
+    int avoidSimdRows = 1 + convolveProcs->fExtraHorizontalReads /
+        (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) {
+            if (convolveProcs->fConvolve4RowsHorizontally &&
+                nextXRow + 3 < lastFilterOffset + lastFilterLength -
+                avoidSimdRows) {
+                const unsigned char* src[4];
+                unsigned char* outRow[4];
+                for (int i = 0; i < 4; ++i) {
+                    src[i] = &sourceData[(nextXRow + i) * sourceByteRowStride];
+                    outRow[i] = rowBuffer.advanceRow();
+                }
+                convolveProcs->fConvolve4RowsHorizontally(src, filterX, outRow);
+                nextXRow += 4;
+            } else {
+                // Check if we need to avoid SSE2 for this row.
+                if (convolveProcs->fConvolveHorizontally &&
+                    nextXRow < lastFilterOffset + lastFilterLength -
+                avoidSimdRows) {
+                    convolveProcs->fConvolveHorizontally(
+                        &sourceData[nextXRow * sourceByteRowStride],
+                        filterX, rowBuffer.advanceRow(), sourceHasAlpha);
+                } else {
+                    if (sourceHasAlpha) {
+                        ConvolveHorizontally<true>(
+                            &sourceData[nextXRow * sourceByteRowStride],
+                            filterX, rowBuffer.advanceRow());
+                    } else {
+                        ConvolveHorizontally<false>(
+                            &sourceData[nextXRow * sourceByteRowStride],
+                            filterX, rowBuffer.advanceRow());
+                    }
+                }
+                nextXRow++;
+            }
+        }
+
+        // Compute where in the output image this row of final data will go.
+        unsigned char* curOutputRow = &output[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];
+
+        if (convolveProcs->fConvolveVertically) {
+            convolveProcs->fConvolveVertically(filterValues, filterLength,
+                firstRowForFilter,
+                filterX.numValues(), curOutputRow,
+                sourceHasAlpha);
+        } else {
+            ConvolveVertically(filterValues, filterLength,
+                firstRowForFilter,
+                filterX.numValues(), curOutputRow,
+                sourceHasAlpha);
+        }
+    }
+}