// Copyright (c) 2006-2008 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 <algorithm>
#include "skia/ext/convolver.h"
#include "SkTypes.h"
namespace skia {
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;
}
// 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 dest_row_pixel_width, int max_y_filter_size,
int first_input_row)
: row_byte_width_(dest_row_pixel_width * 4),
num_rows_(max_y_filter_size),
next_row_(0),
next_row_coordinate_(first_input_row) {
buffer_.resize(row_byte_width_ * max_y_filter_size);
row_addresses_.resize(num_rows_);
}
// Moves to the next row in the buffer, returning a pointer to the beginning
// of it.
unsigned char* AdvanceRow() {
unsigned char* row = &buffer_[next_row_ * row_byte_width_];
next_row_coordinate_++;
// Set the pointer to the next row to use, wrapping around if necessary.
next_row_++;
if (next_row_ == num_rows_)
next_row_ = 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* first_row_index) {
// Example for a 4-element circular buffer holding coords 6-9.
// Row 0 Coord 8
// Row 1 Coord 9
// Row 2 Coord 6 <- next_row_ = 2, next_row_coordinate_ = 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 first_row_index and the negative rows will never be used.
*first_row_index = next_row_coordinate_ - num_rows_;
int cur_row = next_row_;
for (int i = 0; i < num_rows_; i++) {
row_addresses_[i] = &buffer_[cur_row * row_byte_width_];
// Advance to the next row, wrapping if necessary.
cur_row++;
if (cur_row == num_rows_)
cur_row = 0;
}
return &row_addresses_[0];
}
private:
// The buffer storing the rows. They are packed, each one row_byte_width_.
std::vector<unsigned char> buffer_;
// Number of bytes per row in the |buffer_|.
int row_byte_width_;
// The number of rows available in the buffer.
int num_rows_;
// The next row index we should write into. This wraps around as the
// circular buffer is used.
int next_row_;
// The y coordinate of the |next_row_|. This is incremented each time a
// new row is appended and does not wrap.
int next_row_coordinate_;
// Buffer used by GetRowAddresses().
std::vector<unsigned char*> row_addresses_;
};
// Convolves horizontally along a single row. The row data is given in
// |src_data| and continues for the num_values() of the filter.
template<bool has_alpha>
void ConvolveHorizontally(const unsigned char* src_data,
const ConvolusionFilter1D& filter,
unsigned char* out_row) {
// Loop over each pixel on this row in the output image.
int num_values = filter.num_values();
for (int out_x = 0; out_x < num_values; out_x++) {
// Get the filter that determines the current output pixel.
int filter_offset, filter_length;
const ConvolusionFilter1D::Fixed* filter_values =
filter.FilterForValue(out_x, &filter_offset, &filter_length);
// Compute the first pixel in this row that the filter affects. It will
// touch |filter_length| pixels (4 bytes each) after this.
const unsigned char* row_to_filter = &src_data[filter_offset * 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 < filter_length; filter_x++) {
ConvolusionFilter1D::Fixed cur_filter = filter_values[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] >>= ConvolusionFilter1D::kShiftBits;
accum[1] >>= ConvolusionFilter1D::kShiftBits;
accum[2] >>= ConvolusionFilter1D::kShiftBits;
if (has_alpha)
accum[3] >>= ConvolusionFilter1D::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 convolusion 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 ConvolusionFilter1D::Fixed* filter_values,
int filter_length,
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 convolusion,
// 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 < filter_length; filter_y++) {
ConvolusionFilter1D::Fixed cur_filter = filter_values[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] >>= ConvolusionFilter1D::kShiftBits;
accum[1] >>= ConvolusionFilter1D::kShiftBits;
accum[2] >>= ConvolusionFilter1D::kShiftBits;
if (has_alpha)
accum[3] >>= ConvolusionFilter1D::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 larger 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 = std::max(out_row[byte_offset + 0],
std::max(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;
}
}
}
} // namespace
// ConvolusionFilter1D ---------------------------------------------------------
void ConvolusionFilter1D::AddFilter(int filter_offset,
const float* filter_values,
int filter_length) {
FilterInstance instance;
instance.data_location = static_cast<int>(filter_values_.size());
instance.offset = filter_offset;
instance.length = filter_length;
filters_.push_back(instance);
SkASSERT(filter_length > 0);
for (int i = 0; i < filter_length; i++)
filter_values_.push_back(FloatToFixed(filter_values[i]));
max_filter_ = std::max(max_filter_, filter_length);
}
void ConvolusionFilter1D::AddFilter(int filter_offset,
const Fixed* filter_values,
int filter_length) {
FilterInstance instance;
instance.data_location = static_cast<int>(filter_values_.size());
instance.offset = filter_offset;
instance.length = filter_length;
filters_.push_back(instance);
SkASSERT(filter_length > 0);
for (int i = 0; i < filter_length; i++)
filter_values_.push_back(filter_values[i]);
max_filter_ = std::max(max_filter_, filter_length);
}
// BGRAConvolve2D -------------------------------------------------------------
void BGRAConvolve2D(const unsigned char* source_data,
int source_byte_row_stride,
bool source_has_alpha,
const ConvolusionFilter1D& filter_x,
const ConvolusionFilter1D& filter_y,
unsigned char* output) {
int max_y_filter_size = filter_y.max_filter();
// 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 convolusion as the first pixel for the first vertical filter.
int filter_offset, filter_length;
const ConvolusionFilter1D::Fixed* filter_values =
filter_y.FilterForValue(0, &filter_offset, &filter_length);
int next_x_row = filter_offset;
// We loop over each row in the input doing a horizontal convolusion. This
// will result in a horizontally convolved image. We write the results into
// a circular buffer of convolved rows and do vertical convolusion as rows
// are available. This prevents us from having to store the entire
// intermediate image and helps cache coherency.
CircularRowBuffer row_buffer(filter_x.num_values(), max_y_filter_size,
filter_offset);
// Loop over every possible output row, processing just enough horizontal
// convolusions to run each subsequent vertical convolusion.
int output_row_byte_width = filter_x.num_values() * 4;
int num_output_rows = filter_y.num_values();
for (int out_y = 0; out_y < num_output_rows; out_y++) {
filter_values = filter_y.FilterForValue(out_y,
&filter_offset, &filter_length);
// Generate output rows until we have enough to run the current filter.
while (next_x_row < filter_offset + filter_length) {
if (source_has_alpha) {
ConvolveHorizontally<true>(
&source_data[next_x_row * source_byte_row_stride],
filter_x, row_buffer.AdvanceRow());
} else {
ConvolveHorizontally<false>(
&source_data[next_x_row * source_byte_row_stride],
filter_x, row_buffer.AdvanceRow());
}
next_x_row++;
}
// Compute where in the output image this row of final data will go.
unsigned char* cur_output_row = &output[out_y * output_row_byte_width];
// Get the list of rows that the circular buffer has, in order.
int first_row_in_circular_buffer;
unsigned char* const* rows_to_convolve =
row_buffer.GetRowAddresses(&first_row_in_circular_buffer);
// Now compute the start of the subset of those rows that the filter
// needs.
unsigned char* const* first_row_for_filter =
&rows_to_convolve[filter_offset - first_row_in_circular_buffer];
if (source_has_alpha) {
ConvolveVertically<true>(filter_values, filter_length,
first_row_for_filter,
filter_x.num_values(), cur_output_row);
} else {
ConvolveVertically<false>(filter_values, filter_length,
first_row_for_filter,
filter_x.num_values(), cur_output_row);
}
}
}
} // namespace skia