Line Buffer

The LineBuffer class implements the line buffer structure that is commonly seen in image convolution (filtering) operations, where a filter kernel is "slided" over an input image and is applied on a local window (e.g., a square) of pixels at every sliding location. As the filter is slided across the image, the line buffer is fed with a new pixel at every new sliding location while retaining the pixels of the previous image rows that can be covered for the sliding window. The public interface of the LineBuffer class is shown below,

template <typename PixelType, unsigned ImageWidth, unsigned WindowSize>
class LineBuffer {
  public:
    PixelType window[WindowSize][WindowSize];
    void ShiftInPixel(PixelType input_pixel);
};

Below shows an example usage of the LineBuffer class:

• Instantiate the line buffer in your C++ code, with template arguments being the pixel data type, input image width, and sliding window size. The window maintained by the line buffer assumes a square WindowSize x WindowSize window. If you are instantiating the line buffer inside a pipelined function (accepting a new pixel in every function call), you will need to add 'static' to make the line buffer static.

  static hls::LineBuffer<unsigned char, ImageWidth, WindowSize> line_buffer;

• Shift in a new pixel by calling the ShiftInPixel method:

  line_buffer.ShiftInPixel(input_pixel);

• Then your filter can access any pixels in the window by:

  line_buffer.window[i][j]

The figure below illustrates how the line buffer window is being updated after each call of ShiftInPixel. You will notice that the window can contain out-of-bound pixels at certain sliding locations.

For more details about when/why to use the LineBuffer class, see 3.5.2.7 Inferring a Line Buffer.