Box Image Filter

Overview

Box Image Filter is a low-pass filter that smooths the image by making each output pixel the average of the surrounding ones, removing details, noise and and edges from images.

Input	Window size	Output
	5x5

Implementation

Box image filter is implemented as a convolution operation on the input image using the following kernel:

\[ box_{m,n} = \frac{1}{mn} \begin{bmatrix} 1 & 1 & \dots & 1 \\ 1 & 1 & \dots & 1 \\ \vdots & \vdots & \ddots & \vdots \\ 1 & 1 & \dots & 1 \end{bmatrix}_{m \times n} \]

Usage

1. Initialization phase
   1. Include the header that defines the box filter function.

      #include <vpi/algo/BoxImageFilter.h>

   2. Define the stream on which the algorithm will be executed, the input and output images.

          VPIStream stream = /*...*/;
          VPIImage input = /*...*/;

   3. Create the output image.

          uint32_t w,h;
          vpiImageGetSize(input, &w, &h);
       
          VPIImageType type;
          vpiImageGetType(input, &type);
       
          VPIImage output;
          vpiImageCreate(w, h, type, 0, &output);

2. Processing phase
   1. Submit the algorithm to the stream, input, output images, window size and boundary condition.

          vpiSubmitBoxImageFilter(stream, input, output, 5, 5, VPI_BOUNDARY_COND_ZERO);

   2. Optionally, wait until the processing is done.

          vpiStreamSync(stream);

Limitations and Constraints

Box image filter is currently implemented using separable image convolver if it satisfies \(max(m,n) \geq 7\) and the input characteristics (size and/or type) satisfies its constraints.

If \(max(m,n) < 7\) it'll use image convolver if input characteristics satisfies its constraints, or else the operation will fail.

Note

Currently, image convolver has some limitations on its PVA backend that makes some box filter operations fail. This is the case when the box filter has \(mn > 49\) on image types VPI_IMAGE_TYPE_Y8 and VPI_IMAGE_TYPE_Y8I.