Image Resampler

Overview

The Image Resampler algorithm is used to rescale the input image by means of resampling its content to make it conform to the output image dimensions.

No pre-filtering is applied, it's expected that the input content doesn't have frequencies higher than the Nyquist limit to avoid aliasing artifacts when downsampling.

Several interpolation methods are available, allowing trade-offs between quality and performance.

Input	Factor	Output
	\begin{align*} f_x &= 2/3 \\[5pt] f_y &= 3/2 \end{align*}

Implementation

For every output pixel, calculate the corresponding input pixel using the formula:

\[ \mathit{out}[x,y] = P(f_x x, f_y y) \]

where P depends on the interpolation method used.

Sampling operation considers that whole coordinates fall on pixel center.

Usage

1. Initialization phase
   1. Include the header that defines the needed functions and structures.

      #include <vpi/algo/ImageResampler.h>

   2. Define the stream on which the algorithm will be executed and the input image.

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

   3. Create an output image with the required size and matching image type.

          uint32_t w,h;
          vpiImageGetSize(input, &w, &h);
       
          VPIImageType type;
          vpiImageGetType(input, &type);
       
          VPIImage output;
          vpiImageCreate(w*2/3.0f, h*3/2.0f, type, 0, &output);

2. Processing phase
   1. Submit the algorithm to the stream along with all parameters.

          vpiSubmitImageResampler(stream, input, output, VPI_INTERP_LINEAR_FAST, VPI_BOUNDARY_COND_ZERO);

   2. Optionally, wait until the processing is done.

          vpiStreamSync(stream);

Consult the Image Resample for a complete example.

Limitations and Constraints

Constraints for specific backends supersede the ones specified for all backends.

All Backends

• Input and output images must have the same dimensions and type.
• The following interpolation methods are supported:
  • VPI_INTERP_NEAREST
  • VPI_INTERP_LINEAR
  • VPI_INTERP_LINEAR_FAST
  • VPI_INTERP_CATMULL_ROM
  • VPI_INTERP_CATMULL_ROM_FAST
• The following image types are accepted:
  • VPI_IMAGE_TYPE_Y8
  • VPI_IMAGE_TYPE_Y8I
  • VPI_IMAGE_TYPE_Y16
  • VPI_IMAGE_TYPE_Y16I
• The following boundary conditions are accepted.
  • VPI_BOUNDARY_COND_ZERO
  • VPI_BOUNDARY_COND_CLAMP
• Minimum convolution kernel size is 1x1, maximum is 11x11.

PVA

• Not implemented

References

• Daniel Ruijters, Bart M. ter Romeny, Paul Suetens (2008) Efficient GPU-Based Texture Interpolation using Uniform B-Spline
  Journal of Graphics Tools, 13:4 61-69.