Learning OpenCV

Resize

We often encounter an image of some size that we would like to convert to an image of some other size. We may want to upsize (zoom in) or downsize (zoom out) the image; we can accomplish either task by using cvResize(). This function will fit the source image exactly to the destination image size. If the ROI is set in the source image then that ROI will be resized to fit in the destination image. Likewise, if an ROI is set in the destination image then the source will be resized to fit into the ROI.

void cvResize(
   const CvArr* src,
   CvArr*       dst,
   int          interpolation = CV_INTER_LINEAR
);

The last argument is the interpolation method, which defaults to linear interpolation. The other available options are shown in Table 5-4.

Table 5-4. cvResize() interpolation options

Interpolation	Meaning
CV_INTER_NN	Nearest neighbor
CV_INTER_LINEAR	Bilinear
CV_INTER_AREA	Pixel area re-sampling
CV_INTER_CUBIC	Bicubic interpolation

In general, we would like the mapping from the source image to the resized destination image to be as smooth as possible. The argument interpolation controls exactly how this will be handled. Interpolation arises when we are shrinking an image and a pixel in the destination image falls in between pixels in the source image. It can also occur when we are expanding an image and need to compute values of pixels that do not directly correspond to any pixel in the source image. In either case, there are several options for computing the values of such pixels. ...