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 |
|---|---|
|
|
Nearest neighbor |
|
|
Bilinear |
|
|
Pixel area re-sampling |
|
|
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. The easiest
approach is to take the resized pixel's value from its closest pixel in the source image;
this is the effect of choosing the interpolation value
CV_INTER_NN. Alternatively, we can linearly weight the
2-by-2 surrounding source pixel values according to how close they are to the destination
pixel, which is what CV_INTER_LINEAR does. We can also
virtually place the new resized pixel over the old pixels and then average the covered pixel
values, as done with CV_INTER_AREA.[57] Finally, we have the option of fitting a cubic spline between the 4-by-4
surrounding pixels in the source image and then reading off the corresponding destination
value from the fitted spline; this is the result of choosing the CV_INTER_CUBIC interpolation method.
[57] At least that's what happens when cvResize()
shrinks an image. When it expands an image, CV_INTER_AREA amounts to the same thing as CV_INTER_NN.