Pixel density revisited

Started Oct 22, 2008 | Discussions thread
DSPographer Senior Member • Posts: 2,464
Re: Lanczos resampling?

The Lanczos kernel can work well if it is organized for rescaling and not just resampling. That means that the window is applied to the data before resampling but the units of x in the function are in samples at the final spacing. So, if we want to change the image size by a factor of 1/2 with a Lanczos window with a=5 we would apply the following kernel centered at every other point in each of the vertical and horizontal directions:

0.0077 0 -0.0335 0 0.0811 0 -0.1822 0 0.6262 1 0.6262 0 -0.1822 0 0.0811 0 -0.0335 0 0.0077

If a program only uses the Lanczos method to resample the point locations without scaling the kernel to adjust the filter pass band size then it wouldn't avoid aliasing. Too low a value of a would also result in a less effective filter. These effects could easily be tested by making an image with a frequency sweep to Nyquest before re-sampling.

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