Reduce and Expand in Down/Up Sampling without Aliasing

Reduce Operator – DownSampling without aliasing:

Usually a 5  or 7 tap separable 1D filter like  a (1,4,6,4,1) or a (1,6,15,20,15,6,1) filter is used.

We need half the number of pixels:

An easy example , each odd pixel gets 5  separately filtered by (1,4,6,4,1) filter.

 

Reproducing / Expanding Operator – UpSampling without aliasing:

Reproducing / Expanding the Odd pixels  of the upper level:

Use a 3 tap filter – that is the even pixels (2,4)  get a value of zero when upscaled because they are not real, and they are not used (so we only multiply the scale (1,6,1) for the odd pixels.

Reproducing / Expanding the Even Pixels of the upper level: Use a 3 tap filter and use only the odd pixels to scale up by adding them and using half their summed value.