Resampling
This means changing the image resolution or the number of pixels. The idea is to preserve the original image in the best possible way. The techniques involved are split depending on whether going up or down.
Down means taking a big image and making it smaller. Your going to lose information and what we would like to do is lose the least amount. Although all paint packages have the ability to do it a few key presses and one pass they all seem to be universally useless at it. If your going from 1000 to 800 then the best way is to upsample from 1000 to a large multiple of 800 like 3200, or 4000 and then blur and downsample to a 1/4 or 1/5. If changing in much larger ratios like 1000 to 173 or something like that then find a nice multiple of 173 larger than 1000 - i.e. 1730 and upsample to that first then blur 10 pixels into one and then down sample by 1/10th. Many people say - oh always use Bicubic or always use Lanczos but I disagree and I will show the results here with bicubic from 1000 down to 173. Maybe you disagree. Mathematically a sinc resample should be the best possible but I dont think the paint apps implement the full sinc and the windowing makes the resample less effective as more scaling is applied.
Up means taking a small image and making it bigger and here the paint apps seem to do it as well as can be expected so just picking the right algorithm is all thats needed. Again here, people often say that Bicubic or Lanczos is the best and for most images I would agree but there are exceptions. For general work including maps and photos I think its true and below is a sample sheet.
Where the situation changes is with noise and ringing and with small scales. If you are resampling from 997 to 1000 or something very very close to what you want then I would use a pixel/point/nearest neighbor based resample because for about 99% of that image the pixels will not change. Below is a set of lines resampled up very slightly.
You will have to save and zoom up this last image to see whats happening. My monitor makes all three pretty much gray.
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