Digital BW, The Print

Paul,

I've always thought 'stair interpolation' is a very peculiar way to 
interpolate an image, and I don't understand how/why it is supposed to work. 
When you upsample an image by 10%, what happens is that the 10th, 20th, 
30th, etc., lines are 'interpolated' and a new line inserted in front of 
each of them, based on that line and the surrounding lines (depending on the 
method of interpolation). When you then do the second step of another 10%, 
the 10th, 20th, 30th, etc lines are again interpolated, the only difference 
being that the original 20th, 30th, etc lines have all got pushed along by 
the insertion of the previous new lines. And so on. Seems a strange way to 
upsample an image!!

Bob Frost.

----- Original Message ----- 
From: "Paul D. DeRocco" <pderocco@...>
>
> Stair interpolation refers to iteratively computing a spline over and
> over again, usually in fixed proportions, until you get the output size
> you want. For example, if you used bicubic interpolation and increased
> the file size by 10% each time, until you got where you wanted to be,
> that would be stair interpolation. If you used S spline the same way,
> that would also be stair interpolation.

It seems to me that any linear filter applied multiple times is just another
different linear filter, and can be precomputed as such in advance, and
applied in the same amount of time it takes to do any other single filter.
Doing it iteratively seems like the dumb way to do it (if you're writing the
software, that is), and might also build up noise due to accumulated
roundoff errors, especially if computation is done in 8-bit mode.