Digital BW, The Print

> From: Bob Frost
>
> 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!!

That would be strange, but that's not what happens. All pixels are
interpolated. Each pixel in the result maps to some possibly fractional
pixel position in the original, and its value is computed by filtering the
original pixels in that region. Even if an output pixel lines up perfectly
with an input pixel, it is still computed by filtering that input pixel with
its neighbors, so that it won't look different from those pixels that don't
line up. Yes, this does soften the image, by discarding some high spatial
frequencies, but this is often masked by including a bit of high frequency
boost--I think that's what the new "sharpen" option in PS Bicubic is.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...