Digital BW, The Print

> From: tvalleau [mailto:tracy@...]
>
> Please don't mention that to the millions of photoshop users who do
> anti-aliasing every day... ;-)

Whenever a digital image is reduced in resolution, the image must be blurred
before resampling, to the point where there are no longer any spatial
frequencies so high that they can't be represented at the lower resolution.
This is the purpose of Photoshop's "bicubic" interpolation algorithm. If,
instead, you use the "nearest neighbor" algorithm, which does no
pre-blurring of the image, you risk aliasing artifacts, if there are sharp
edges or fine repetitive patterns in the original image. Once you've reduced
the resolution using "nearest neighbor", and you see the resulting moire and
jaggies, it's too late to fix them. They _must_ be fixed by filtering the
image _before_ resampling, which, again, is the purpose of "bicubic"
interpolation.

Similarly, if you start with an optical image, and you want to sample it an
turn it into a digital image, you _must_ filter the image _before_ sampling
it. In this case, it must be done with a diffuser in front of the sensor.
Even lens blur, while slightly helpful, isn't the right kind of
filter--what's needed is something more like a Gaussian blur. If you don't
have this diffuser, and you get moire or jaggies, it's too late to fix them.

So the confusion is this: an anti-aliasing filter, whether in the optical or
the digital domain, is something that prefilters a high-resolution image
_before_ resampling down to a lower resolution, thus preventing aliasing
artifacts from occuring. It's not something that removes artifacts after
they happen--that's impossible.

> I don't believe that's correct, and that anti-aliasing is, in fact, a
> pretty darned (read: NSA) sophisticated algorithm (I learned to spell
> it... :-) There are both optical and digital approaches to anti-aliasing.

Anti-aliasing isn't particularly sophisticated. It's just a two-dimensional
spatial low-pass filter. There are some fancier "smart" algorithms, like the
wavelet processing used by Genuine Fractals, but they're used when going in
the other direction, when increasing the resolution, in order to
artificially preserve edge sharpness.

--

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