John Moody wrote:
> We sound like a broken record here but... Qimage..excellent. CS2 is pretty
> good as well assuming you are not trying to scale too excessively.
Since I have a few minutes free, John's comment seems like a good
opportunity to talk a little more about splines....
What spline(s) does Qimage implement? According to the website, they
support "Lanczos, Vector, and Pyramid" interpolation. Are these the only
three? I understand 'vector' and 'pyramid' to be spline classes, which
makes it hard to understand exactly what Qimage is claiming. There are
good and bad pyramid splines (when it comes to applying them to images,
at least), and I expect the same is true of vector splines. Lanczos,
however, is a specific way to compute the spline, and it is quite good
with a typical photographic image and doesn't do at all poorly on
graphics designs with lots of smooth lines, either. I'm also a little
unclear on whether Qimage will allow you to open a TIFF, compute its
spline, and save it back as a TIFF for more work in Photoshop, or
whether it is a print output tool only.
Photoshop is strictly 18th and 19th century in terms of its spline
support. And I mean that quite literally - the math behind all three of
their methods is at minimum well over a century out of date. The three
methods are bicubic, bilinear, and nearest neighbor. Bicubic is an
excellent way to undersample, but is a rather problematical algorithm
for oversampling - more on this in a moment. Bilinear is something of a
special-case spline, not usually appropriate for upsampling photographs.
And nearest neighbor is a blunt instrument that I think I independently
invented when I was 16 for interpolating values from Cepheid light
curves on an Atari 400 in CPM/BASIC. It is possible this spline was
invented prior to the year 1500. You don't want to go there. Photoshop
is in pretty serious need of spline improvement.
I think it is still true that Genuine Fractals is proprietary (hence my
previous reference to "mystery math"). If so, nobody really knows what
it does. I do agree with another poster that GF is almost always better
than bicubic for oversampling, especially at certain ratios where
bicubic tends to resonate.
Photozoom Pro supports the S-spline, Lanczos, Hermite, Bell, Mitchell,
Catmull-Rom, B, Nearest-Neighbor, Bicubic, and Bilinear splines. I can't
say my experience is extensive with most of these.
Part of the "problem" with some splines is that they are either patented
(for example, S and B) and rarely or never licensed, or they are trade
secrets (for example, Genuine Fractals). Also, spline computation is a
very fast-moving area of math these days. All this accounts for the
fragmentation of the spline market and the reason there are a dozen
tools out there. However, the patents also mean you can go read all
about the method, sometimes including the math.
S spline is particularly interesting in this respect. It avoids aliasing
entirely, as far as I can tell (actually, I think it would alias at
ridiculously high frequency proportions, well beyond what would ever be
encountered in real life). The technique does not make any changes to
the proportional tonality of the image (some other interpolations, but
by no means all, do). It almost fully preserves, but does not enhance,
acutance (this avoids the excessive shadowing along tonal boundaries,
similar to overusing USM, that some splines introduce). Mathematically,
the image coming out of s spline has bulk properties (acutance, edge
sharpness, histogram) nearly identical to the original.
From my experience with it, I think the S spline is currently the gold
standard against which all other methods of computing a spline should be
compared. It can be used pretty effectively at any point in the
workflow, including (I believe, somewhat heretically) being applied
after sharpening if you are using something other than USM. In the
spirit of making a meaningful comparison to S, I'll say:
Lanczos: comes fairly close to S, tending to introduce certain artifacts
on medium-contrast boundaries but doing very well with sharp graphic
design edges and not bad at all on most photographs. Does not preserve
image acutance as well as S, but it is a small difference. If you have a
tool that does Lanczos, you probably don't need to move to a tool that
has S.
Genuine Fractals: notably inferior to S based on my experiences a few
years back. It fails to preserve acutance as well as S or Lanczos, and
bulk tonality changes can result in images with multiple high contrast
boundaries. However, it is far better than Bicubic.
Bicubic: really, really stinks compared to S, Lanczos, and GF. It
aliases, resonates, and introduces several different kinds of artifacts
including one which appears similar to noise, and another which is
similar to a ghost image of brightly toned image elements. It also fails
to preserve image acutance, doing far worse than GF in this respect.
I'd be curious to hear what people have thought about Lanczos or GF in
comparison to algorithms such as Qimage's Pyramid and Vector, and the
T-spline (if anyone is using it). Also, I'm perfectly open to being told
my impressions of the four splines above are wrong - there are some
unavoidable mathematical *facts* about all of them, and I really like
dealing in facts; but their application is aesthetic in this context,
and that makes all the difference. I'm not married to my spline by any
means.
--
Jeff Medkeff
Eagle River, Alaska