Different scaling algorithms induce different amounts or ringing.
If a screenshot of a DVD is scaled with one of those algorithms,
another layer of ringing is induced 'on top' of the EE/ringing that
is encoded on the DVD. Is it then still valid to even use 'scaled'
screenshots to in reviews to analyze EE on DVD?
The scaling algorithms in external scalers or HTPCs also induce
such another layer of ringing. Since DVDs are mostly plagued by
source encoded ringing/EE anyway, wouldn't it be best choose the
scaling algorithm with the least amount of ringing?
Both valid concerns that i am trying to tackle in this editorial.
The question why to scale at all is a different topic that i won't
I will try to show that this 'additional' ringing layer, that is
introduced through scaling, is basically irrelevant under real world
situations, and thus any concerns that DVD reviewers use these algorithms,
is only 'theoretically' valid, and that it is as such more of a
moot point really.
I prepared a comparison picture that demonstrates the effect that
different scaling algorithms have on various amounts of source inherent
ringing on DVD transfers. For the comparison, i upscaled frames
from 6 different DVDs from NTSC SD resolution (720x480) to HD (1920x1080),
mimicing a high-end HT application (e.g. 9" CRT with scaler),
or a DVD review that uses upscaled screenshots to show deficiencies
(like i do here on my site). I then used equally sized 200x225 pixel
big crops from these upscaled frames to focus on 'critical' areas,
where the level of ringing is clearly visible.
Here is the picture. Its probably best to open it in a different
[Click image to enlarge]
From left to right, the amount of DVD inherent ringing increases
No ringing at all. These are black lines on a gray background.
2.) Fast and the Furious
Very high level of detail, yet basically no ringing. Great.
A tad more ringing. Still very good.
Quite thick and strong halos, equally in both dimensions. Bad.
5.) Star Wars: The Phantom Manace
A real classic. Ugh!
6.) Die Hard 3, first edition
I used 4 different scaling algorithms to upscale these 6 samples.
They too differ considerably in regard to the amount of ringing
they induce, only the first one doesn't induce any ringing at all.
Why would someone choose any other scaling algorithms than the first
one then, you might ask?
Well, its not that easy. Going into more detail on scaling is on
my to-do list, but thats a broad and complex topic. But it should
be suffice to say that the scaling algorithms are always a tradeoff
between 4 characteristics: ringing, response/detail, postaliasing
and computational demand. Bilinear, which is the filter that inherently
doen't induce any ringing of its own, is also very easy to compute,
but isn't optimal postaliasing wise and its transfered response
is the most soft/blurry of the commonly used scaling algorithms.
So for HT scaling applications, bilinear filtering is a rather
bad choice. For DVD reviews, it would do, but as is my very intend
to demonstrate in this post, the ringing induced through the other
filters does't really affect the screenshots sufficiently to invalidate
From top to bottom, the amount of scaling induced ringing
[Note: To inspect these differences in ringing, its advised
to focus on the leftmost AVIA pattern, since its the only one without
any source inherent ringing. Lateron, i will discuss the effect
this has on the actual movie samples!]
1.) Photoshop Bilinear Filter
The topmost row is upscaled with Photoshop's bilinear filter. As
already mentioned Bilinear scaling doesn't induce any ringing of
its own, as can be seen on the AVIA crop on the left. Its incapable
of transfering the frequency response properly, though, resulting
in a blurry picture (e.g. Van Diesel's face).
2.) Bell Filter
This is the filter that i use for my reviews. It does induce the
very slightest amount of ringing (again, see AVIA pattern), so little,
that most probably wouldn't notice. Response is a bit better with
3.) Photoshop Bicubic Filter
This is probably the most common upscaling algorithm. Graphic cards
like Radeon/Geforce are using this since years, some external scalers
as well. Resampling in Photoshop and other tools is very commonly
done with it. And its probably also used by most of the other DVD
review sites that use scaled screenshots like i do. The reason for
its popularity is the good 'balance' between the 4 characteristics
that i mentioned above.
If you inspect the AVIA pattern, you can see that it has indeed
quite a bit more ringing than the earlier 2 filters, but its still
rather simple to compute (at least with reasonable support/neighborhood)
and has very good response.
4.) Lanczos-windowed Sinc Filter
The family of Sinc filters are 'ideal' in the sense that they theoretically
transfer the response flat. In its pure form, a sinc filter would
have infinite support, thus being uncomputable. Even with restricted
support, and using a Lanczos-window is one such restriction, they
are very compute intense.
The price you pay for the 'perfect' response is considerable ringing.
Now, if you only look at the AVIA pattern, you could think that
the ringing that is induced through the different filters is rather
severe, especially in case of the Lanczos filter. Yet, if you look
at the second sample (Vin Diesel), you'll see that there is not
that much if any actual difference in ringing.
The tree in the top left for example is a prime 'potential' candidate
for ringing. Heck, on 90% of all DVDs, this would be a mess no matter
how you slice it! Yet, even the Lanczos filter doesn't manage to
turn this into anything but smooth, ringing-free bliss. [Hey,
i am not talking about the movie here :-) ]
Why is it that the AVIA pattern shows the difference in ringing
so blunt, yet that tree doesn't? The reason is, that the the resampling
filters mostly produce ringing from frequencies close to the passband
limit (e.g. 6.75Mhz in case of DVD). The pixel perfect lines in
the AVIA pattern HAVE response flat to that limit. Thus, they induce
All 'real' DVDs are heavily filtered, though. In the horizontal
direction, the Superbit titles are the closest to being unfiltered,
although at the cost of aliasing. Vertically, the Superbits are
just as much filtered as any other title to minimize interlace flicker.
If specially tailored for progressive display, the Superbit concept
could also be used vertically (opening up the filter), yielding
yet another gain in picture detail. 'Superbit squared'
Anyway, because DVDs are so heavily filtered, they don't contain
much, if any, response close to the passband limit and are thus
less likely to cause much scaling related ringing. So basically
the primary tradeoff of scaling algorithms: response/detail vs ringing
is slightly tilted with 'real world DVDs'.. The negative
aspect ringing, is not as much of a problem. The positive
aspect, higher response, preveils.
So what you basically see when you inspect all the 5 movie samples,
is that the different filters don't really 'add' any ringing that
isn't there. What it does, is make the ringing that is there, 'stand
out' more, since the transfered frequency response is higher towards
higher freuencies and 'ringing' is mostly higher frequencies.
No matter which row of samples you watch, its always apparent that
Furious has none, Unbreakable has considerable, and the last 2 have
excessive ringing. The 'relationship' among these DVDs in regard
to ringing is always the same.
And thats the important thing here. ALL upscaled screenshot samples
i have ever seen posted, always really really actually demonstrated
the deficiencies that are present ON THE DISC. No matter what upsampling
filter used. And since bicubic isn't even the filter that introduces
the most ringing of its own, yet its the most used one, makes this
a complete non-issue.
Again, the reason you see a bit more ringing for example in the
Lanczos Unbreakable sample is NOT because it 'adds' that ringing.
Its because the transfered frequency response is much better with
that filter, thus those higher frequencies of which the ringing
consists, are less subdued, thus more visible. So the bilinear filter
'blurrs away' some of that ringing. The problem is, that 'real detail'
in that freqency range are blurred away as well.
I laid out my case, honorable jury. I vote 'not guilty' :-)