In praise of AA filters

Started Oct 23, 2013 | Discussions thread
crames
Regular MemberPosts: 192
Like?
Re: ready to see your artifacts?
In reply to rovingtim, Oct 25, 2013

rovingtim wrote:

crames wrote:

rovingtim wrote:

So, here we go:

1) purple tree phenomenon

Leaf-less trees are not this colour ... as you can see from the leaf-less trees in the foreground of your own image.

Hmm, why would aliasing be responsible for the purple color?

Because cameras with good AA tend not to do this, as per my link. What is your explanation that cameras without AA's tend to do this?

I'd say you were jumping to a conclusion. I've never heard of this purple issue before, not with medium format, D800e, etc. There was an issue with the Leica M8 but that had nothing to do with the lack of AA filter, rather it was due to weak filtering of IR.

2) False detail

If you're talking about the pattern of the tiles on the right, it seems to be correct, parallel to the edges of the window covers. What detail do you see as false?

Seriously? You're proving my point in my original post. Well, the detail on the right is false. We know this because we can see the tile detail at other angles is the same and makes sense if you know how tile roofs are built.

Directly to the right, the next building has a roof that is oriented the same and has the same pattern. The tiles are oriented exactly the same, parallel to the sides of the "shed dormer windows." There's no way that detail above nyquist could alias to that pattern.

Notice how the detail on the roof suddenly changes as the angle changes. This is detail being invented out of whole cloth. There are lots of examples of this in your image.

Invented detail? On the left it seems to be the actual tile pattern at the edge of detectability.

Except ... it's the same pattern as the false detail on the image above ... and the false detail even follows through on the corner trim as if the corner trim was solely created to match the 'proper' detail on only one side of the roof that you say is within Nyquist. Funny that.

Let's zoom in a little and take a close look:

roof zoom 300%

Looks like the tiles are in columns, parallel to the edge, just like everywhere else.

On the right the pattern is not resolved - above Nyquist, apparently, and not showing up as aliasing.

Sure.

3) Jaggies

Yes, jaggies there. Could be due to sharpening.

How does sharpening produce jaggies?

By changing the contrast across the edge.

Look, I'm not saying that Sigma Foveons never alias. There are other things happening to create artifacts that are strictly not aliasing. Without an AA filter, not only can detail from above Nyquist be aliased down, but also detail below Nyquist tends to be greatly boosted. Boosted detail in the range between Nyquist and 1/2 Nyquist often don't "look right" because of the sampling process itself, even though they are perfectly valid (not aliased). In order for those higher details to look right, it's necessary to invoke the part of the sampling theorem that is usually ignored with images: reconstruction. The zooming examples I gave are a way to "reconstruct" those higher details below Nyquiest.

I'm not a Foveon advocate, but I think they get a bad rap because anything that doesn't look right is blamed on the lack of AA filter.

Reply   Reply with quote   Complain
Post (hide subjects)Posted by
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow