In praise of AA filters

Started 6 months ago | Discussions thread
crames
Regular MemberPosts: 181
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Re: Wait a minute: Foveon example not so aliased after all...
In reply to John Sheehy, 6 months ago

John Sheehy wrote:

crames wrote:

Take a look at the foveon example zoomed 300% with sinc interpolation. There is actually only a few small aliased patches, mostly in the upper left.

Huh? I see moire in the right edge, second patch from bottom, the right bottom of the first full patch in the lower right corner, the patch just a little left and south of center, and I also see inconsistent rendering of the darker spots in the areas where the three-toned lines cross the other lines, due to luck of alignment. This is a clearly aliased capture. I don't see just fabric here; I see poor capture, which ruins the fabric. Face it, we need cameras with a hell of a lot more pixels to properly record things like this. Shortcuts do not work.

Yes, I said "mostly" in the upper left. It's nice that you were able to find some artifacts in other areas, but without knowing the origin and processing history of this image it's hard to say what is the exact cause of some defects. For sure I would say that the moire that occurs at angles that are inconsistent with the direction of the weave is aliasing.

The point is that most of the "aliasing" goes away with a little interpolation, which means that most of the so-called aliasing is in fact not aliasing.

Lee Jay said that the image is "covered with aliasing." "The entire thing is full of jaggies, false detail and patterns that don't exist."

Here is the original X3 image again, this time zoomed with nearest neighbor.

NN 300%

There you go, jaggies, "snap to grid," moire patterns, etc.

Here it is with Photoshop bicubic smoother @300%:

Bicubic smoother 300%

Would you say that it is still covered with aliasing, or would you agree that most of the artifacts have disappeared? True aliases do not go away so easily.

Foveon zoomed 300% with sinc interpolation

You can't really evaluate aliasing without doing some interpolation - has to do with the sampling theorem. If it goes away when you interpolate, it can't be aliasing.

What people often think is aliasing is really caused by the way the image is displayed, rather than the way it is captured.

Not in a 1:1 display; then there has to be a difference in capture for there to be a difference in output. A converter does not randomly increase micro-contrast in some areas and decrease it in others.

I'm not understanding what you are saying here. Reconstruction artifacts are especially visible in a 1:1 display, since no reconstruction occurs, other than representing pixels as little sharp-edged squares or rectangles, a distortion that occurs regardless of whether the image was captured properly or not.

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