For David Millier

David,

You have serious misconceptions about "aliasing" .... To continue our discussion - here is your last response:

This is absolutely wrong.

The Nyquist limit is a hard stop. You cannot reconstruct anything above Nyquist. Any energy available above Nyquist cannot be rendered. What happens is that energy folds back to lower frequencies as a kind of beat pattern.

There is no partial aliasing, partial real detail, as you suggest; no slight loss of accuracy as you move further beyond Nyquist, that is not how it works. All meaningful information above this point is lost. This is sampling in action. The reason for this is that without enough samples, things become ambiguous, there is no unique reconstruction of the details: is that a 2000 lp/h detail or is it 20 lp/ph or 40 or 80 or 500. The sampling mechanism can't tell.

What you get instead is distortion overlayed on proper detail at lower frequencies. The damage is not done above Nyquist as you suggest, but lower down the frequencies where it corrupts detail that would otherwise be fine. That is why it is objectionable (eg jaggies).

Anything you think you see above Nyquist isn't remotely detail, it's noise. When we say it hints at texture we mean exactly that: it's not a slightly distorted version of what's there, it's garbage.

Which brings us to the central argument. Does this noise convince the eye that it is real detail or is it just noise. You argue that it does. If this is true for most people, what we rely on perceptually is that the garbage is rendered sufficiently small that you can't tell its garbage, it just looks like a suggestive roughness.

An example of this is my pier shot I've use a few time over the years. There is a lifeboat boathouse on the pier with a tiled roof. The tiles are standard tiles that run horizontally. If you look at the aliased image, the Sigma renders the roof as a series of diagonal stripes fading in and out in brightness alternately. Nothing the sensor is doing has anything to do with the real tile pattern, it's pure distorted aliasing. However... if you print it, this area of the image is so small you can't really see it properly. The vaguely stripy pattern can be perceived as being what you might expect: a roof pattern. It's the wrong pattern but it fools the eye. If you up close, it looks plain wrong, a roof doesn't look like that but at a distance you can get away with it.

And that's the crux of the argument in favour of aliased images.

My conjecture is simply that if aliasing seen like this provides some textural benefit, maybe other textures (like for example grain) can also do the same job. Perhaps a distant rock surface whose detail is beyond the sensor, can be made to look a little better by adding some grain if the grain gives the impression of a surface texture (even fake as it is).

It's an idea worth exploring - I've seen suggestions before that added grain can make digital prints look less plastic."

First, "jaggies" are not caused by anything remotely resembling "distortion overlaid on proper detail at lower frequencies."

Jaggies are caused by attempting to construct smooth lines and curves with square or rectangular building blocks of light (pixels). When the contours are small enough that they are represented by very small numbers of these "pixels" then the irregular "stairsteps" are seen in enlargements. Antialiasing filters simply blur this jagged appearance by interspersing additional and softer (blur) pixels offset sufficiently to fool the eye.

Second, above Nyquist as indicated on the SD1 photographed resolution chart what do "you" see? If you see "noise" then you and I have essentially quite different eyesight. What I see are quite plainly "lines." Lines just like the ones below Nyquist only fewer of them.

Do you "really" believe in magic? Do you think that these "lines" appear on every photograph by the SD1 above Nyquist regardless of whether the subject of the photograph has any "lines" in it?

Funny thing - when I shoot a photograph of a distant pine tree with one of my Foveon based sensors and greatly enlarge it, I see no black and white "lines" where the image begins to break down. What I do see are pine needles. Now pine needles somewhat resemble "lines" in shape so do you actually think that they magically become black and white lines such as the ones seen in the resolution chart above Nyquist, or do you think that perhaps fewer pine needle than actually exist in reality might be seen? Or do you believe that the photography fairy has duped me into believing that black and white lines actually look like pine needles?

Choosing to refer to what is seen above Nyquist in Foveon photos as "grain" is simply your way of avoiding reality in attempting to support your opinions. I'll choose to believe my eyes and you continue to believe your "theory" about what is going on and we will simply have to agree to disagree.

Best regards,

Lin

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