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[filmscanners] RE: 8bits vs. 16bits/channel: can the eye seethe difference



Hi Paul,

> > Whether the 182.75 gets picked up as 182 or 183 is a form of
> aliasing, not
> > dithering.  If in fact, the original image has that kind of tonal
> > resolution, then it is not being sampled at a high enough frequency to
> > maintain that level of detail.  It's being undersampled, and as
> such, the
> > result of the sampling of the higher frequency components is aliasing.
>
> No, it's not aliasing.

As I said in subsequent posts it is more aptly called quantization error,
bit it is still also aliasing, and it is caused by quantization error.

> Aliasing
> happens when you take something with one sample rate (possibly infinite,
> i.e., analog) and translate it to a different sample rate, resulting in
> frequencies present in the original that exceed half the new sample rate
> (the Nyquist) being translated into new, usually unwanted,
> frequencies. That
> has nothing to do with what happens when 16 bits are reduced to eight.

Agreed, and the translation of 1/2 LSBs is in fact also aliasing.  It's the
same aliasing that happens when you are trying to print a straight line and
you get stair stepping, which is aliasing.  If you think about it a bit,
and/or draw it out on a piece of paper you will see that it is.

> > But it does NOT contain "more than 8 bits of useful
> information".  You can
> > not consider noise valid data.  I understand the effect you are talking
> > about, and it is not dithering.  The effect is caused by post
> > data filtering
> > as done by your optical system, which is sampling at a lower frequency.
>
> It isn't the noise that's the useful data. It's the average across many
> pixels that's the useful data. The presence of noise is what allows this
> information to leak through the truncation process.

Actual "information" doesn't "leak through", the information is random, no
additional fidelity is kept.

> Austin, sometimes I despair at your peculiar ideas. Digitizing
> with a finite
> resolution is not aliasing, it's merely quantization.

It IS quantization error, but it is the quantization error that causes the
aliasing, in this case.

> We're talking about smooth areas of
> an image such as blue sky, whose frequency content is nil.

No.  Any time there are changes in data, there is frequency content.  If you
have 182, 182.5, 182, 182.5, 182 you have a frequency.  If that gets
translated into 182,182,182,183,182 you have aliased the frequency.

> Quantizing can
> create posterization,

Agreed it CAN.

> which can be eliminated by the addition of noise
> before the quantizing.

No...not necessarily.

> That ain't aliasing.

No, in that case it isn't.  I didn't say adding noise was aliasing.  I said
quantization error was noise, and I said quantization error CAN also cause
aliasing.

I'll read the rest of your post when I have time.

Regards,

Austin

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