[filmscanners] RE: 8bits vs. 16bits/channel: cantheeyeseethedifference

["Austin Franklin" <austin@darkroom.com>]

Hi Frank,

> > >That's what PS
> > > actually does? Of course that IS dithering, not aliasing.
> >
> > But it doesn't do that.  It simply chops off the lower 8
> > bits.  That is not dithering or aliasing.  Converting 0x1234
> > to 8 bits is simply 0x12.
>
> Hmmm. We seem to have a disagreement here. I wonder who is right?

Now that I understand what he was saying (and note that it is different than
what I believed he was saying in the first place) PS may very well do
something to the LSB, though I still contend it is not dithering, unless
it's random.

> > Now, if you are processing some data, and have to "split the
> > difference", happening to arrive at 127.5, that is
> > quantization error, not dithering.
>
> True, if that's what PS does. The other guy seems to disagree with you,
> though.

Agreed.  There are a few issues here:

1) How does PS convert from 16 bits to 8 bits?

2) If in fact it does choose the LSB algorithmically (meaning more than
simply lopping off the lower 8 bits), is what it does dithering or not.  If
it is not random, it is not dithering.  If it is simply rounding, it is not
dithering.  I don't know what it's doing, and Roy says he can prove it's
dithering...so we'll see.

3) If it does in fact "dither", is it even significant?.  I contend not, as
no image in the real world will have 127.5 across a significant area to make
a visually appreciable difference.

Regards,

Austin

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