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

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

Hi Frank,

> > The concept is always called DITHERING in the imaging world.
> > You can quote DSP definitions, but unless you apply them as
> > is standard in the imaging industry you are not going to
> > understand and be understood. Your DSP definitions can apply
> > quite easily -- the dithering is simply adding that 1/2 LSB
> > noise to the data and then truncate to 8bit.  So: 127 goes to
> > 127 +/- .5 goes to 127 127.5 goes to 127.5 +/- .5 goes to
> > half 127, half 128 128 goes to 128 +/- .5 goes to 128
>
> This sounds like in going from 48 bit color to 24 bit color, you're
> losing spatial resolution. I never realized that before.

Why do you believe you are losing spatial resolution?  If you have an image
that is 2k x 3k 16 bits/channel and convert it to a 2k x 3k 8 bits/channel
image, you still have the same spatial resolution, 2k x 3k.  You've reduced
the TONAL resolution, but not the spatial resolution.

>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.

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.

> Dithering is
> something engineering deliberately does. Aliasing is something
> engineering tries to deliberately avoid.

You are absolutely correct about both!

Regards,

Austin

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