on 3/17/03 5:47 PM, Frank Paris at frankparis@comcast.net wrote:
>> 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. That's what PS
> actually does? Of course that IS dithering, not aliasing. Dithering is
> something engineering deliberately does. Aliasing is something
> engineering tries to deliberately avoid.
>
Yes, PS really does do this. It isn't too hard to demonstrate.
(if you are curious enough I'd be glad show this).
But don't worry about the resolution at all. I'm only talking about
differences less than the least significant bit -- very small by
any measure. No matter what PS did you'd lose data below the least
significant bit. The bad thing would have been to just truncate --
in that case the average value would actually drop by 1/2 LSB.
If you have a value of 127.5 is HAS to go to either 127 or 128 so
it makes sense to split the difference. If the value was 127.3
30% would go to 128 and 70% would go to 127 -- much better than
just truncating and even better than always rounding as well.
Roy
Roy Harrington
roy@harrington.com
Black & White Photography Gallery
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