on 3/21/03 10:58 AM, Paul D. DeRocco at pderocco@ix.netcom.com wrote:
>> From: Roy Harrington
>>
>> I tried your particular construction and by sheer coincidence the numbers
>> you get happen to be an almost exact match of 16 bit versus 8 bit.
>> 0xAAAA is the exact mapping of 0xAA so when you converted, it was
>> extremely
>> close. When I tried it I actually got .26% of the pixels to come out 169
>> with the rest all being 170.
>>
>> 8 bit goes from 0 to 255 (in hex 0xFF)
>> 16 bit goes from 0 to 65535 (in hex 0xFFFF)
>>
>> Both the black and the white points must match in mapping, 0 to 0 is
>> trivial but 65535/255 = 257 so you need to multiply 255 by 257 to convert
>> from 8 to 16 bit.
>
> You're right! I had assumed that they divided by 256, not 257. I stand
> corrected. When I repeated the test to produce numbers that are not
> multiples of 257, it did indeed do some dithering.
>
> What's odd is that in RGB mode it dithers all the colors with the same
> pattern. You'd think they'd use uncorrelated dither noise for the three
> colors.
Interesting. Maybe they figured there was a possibility of color
artifacts showing up. I assume you mean an RGB where R=G=B ? If they
were different the dithering would have to be different, too, wouldn't it?
>
>> Here's a much easier construction that will show what you want. Fill the
>> canvas in 8 bit mode -- say Gray=128. Convert to 16 bit now you have an
>> exact 16bit representation of an 8bit value. Now do a very minor Levels
>> change. Set the white point from 255 to 254. That will nudge every gray
>> value X = X*(255/254). So 128 will go up by a half, if you had picked
>> Gray = 85 to start it would go up by a third, ... etc.
>>
>> Now convert to 8-bit and see the dithered pixels.
>
> What I'd like to know is how you managed to convert from 8bpc to 16bpc. You
> must have a different version of PS, because mine doesn't let you do that.
> I'm using v7.01 for Windows.
That's strange. I've always been able to change the mode to 16bit. You
can't have any layers of course. I'm using PS 6 and 7 on both Mac OS 9 and
OS X.
>
> --
>
> Ciao, Paul D. DeRocco
> Paul mailto:pderocco@ix.netcom.com
>
Roy
Roy Harrington
roy@harrington.com
Black & White Photography Gallery
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