Digital BW, The Print

on 12/5/01 5:11 PM, Austin Franklin wrote:

>> in PS, even if it means
>> going from 8bit
>> output to 16 bit in PS, before doing them.
> 
> Well, I don't know that that will work as you think it will.  Let me think
> this through  When you convert from 8 bit to 16 bit, you are just using the
> top 8 bits of data, and the bottom are now all zeros.  Now there are 255
> "gaps" between each of the valid image data points...and when you make the
> moves, you then re-map them to new 16 bit values, so the lower 8 bits will
> now be being used...BUT you have only 256 discrete values in the first
> place.  When you go back to 8 bits though, you are just lopping off the
> bottom 8 bits and any of the values that were re-mapped into values that the
> top 8 bits are the same, will now not be distinct...so I believe this method
> does not necessarily work, and can cause losing some of your 256 original
> values.  Let me think more about it...unless you or someone else has a
> thought on this...


I don't know what really happens numerically when you do an 8-bit to 16-bit
conversion, but I don't think it helps. When you do a rotation, or an
upsampling, or gaussian blur, you smooth your histogram, but what you don't
do is add real discrete values, values which have meaning as detail. Those
operations cause some averaging (Austin, keep me honest on my terminology
;-), but they do not increase dynamic range, or detail (detail defined as
the ability to distinguish between discrete tones). In fact, they increase
the likelihood of banding.

I still don't know exactly why that's true, but, I remember earlier on I was
printing smooth gradients and getting banding, both onscreen and in print. I
kept trying to blur the gradient to smooth it out, but the more I did the
worse it got. Finally I asked for help on some PS lists and was shocked to
learn that noise was the common cure for banding.

I guess it's that something needs to fill in between those missing tones and
the randomness of noise breaks the edges. Somehow the purity of averaging
doesn't hide as well. But that's why images with lots of detail fare better
in 8-bits than very smooth images.

Having just said that Austin, it occurs to me that possibly that is one
reason your images tend to posterize. You say your images are virtually
grainless, you don't sharpen, and I see your tonalities are very smooth, so
your type of image might just be prone to that sort of thing. They always
look good though.

Nice picture of your kid BTW. He's a good looking kid.

Todd