U of S | Mailing List Archive | alt-photo-process-l | OT: 16 bit editing myth or reality?

OT: 16 bit editing myth or reality?

  • To: alt-photo-process-l@usask.ca
  • Subject: OT: 16 bit editing myth or reality?
  • From: Yves Gauvreau <gauvreau-yves@cgocable.ca>
  • Date: Fri, 14 Dec 2007 11:39:17 -0500
  • Comments: "alt-photo-process mailing list"
  • List-id: alt-photo-process mailing list <alt-photo-process-l@sask.usask.ca>
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  • Reply-to: alt-photo-process-l@usask.ca


I was curious to verify that editing in 16 bit mode (16/48) was really a
plus or a myth especially when considering a printed image.

So I generated a large number (1 million) of normally distributed real
numbers (doubles) in the range of [0..1]. If you made an histogram of these
numbers it would look pretty much like what you would observe with a regular
image. I can add that this data set is similar to a black and white image.

I then converted these real numbers to a set of 8 bit integers [0..255] and
also to a set of 16 bit integers [0..65535] thus creating 2 sets of values
from the same original set of real numbers.

I then applied 5 different curves to each set of values. The simple curve
was y = x^1/g where x is the set of either the 8 or 16 bit integers, y is
the resulting set of new values and g was either 1.5, 1.8, 2.0, 2.2, 2.5.
These curves represent a mild to heavy raise of a single control point near
input 50. All this created 5 sets of new values for both the 8 and 16 bit
sets or 10 new sets if you prefer.

I then counted the unique levels of the original 8 bit integer set and of 16
bit integer set that was converted to 8 bits assuming this would happen if
we printed our 16 bit original file.

Next I counted the unique discrete levels present in each of the 10 data
sets. Don't worry the software I used for this counted them for me. This
counting would be similar to counting the bins of an histogram having a
count of 1 or more in them. In simpler terms, I was interested to find out
the number of levels we loose by applying a curve to an image, this would
look like holes on an histogram.

Here are the results:

The 8 bit integer set as 238 out of the possible 256 levels

curve 1 => 203 or 85.3% (203/238)
curve 2 => 188 or 79.0%
curve 3 => 178 or 74.8%
curve 4 => 170 or 71.4%
curve 5 => 159 or 66.8%

The 16 bit integer set as also 238 out of the possible 256 levels when
converted back to 8 bits

curve 1 => 216 or 90.8%
curve 2 => 203 or 85.3%
curve 3 => 192 or 80.7%
curve 4 => 185 or 77.7%
curve 5 => 173 or 72.7%


Though I started with random numbers which mean the data is not a real
image, I took great care to use numbers that would be representative of a
real world B&W image. Also the curve I use may not be representative of an
actual transformation one would use on a real image but it is actually an
inverse gamma transformation and this type of transform is used practically
all the time in color managed environment. I think we can say that both the
data and the curve are representative of actual editing that could be done
on real world images though this particular data set is similar to a B&W
image and it may not be the same with a color image.

I think we can safely say that this particular editing simulation shows that
we would benefit from working in 16 bit mode.

But I remind you that other types of editing may or may not allow us to come
to the same conclusion.

Hope this was helpful,