Re: faking bit depth
Re: faking bit depth
- Subject: Re: faking bit depth
- From: Robin Myers <email@hidden>
- Date: Thu, 28 Mar 2002 10:40:11 -0800
- Organization: Robin Myers Imaging
Chris Jordan wrote:
>
Unfortunately the faking-bit-depth concept doesn't work. The reason is,
>
if you start with 8 bits per channel, then when you convert to 16 there
>
is no way to "fill in" the rest of the levels that the 16-bit colorspace
>
has available. For example, imagine that your file has pixels at the
>
0,0,0 level and 1,1,1 level in 8-bit; in 16-bit it would convert to
>
0,0,0 and 256,256,256 (out of a possible 65,536 levels per channel), but
>
there would be no way to fill in the spaces between 0,0,0 and
>
256,256,256 to get any use out of those extra levels in 16-bit mode. So
>
your edits in 16-bit will have exactly the same effect as if you had
>
done them in 8-bit, and when you convert back to 8-bit you'll see the
>
histogram is just as broken up as if you had done the edits in 8-bit.
Sorry, but part of this is wrong. When you upsample an image from 24-bits/pixel to
48-bits/pixel, the new values are indeed separated from each other by 256 values.
However, you are overlooking the effect of the image operations then performed on the
48-bit/pixel image. Subsequent Levels, Curve, Color and other operations "fill in" the
spaces between the original values. When the image is converted back to 24-bits/pixel,
the spaces existing in the original 24-bit histogram are "filled in", making a smoother
histogram.
You can confirm this with a simple experiment. Take an 24-bits/pixel image, use Levels to
increase the contrast significantly, then examine the histogram. There will be gaps. Take
a copy of the original image, convert to 48-bits/pixel, use Levels to increase the
contrast to the same amount, then convert to 24-bits/pixel and examine the histogram.
There will be fewer or no gaps.
Converting to higher bit representations before performing most image operations,
especially those involving tone curves, is a common practice in high-quality imaging and
has been for years (even before Photoshop). It is also a common sampling technique and is
used in many other applications besides imaging.
Robin Myers
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