Re: 16 bits = 15 bits in Photoshop?
Re: 16 bits = 15 bits in Photoshop?
- Subject: Re: 16 bits = 15 bits in Photoshop?
- From: Richard Wagner <email@hidden>
- Date: Wed, 13 Apr 2005 10:04:59 -0700
on 13/4/05 14:05, Andrew Rodney at email@hidden wrote:
> The high-bit representation in Photoshop has always been "15 1" bits
> (32767 (which is the total number of values that can be represented
by 15
> bits of precision) 1). This requires 16 bits of data to represent
is
> called "16 bit". It is not an arbitrary decision on how to display
this
> data, it is displaying an exact representation of the exact data
Photoshop
> is using, just as 0-255 is displayed for 8 bit files.
Well, I don't know that I'd call it "arbitrary" but it was a decision
that Thomas Knoll made a long time ago, likely for good reason. It may
be "an exact representation of the data that Photoshop is using" but it
means that PS cannot display true 16-bit data. This is in contrast to
8-bit data, which displays the expected 256 levels (0..255 for an
unsigned byte). I seriously doubt that Adobe would change the
underlying representation of 16-bit data at this time, though.
From Bruce Lindbloom's site (http://www.brucelindbloom.com), under RGB
Reference Images:
Adobe Photoshop represents 16-bit image data as a 16-bit unsigned
integer, in a form that has the binary point between bits 14 and 15,
and therefore can only represent 32,769 unique levels (binary
0000000000000000 through 1000000000000000). This means that simply
opening and saving one of the 16-bit reference images will reduce the
number of unique levels per channel by about half (which will also
reduce the total number of unique colors).
--Rich Wagner
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