Re: 16 bits = 15 bits in Photoshop?
Re: 16 bits = 15 bits in Photoshop?
- Subject: Re: 16 bits = 15 bits in Photoshop?
- From: Graeme Gill <email@hidden>
- Date: Thu, 14 Apr 2005 08:54:29 +1000
- Organization: Argyll CMS
bruce fraser wrote:
Photoshop's 16-bit implementation allows 32,769 levels, from 0 to 32,768.
As to why they did it this way, you'd have to ask the Photoshop
engineers, but
a) having an explicit midpoint is a significant advantage for many image
calculations
b) until very recently, there were no capture devices that even claimed
to capture more than 32,769 levels, let alone ones that actually did so.
One obvious advantage of a 16 bit representation with a maximum value
of 2^15, is that it's fast to multiply or divide by the maximum value
(or a power of 2 multiple of the maximum value) You simply use a shift.
Something like MMX/SSE doesn't have a vector divide or multiply instruction
that is as fast as simply shifting.
Graeme Gill.
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