Re: Color Conversions and Dither
Re: Color Conversions and Dither
- Subject: Re: Color Conversions and Dither
- From: Graeme Gill <email@hidden>
- Date: Mon, 19 May 2008 10:56:22 +1000
Roy Harrington wrote:
evidence of this is difficult if not impossible. My gut feeling was
that since the printers
themselves (Epsons) were essentially just 2-bit at the dot level an
intermediate stage of
16-bit --> 8-bit --> 2-bit would not hurt versus 16-bit --> 2-bit
directly. So rather than
That's a bit misleading. The screening does convert to very low
bit, but it doesn't necessarily loose 6 bits of information, the
information is just transformed from intensity levels to spatial
information. The loss of information (if any) in the screening
depends on the screening algorithm. In classical line algorithm
it depends on the line spacing. In something like a stochastic
screening, it depends on the cell size. For instance, stochastic
screening to 2 bit is capable of representing 2^16 levels using
a 128 x 128 cell.
The important aspect is actually what color space we're in.
In a well behaved (that is visually linear, where any 1 bit difference
in color value is a proximately visually uniform) space, 8 bits
per component is usually quite adequate. In something like
an uncalibrated printers device space, particularly if it's
an ink jet with a high dot overlap factor, then 8 bits is
going to be completely inadequate.
So a high quality print path will actually be prepared to
boost the bit depth in the process of transforming from
one colorspace to another (ie. through a calibration curve),
if it is attempting to maintain adequately low quantization
levels in each colorspace. Not all printer drivers pay attention
to such detail though.
Note that if you're driving the printer via the vendors
"RGB" printer interface, you're essentially driving
it in an "sRGB" like space, in which their driver is
doing a lot of work to linearize the device, and make
the color space behave reasonably.
Of course, doing all your work in 16 bit gives much greater margin
for error, dynamic range, and freedom from worry about exactly which
colorspace things are in.
Graeme Gill.
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