RE: Grayscale perception
RE: Grayscale perception
- Subject: RE: Grayscale perception
- From: "Mark Rice" <email@hidden>
- Date: Tue, 6 Dec 2005 02:54:23 -0500
- Organization: Zero One
Yes, the 12 bit to 8 bit luts were meant take the logaritmic output from the
D to A converters and convert it to linear 8 bit output. That is the point -
the 8 bit output was measured on the "equal brightness" scale, meant to make
all step appear to have equal brightness difference from the previous step.
Regarding monitors, all monitors have dithering in the algorithms. If they
didn't, how would one produce a scale containing 16.7 million colors in only
1280 lines of resolution? Theoretically, you could only produce 1280 colors,
which would definitley show quantization. Dithering is used to achieve the
effect of a smooth gradation.
Mark
-----Original Message-----
From: Ray Maxwell [mailto:email@hidden]
Sent: Tuesday, December 06, 2005 2:00 AM
To: Mark Rice
Cc: email@hidden
Subject: Re: Grayscale perception
Hi Mark,
I think there are three possible reasons why you might be seeing steps in
your film recorder.
The first is that the 12 bit LUTS may not have steps of equal visual
perception. It may be very non-linear with respect to vision. I would
guess that the output of the LUT is linear with respect to laser or light
head power. This means that it will be very non-linear with respect to
density on film.
The second is that you may be seeing banding in certain regions of range.
The third is that I qualified my remarks by specifying reflective media
which usually has a maximum density of 2.2 to 2.4. Transparent media can
reach densities of 3.0 or more. Therefore, it has a longer tonal scale and
can do a few more steps.
In the early days of Creo when they were building film recorders it required
extreme mechanical accuracy and a steep energy profile of the laser beam to
get rid of all banding. They patented the steep energy profile and used the
trademark "SquareSpot". Almost all other laser heads or light heads used a
gausian spot. It is very tricky to align this type of head so as to not get
any banding at certain power levels.
The focus on the imaging head requires temperature compensation and servo to
keep it in focus as the drum turns. If it is an internal drum imager the
same applies except you can substitute mirror rotation for drum rotation.
Because your device has 12 bits and can produce 4096 steps does not mean
that they are equal visual steps.
There are lots of monitors around that have 8 bit LUTS that produce
posterization at certain levels. However, if you get a high-end monitor
with 12 bits going to the D/A, and you calibrate it carefully, you can
produce smooth gradients with 100 steps of 1% in the CMYK color space.
Digital quantization steps do not necessarily equal even visual steps.
Ray
Mark Rice wrote:
>Ray, I have to disagree. I have been using an LVT film recorder for 20
>years. It has internal 12 bit LUTs and 8 bit output. Since the
>beginning, we have been plagued by quantization in the highlight
>regions. And the LVT is the best film recorder - I have used the
>Lightjet as well, and it does not have as smooth a gradation or as
>consistent calibration. I have measured the gradations we tested to
>assure that there were no "jumps" in bit values. The eye can detect
>this on a high quality film recorder. The only way to get rid of it was to
introduce noise, which is very similar to dithering.
>
>One doesn't see this effect very often on printers for two reasons:
>1. The dynamic range is about a factor of 10 lower than a transparency
>film recorder.
>2. Dithering is almost always introduced by the RIP, the driver, or the
>device itself.
> One of the things I have always wanted to try was to print a very
>large sheet of paper on a Lambda or Lightjet with a perfect gradation
>in 8 bit grayscale, with no dithering, and see if any banding can be
observed.
>But with most RIPs and print drivers, and even inside the black box
>that is the printer, it is usually impossible to tell if dithering is
occurring.
>
>Mark
>
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