Video LUT Question
Video LUT Question
- Subject: Video LUT Question
- From: David Remington <email@hidden>
- Date: Wed, 18 Dec 2002 07:35:35 -0500
Bruce,
How do you "Load the video look up tables (VLUTs) with ramps"? I don't
recall ever hearing this technique mentioned before.
Thanks,
David Remington
Monitors:
--------
Within a color managed environment like Photoshop: Load the video look
up
tables (VLUTs) with ramps. Use the monitor's brightness and contrast
knobs
(or whatever means are available to control the monitor's electronics)
so
that you get as close as possible to your desired white point, a good
black
and a gamma of 2.2. (It's not important that you actually achieve 2.2,
it is
just a target goal.) Do not use VLUTs to do this. Then profile your
monitor
while it is in this state. Do not make or use use monitor profiles with
vcgt
tags. Leave your VLUTs with ramps in them. The reason for the ramped
VLUTs
is that for any 8-bit to 8-bit LUT (like you'll find in most VLUTs), you
will lose levels by loading in a curved function. This is not a
trade-off
situation where you give up something here to gain something there.
When you
load a curve into a VLUT you give up levels and gain nothing (remember
we're
in a color managed environment -- Photoshop will compensate for any
differences between the working space gamma and the monitor gamma). For
example, if your native monitor gamma is 2.5 and you use a vcgt tag to
make
it appear as though it has a gamma 1.8, you have reduced the number of
levels driving the monitor from 256 down to 202. You can explore this
yourself here: <
http://www.brucelindbloom.com/LevelsCalculator.html>.
David Remington
Collections Reformatting Photographer
D 40 Widener Library
Cambridge, MA 02138
617-496-9346
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