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matting concerns (suppress)



I'm called upon occasionally to do transfer of bluescreen or
greenscreen shots for which I am asked by the compositor (Flame or
Henry artist) to provide the following for each scene:

1) 'native' color with regard for later matte of the blue or green

2) matte (hicon)

3) 'suppress' pass --object to be matted, with background color
removed or suppressed-- this is used in the best case scenario by the
compositor, along with the matte and background, to build the
composite; purpose is to avoid 'fringing' and contamination of object
to be matted with backing color ...background should have texture
and luminance of the original background, with only the chroma removed
(sometimes compositor will ask for slight amounts of chroma in a hue
to match eventual composite background)

4) beauty pass --this is a pass done on the shot with regard only for
the color of the object to be matted, e.g. a car.  Designed to be used
with the matte and suppress pass to recover bits of the backing
color that occur in the object to be matted (sometimes not
needed or requested--pass #1 can provide)

The method of obtaining the 'suppress' pass seems to vary among some
colorists with whom I've spoken-- one way is to process the shot
through the Ultimatte, and, using the Test Signal as background input,
match the luminance of the original color backgound.  Disadvantage:
because the matte is processed, noise can be a problem.  Another way
to derive the suppress pass is to re-correct the scene through the
color-corrector, i.e. layoff the shot to DDR and then run the DDR
through the color-corrector, using secondaries to pull out the backing
color.  

Is anyone aware of any other methods for deriving the 'suppress' pass?
Is anyone besides me even doing this at all?

I found, the other day when trying to suppress the backing blue on a
shot done of a dark green truck, with 'Fisher' light above, rotating
on a turntable, that I wasn't able --despite nearly an hour of vector
sharpening and secondary wrestling-- to remove, completely, slight
bits of blue in the sheen and reflections on the truck's windows,
though I had the background a fairly satisfactory un-artifacted grey.
I was using the DDR-through-the-color-corrector method.

With the daVinci 888/DUI to do the 'suppress' pass, and when
presented with a backing color to remove that has varying luminance
and chrominance, pulling back the vector to center-of-scope results in
a somewhat 'sausage-shaped' vector that, depending on the saturation
level, either includes partial opposite-color components or some of
the original color.  I've found I can reduce this effect somewhat by
playing with the width and saturation controls simultaneously, but
usually have to run the width so high --up to 120 degrees-- that other
undesired artifacts are achieved.

Can anyone commiserate, or instruct me a bit in this?  thanks.



-- 
Rob Lingelbach          |  2660 Hollyridge Dr., Los Angeles, CA 90068
rob at alegria.com  	| "I care not much for a man's religion whose dog or 
rob at info.com		|  cat are not the better for it."  --Abraham Lincoln
rob at cloister.org		KB6CUN   http://www.alegria.com

** thanks to 525 Post for contributing to the TIG in 1997
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