Thanks for the explanation Dennis.
Dennis' explanation is more what I was trying to find out.
The demo was an Avid User's group in New England (actually at
Avid) with some local colorists including one from Frontline. The
focus was less on artistic uses of grading and more on dealing
with troublesome doc footage shot on all kinds of gear relatively
quickly. Auto-contrast and auto color were both billed as
ballpark first steps not the final word. Understanding the
principles behind auto contrast was easy, auto color balance less
so. Part of what was surprising was that the repeated use of
auto color in Curves, hitting it two or three times in a row, led
to slightly different results.
D.
On Sat, Mar 31, 2012, at 06:39 PM, Dennis Degan wrote:
On Mar 31, 2012, at 2:57 PM, Dan McCabe wrote:
> Anyone know how auto-color balance works in the CC? Saw an Avid
CC
correction demo this week and it was never really explained.
I say:
I can tell you how auto balance works in general. A color
imbalance
may exist in scenes that contain unequal RGB gains and ped
levels.
IOW, 'white' isn't really neutral white and 'black' may contain a
bias
towards one or more colors). These imbalances are usually caused
by
the lack of proper camera white- and black-balance when the scene
was
shot. Auto-balance compares the relative levels of R,G, and B in
the
overall scene and adjusts the gain and ped of the 3 color
channels in
order to minimize any color bias at the white and black levels.
Since
auto-balance cannot take into account any errors in gamma (it
cannot
measure gamma nor many other parameters), the use of auto balance
is
fairly crude and limited. All it sees is the relative peak and
pedestal differences between red, green, and blue. Using that
information, auto-balance then adjusts peak and ped levels of the
3
color channels in order to minimize measurable color bias in the
blacks and whites. Its use is not intended to replace a serious
careful color grading done by an experienced specialist.
Dennis Degan, Video Editor-Consultant-Knowledge Bank
NBC Today Show, New York
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