Image Processing Reference
Light and Shade
Now that the preprocessing has sorted out the temporal and spatial adjustments, we can
turn our attention to the overall lighting and coloring of the image. It is helpful to sepa-
rate adjustments to the brightness from those that modify the color, so first you'll need
to get the luma values right. Then the color can be tweaked if necessary. The luma
may need to be checked again and very slight adjustments made after color correction is
taken care of.
Correction Is Sometimes Mandatory
Sometimes the footage you are processing requires some color correction simply because
of the kind of material involved.
Old home-movie film is almost certain to have a purplish cast to it because some of
the dyes in the emulsion will have faded more than others. The magenta dye seems to hold
an image for the longest time.
Shooting underwater lends a bluish cast to the whole color balance, which darkens
with the depth that the diver is at when the film or video is shot. This is because the reds
in the illumination are lost faster than the greens and blues. Some experiments have been
done with logging the depth in a data track stored with the video. This is then used to
select a color-correction filter according to the depth that was recorded for that frame. This
seems to work very well, although it probably requires some calibration.
Compensating the Luma