Mixed Artificial Lighting


Dan Margulis <76270.1033@...>
 

I deal with lots of images from museums and art galleries. Often these
have very strong mixed casts which I find impossibly difficult to fix with
any sort of success rate. A typical recent example is a dark bronze statue
lit in a mixture of daylight, mercury halide and possibly something else.
Its cast is bright yellow in the top half, a strong magenta in the lower
half and some very blue highlights for good measure. The nature and
distribution of the various casts make even a selection, erm,
challenging.>>

You are on the right track by realizing that a selection is necessary. This
type of work is not like altering the general appearance or lighting, which
can be done with curves. But here you are taking something with two
different light sources and making them appear to merge into one. This is a
move away from the art, just as much as if you had decided to change the
statue from bronze to silver. The only thing is, it's easy to select the
statue, but very difficult to select the cast area unless you know the
secret.

Assuming that there are only two casts, the way to tackle this is to
correct for one of them by means of curves. Then load a mask for the area
affected by the second cast, and desaturate the area with Adjust
Hue/Saturation.

That mask can be made easily from the A or, more commonly, B channel of
LAB. For example, suppose you are attempting to eliminate a blue cast in a
certain area.

1) Make a copy of the image, and convert the copy to LAB. Make a copy of
the B channel separately and throw the LAB away.

2) This B channel will look like a gray blur, but the cast area will be
marginally darker than its surroundings. Apply a very steep straight line
curve to the channel, trying to darken the cast area and blow everything
else out. You will probably need to apply curves twice to do this, but the
result should be a white background and a dark gray gradation where there's
a cast.

3) In the event that there's something in the image that's *supposed* to be
blue, erase it from the channel.

4) Apply a Gaussian blur to further soften the edges.

5) Return to the original image, and with Select: Load Selection, load this
other channel. In this particular case, where the area you want to select
is black in the mask rather than white, you'll need to click the "Invert"
box in the Load Selection dialog.

6) With the mask loaded, you can desaturate or apply curves or selective
color, depending on the character of the iamge.

Dan Margulis