cal = UpdateAmbient(cal,newP_ambient,[ADD])

Update the ambient light used in the conversions. The
value of P_ambient in the structure is replaced with the
passed value and the computed quantities that depend on
it are updated.

The ambient must be specified in the same measurement units as it
was in the cal at the initial call to SetColorSpace. If different
units are desired, all ambient fields in the structure must be updated
and SetColorSpace called again. I have never wanted to do this,
so I haven’t written a separate routine.

It is sometimes useful, however, to update the ambient in the
linear color space defined by the call to SetColorSpace. To
do this, use UpdateAmbientLinear.

If flag ADD is true, passed ambient is added to current
value. Otherwise passed value replaces current value.
ADD is false if not passed. Use caution when setting ADD
true – if the ambient is changing during the experiment
you typically don’t want to keep adding multiple times.

11/17/93 dhb Wrote it.
8/4/96 dhb Updated for modern scheme.
8/21/97 dhb Update for structures.
3/2/98 dhb Fix bug in checks introduce 8/21/97, pointed out by dgp.
3/10/98 dhb Change T_ to P_.
10/26/99 dhb, mdr Fix bug in checks. There was also a variable name
glitch. I don’t think this could have worked the way
it was. Perhaps no one calls it.
5/2/02 dhb, kr Add ADD flag.

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