20011004: cursor value

>From: Dave Santek <address@hidden>
>Organization:  Space Science & Engineering Center
>Keywords:  200110011803.f91I35128630 McIDAS-X grayscale cursor values

Dave,

>JohnB reminded me that the graphics take some of the levels, so
>the original 0 to 255 gray levels get compressed into (256 - graphics_levels )
>space.

Also, the cursor uses at least one color.  This means that there is
never a one-to-one mapping between display object values and original
image brightnesses.  This is not a huge problem for display level
tracking, but it can be if one then tries to take that value and return
its calibrated (e.g., T) equivalent.

Even though I have not yet had time to look back through the code, I
have the impresion that the situation is even more complicated than
that.  What is niggling my memory is something to the effect that the
frame object values are indicies into the frame enhancement, not
brightnesses or scaled brightnesses.

Tom