.PP
The LUT subform provides a graphical representation of how the primaries
are mapped to the image pixel values; the shapes of these primary color
curves can be interactively changed by the user.
.PP
The "Lut" window displays three LUT plots, each of which has a vertical axis 
which represents the range of image pixel values, and whose horizontal axis 
corresponds to the values of one of the three display colors.
The minimum and maximum values for each axis are given, along with boxes that 
numerically show the current mouse cursor position with respect to each axis.
.SH
Using Interactive LUT to Alter the Look Up Table
.PP
Changing the look up table is an easy process.  Position the cursor in the
plot window which is labeled with the desired color. Large white
crosshairs will mark the position of the mouse in the window, indicating
that that you are ready to change the values of the primary color curve
displayed in that plot. Click once at the position where you wish to begin
changing the color curve. This marks the beginning of the segment of the color
curve that you wish to change.  The color curve will follow the mouse as it is
moved within the plot window; as you move the mouse back over the same range,
the original curve will be restored. While the process is in action, the
currently displayed image will immediately reflect changes to the color curve.
.PP
When the segment of the color curve appears 
as desired, click again.  A straight line will be drawn from the first point
to the second, and the changed segment of the curve will be "set" at 
that point. Now, the process is repeated, until the color ramp for that 
primary appears as desired.  When finished, click the mouse twice at precisely
the same point.  The large white crosshairs will appear again, indicating 
that you are now finished with that color curve.  At this time, you may go 
on to change one of the other two color curves, or to exit LUT.
.PP
To determine how a pixel value is mapped using the "LUT" window, 
locate the image pixel value on the vertical axis by positioning the
horizontal cross hair at the pixel value and then project across until it 
intersects a color curve.  The point on the horizontal axis where this
intersection occurs is the value for that display color that is mapped to the 
image pixel value.  This can be done for all three parts of the current
colorspace.  For grey level images, all three colors will follow the same curve.
.SH
Undoing Changes
.PP
At any point, the "UNDO" button may be pressed, undoing the latest change 
to a color segment.  Since all color changes are kept in a stack,
the "UNDO" button will simply pop the stack; this implies that
successive clicks on "UNDO" will eventually restore the color
curve to its original state.  
Warning:  Since all colormap operations are tied together, 
"Undo" will also reverse color changes made using "PseudoColor" or 
"Windowing/Thresholding".
.SH
Histograms
.PP
If desired, a histogram which shows the distribution of the image pixel values
may be plotted in the same window as the colormap.  The default action is to
display the histogram.  If no histograms are desired, click on the logical
"Histogram" logical selection so that "Shown" changes to "Hidden".
.PP
When a histogram is displayed, three types of histograms are available. The
histogram type may be continuous, filled, or discrete.  Toggle the "Histogram
Type" cycle option until the desired histogram type is displayed.
.PP
For the histogram plot, the horizontal axis represents the image pixel values
and the vertical axis represents the number of occurrences.  Disregard the
values on the vertical axis when examining the histogram since they 
apply only to the colormap plot. 
.SH
Colorspace Models
.PP
The LUT function may use any one of eight color systems available with
Editimage: RGB, CMY, HSV, HSL, YIQ, XYZ, UVW, or GREY.  One can change the 
current colorspace model that is used with the LUT function by clicking on
the desired colorspace model.  The three plots which represent the colormaps
for each of the three colors of the chosen color system will update 
immediately when the colorspace model is changed. WARNING: if the color curves
in LUT are initially changed in one colorspace, and later the colorspace model
is changed, it is IMPERATIVE that the user be aware of the relationship between
the former colorspace and the latter colorspace.  If color curves are set in
the first colorspace, and further changes to the color curve are made in a
second colorspace \fIover a region which is undefined in the second space with
respect to the first,\fP the color curves exhibited in the plot windows will
exhibit erratic and unpredictable behavior.  This is to be expected.
For a detailed explanation of each colorspace model, see the section
on "Colorspaces".
