Chagall Reference   LUT-graph              12.5.93
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   The LUT-graph is used to alter the brightness,
   contrast and colour rendering of a picture, or
   of a cut-out portion.

   On principle the use of this function only 
   makes sense with colour and halftone pictures.

   The central part of the LUT-graph is a two-
   dimensional coordinate system whose X-axis
   shows the input value of the basic colour 
   identified at the top of the 'Curves' field.
   The Y-axis determines which value the output
   will have for the corresponding input value.
   A linear curve from bottom left to top right
   represents no change, i.e. a neutral behaviour.

   Depending on the colour-model chosen (Display..\..\OPTIONS\DISPLAY) 
   the values will:
   - for RGB have white as colour value 255 at extreme 
     right and top
   - for CMY have white as colour value 0% at extreme 
     left and bottom.

   At centre-right you can set the type of the colour
   curve:

   If you decide on a freely definable curve, then
   for every input value you can set practically any  
   output value manually on the graph by drawing the
   desired curve with the mouse cursor. Incidentally
   with free definition you can also draw above and
   below the actual coordinate system which makes it
   possible to set values such as 0 and 255.

   One also very often works with gamma-curves, which
   make use of a value  (Gamma) that produces a curve
   in the form of

                     Output = Input ^  
          (where ^ means 'raised to the power of').

   Gamma curves are relatively well suited to describe 
   the colour behaviour of various input- and output-
   devices and hence for the calibration of such devices.
   You can set the value of  between 0 and 7.0 in 
   variously-spaced steps by clicking on the arrows 
   or by direct numeric input after clicking on the 
   individual R, G or B (or C, M, Y) values; the 
   PopUps allow you to get from one end of the scale 
   to the other very quickly without having to wait 
   for the calculation of the intermediate steps.

   'Neutral' sets the curve to a linear relationship,
   whereby a curve with  = 1.0 is selected.

   'R=G=B' ensures that the shape of the current curve
   is also transferred to the other two curves, so that
   all three colours are affected equally. If the current
   picture is a halftone picture then this function is
   not selectable since only a single curve (for black)
   exists. You also cannot use the 'R=G=B' function if
   more than one colour curve has been modified. 

   The 'Save' and 'Load' buttons permit all three curves
   to be saved and re-loaded together respectively.
   This permits the same changes of the LUT-graph to be
   applied to several pictures without problems.

   If you don't want to involve yourself with the 
   colour curves in detail but simply wish to alter 
   the brightness or contrast of a picture, then Chagall
   offers for this purpose two easy to use sliders that
   permit adjustment of the two parameters for each
   currently active (!) colour curve. By the way, this
   does not change the shape of the colour curves but
   only moves them appropriately, so that no information
   is lost.

   This function too allows its effective range to be
   controled. During adjustment of the parameters all 
   changes seem to operate immediately over the whole (!) 
   picture, but are only finally applied after 'Activate'
   is selected. If the effective range is limited to a 
   frame then on activation only the cut out portion
   will be altered and the remaining picture displayed 
   in its original form again.

   This transformation too respects an existing 8-bit mask. 

   Finally it should be noted that Chagall memorises the
   settings of the LUT-graph for each picture even if 
   they were not actually applied to it with 'Activate'.
   Actually Chagall does not modify the picture itself,
   but only the display of the picture in the window. 
   This has the consequence that tools that have been 
   used in the picture will also be affected by changes 
   to the LUT-graph, which at first may seem somewhat 
   confusing. The advantage of this procedure is that 
   you can amend the LUT-graph of a picture while 
   occasionaly switching over to a different (reference) 
   picture. It is also possible to save the modified but 
   unused LUT-graph with the unmodified original picture, 
   as long as the picture format you are using supports 
   this facility (e.g. TIFF).
