
   FULL-COLOR DISPLAYS

      A)  Introduction
      B)  To run a color simulation
      C)  Pixel saturation control

   
   A) INTRODUCTION

   HALOSIM3 does full-color simulations.  

   All spectrum colors are used between wavelengths of 
   0.37 (deep violet) to 0.80 micron (deepest red) via a 
   continuous selection algorithm rather than the use of 
   just a few limited sample colors.

   The HALOSIM color model takes account of the spectral 
   distribution of sunlight and eye color sensitivity.   

   Some halos resulting from refraction through crystal 
   faces inclined 90 degrees to each other can be highly 
   coloured.  These are:

      46 deg circular halo
      Circumzenithal arc
      Circumhorizontal arc
      Infralateral and supralateral arcs

   The colors of these halos are well dispersed and the 
   halos look more diffuse than in a 'monochrome' single
   wavelength simulation.   It follows that they also 
   need more rays to show brightly.  Be patient!

   
   B)  TO RUN A COLOR SIMULATION

   Set up a simulation in the usual way.

   From the 'Plot syle' box select "Color shades on color".

   Set 'Levels' to a value between 2 and 8.

   Run the simulation.

   Increase the number of 'Levels' and the number of rays
   until the simulation is smooth enough.   More rays can 
   always be added using the 'Add' button.


   C)  PIXEL SATURATION CONTROL

   PCs display colors as combinations of red (R), green (G) 
   and blue (B).  Each colour can take only one of 256 
   levels in a 24 bit display.

   This is quite limiting to HALOSIM which increments the 
   pixel brightnesses as it proceeds.  It means, for example,
   that only 256 pure red rays can 'strike' a screen pixel 
   before it is saturated in red.

   What happens when more rays 'strike' a saturated pixel?
   HALOSIM3 has two preset options accessed at the lower right 
   of the extended Control Panel when color plotting is 
   selected.

   The options on the saturation control are 'FREEZE' and 
   'PHOTO'.

   FREEZE - Once R,G or B saturate the pixel 'freezes' and 
            the impact of more rays is ignored - the pixel 
            color does not change.   Colors are preserved
            in regions 'over-exposed' by large numbers of 
            rays.

   PHOTO -  Once R,G or B saturate, that particular color 
            remains fixed - BUT - the other pixel colors 
            can become more and more intense as rays 
            containing other colors strike it.

            The pixel color usually tends to white and the 
            simulation behaves like a color photograph as
            it becomes over-exposed.

   Which to use?    The choice is yours.   The differences 
   are small.   Les Cowley prefers FREEZE and this is the 
   default setting.   He uses PHOTO for parhelia.

   The saturation behaviour is a compromise forced by the 
   limitations of 24 bit color displays.   It is present in
   grey scale simulations also but is less noticable.

   A solution is planned for HALOSIM4 !
  



