
                                                        JEAN VAN MOURIK
                                                        THIRD MILLENNIUM 93

    
                       COLOUR MODEL TUTORIAL
                       ---------------------

    THE MOST IMPORTANT CONCEPT TO UNDERSTAND IN THE USE OF COLOUR WITH
 COMPUTER GRAPHICS IN RELATION TO PAINT OR INK IS THE DIFFERENCE BETWEEN THE
 ADDITIVE AND THE SUBTRACTIVE COLOUR MODELS.
    THE HUMAN EYE CAN PERCIEVE A COMPLETE COLOUR GAMUT WITH JUST 3 
 PRIMARYS. WITH PAINT THIS IS RED, YELLOW AND BLUE. WITH LIGHT THIS IS
 RED, GREEN AND BLUE. LOOK CLOSELY AT A TELEVISION SCREEN IN A WHITE LIT
 AREA AND YOU WILL SEE ONLY RED, GREEN AND BLUE DOTS. WHEN ALL 3 TYPES 
 OF DOTS ARE BRIGHT, FROM A DISTANCE THIS GIVES THE IMPRESSION OF WHITE.
 '!RGB' ON THIS DISK EXPLAINS THIS GRAPHICALY.
    WHEN WHITE LIGHT SHINES THRU A PRISM A COMPLETE SPECTRUM OF COLOURS 
 APPEARS, WHICH PROVES THAT WHITE LIGHT IS A MIX OF A RANGE OF COLOURS.
 THE ADDITIVE COLOUR MODEL IS APPROPRIATE HERE.
    WHEN 3 PIGMENT PRIMARYS ARE MIXED A VERY DARK SHADE OF COLOUR 
 IS CREATED. THE PERCIEVED COLOUR OF AN OBJECT DEPENDS ON THE WAVELENGTH
 OF THE COLOURS REFLECTED FROM ITS SURFACE  (AND THE COLOUR OF THE LIGHT
 ILLUMINATING IT). ALL THE OTHER WAVELENGTHS IN THE LIGHT WHICH ILLUMINATES
 THE OBJECT ARE ABSORBED BY THE OBJECT'S SURFACE.
 WHEN ALL THE VISIBLE WAVELENGTHS ARE ABSORBED, THEN A SURFACE IS PERCIEVED
 AS BLACK. SO BY SUBTRACTING WAVELENGTH (THRU ABSORBATION OF LIGHT)
 THE (PAINTED) SURFACE AQUIRES ITS PERCIEVED COLOUR.  THE SUBTRACTIVE COLOUR
 MODEL IS APPROPRIATE HERE. 
    
    BECAUSE IN COMPUTER GRAPHICS ONLY 3 PRIMARIES ARE USED, A COLOUR CAN
 BE DEFINED AS A POSITION WITHIN A CUBE WHERE EACH PRIMARY IS RELATED
 TO AN AXIS; E.G. RED TO THE X AXIS, GREEN TO THE Y AXIS AND BLUE TO
 THE Z AXIS (X,Y,Z). THE DISTANCE TO THE ORIGIN (0,0,0) ALONG ONE AXIS
 RELATES TO THE BRIGHTNESS OF THE RELATED PRIMARY. (LOOK AT 'MODEL-A')


   
   ABOUT THE 2D COLOUR PALETTES;
  
   THE ADVANTAGE OF A 2D PALETTE IS THAT THRU THE METHOD OF DITHERING, THE
   APPARENT RANGE OF COLOURS CAN BE INCREASED.
   THE 2D PALETTES ARE DESIGNED WITH 'DESIGN' WHICH IS ALSO ON THE 
   'COLOUR-MODELS' DISC. ALL COLOURS OF ONE PARTICULAR PALETTE ARE CHOSEN
   WITHIN ONE PLANE OF THE 3D COLOURMODEL IN 'DESIGN'. THIS CAN BE
   VISUALISED AS A CUBE STANDING ON ITS VERTEX (WHERE BLACK IS).
   IN 'DESIGN' ONE PLANE IS DISPLAYED WHICH IS PERPENDICULAR TO THE
   BLACK/WHITE DIAGONAL AXIS OF THE COLOUR CUBE. IT IS POSSIBLE TO MOVE UP
   AND DOWN PLANES WITHIN THE MODEL.
   THE COLOURS OF A 2D PALETTE ARE ALL CHOSEN WITHIN ONE OF THESE PLANES
   AND IN SUCH A WAY THAT THEY ARE ALL EVENLY SPACED. THEN USING A DATAFILE
   CREATED WITH 'DESIGN' THE RGB VALUES OF EACH COLOUR WITHIN THE PALETTE
   DESIGNED CAN BE SAVED TO DISC. 
   IT IS NOT! THE STANDARD PALETTE FILE BECAUSE I DON'T KNOW HOW TO DO THIS
   FROM WITHIN BASIC.
   IN THE 2D PALETTES THE BACKGROUND COLOUR (0) IS AN INTERGRALE PART OF
   THE PALETTE AND IS ALWAYS IN THE CENTRE.
   I WILL NOT ATTEMPT TO EXPLAIN HOW DITHERING WORKS EXACTLY AS IT IS QUITE
   COMPLICATED. IF YOU REALY NEED TO KNOW PLEASE READ THE MANUAL.
   SUFFICIENT TO SAY HERE THAT IN THIS SOFTWARE THE DATA FILE 'DITHER' IS 
   USED TO MAKE THE DITHERING METHOD OF MIXING COLOURS WORK CORRECTLY.
   THE 2 COLOURS USED IN THE DITHER METHOD SHOULD NOT BE TOO DIFFERENT ELSE
   THE DITHER BECOMES APPARENT. 
   ALSO SEE THE PROGRAM 'DITHERING' ON THIS DISC.
      
   'PALETTE+' IS A DISPLAY OF A 2D-PALETTE WITH DITHERING OF 169 SHADES OF
   COLOUR IN A 16 COLOUR MODE. WITHIN A TRIANGLE WITH 'PURE' COLOURS AT ITS
   VERTEXES (I.E. NO DITHERS), IT HAS ANOTHER 12 COMBINATIONS OF DITHERED
   COLOURS BETWEEN THESE 3 'PURE' COLOURS. THESE ARE GENERATED USING EITHER
   2 OR 3 'PURE' COLOURS.

           

 THIS IS THE GENERAL ACCEPTED COLOUR-MODEL AS APPLIED IN COMPUTER GRAPHICS.
 SPECIFICALY TO THE ARCHIMEDES COMPUTERS THE BRIGHTNESS LEVEL OF EACH
 PRIMARY CAN BE SET FROM 0 TO 230 (MAXIMUM), IN STEPS OF 16.
 THIS RESULTS IN 16*16*16 DIFFERENT COMBINATIONS FOR RED, GREEN AND BLUE.
 THE SETTING OF THE PRIMARY BRIGHTNESS LEVELS CAN ONLY BE DONE IN THE
 16 COLOUR MODES. THE PROGRAM 'DESIGN' ON THIS DISC HELPS YOU TO CHOOSE
 THE 16 COLOURS OUT OF A RANGE OF 16^3=4096 COLOURS. 
 WITHIN THE COLOUR CUBE BLACK IS AT (0,0,0), WHITE IS AT (15,15,15)
 AND ALL THE GREYS ARE ON THE DIAGONAL OF THIS CUBE.
 'MODEL-A' IS ABOUT 256 COLOUR LAYOUT, USE THIS PROGRAM AND YOU WILL
 GAIN A VISUAL UNDERSTANDING THIS 3D COLOUR-MODEL.


                                                  WALES, JULY 1993

    JEAN VAN MOURIK
    THIRD MILLENNIUM
    4 PANTLLYN
    LLANDYBIE
    AMMANFORD
    DYFED  SA18 3JT
    UNITED KINGDOM.


    PHONE   0269 851044 


 **************************************************************************