Subject: The Complete And Essential Guide To Bitmaps
        Part Three: Corrections!
                By: James Collett (Professor)
       A Member Of: VIRTUAL INFINITY
              Date: May 1994
           Address: Park View Farm, Carlton, Nr Nuneaton, Warks, CV13 ODA
          Internet: s6005146@brookes.ac.uk
      Mono Account: bcc


    "To Err Is Human..."
    ====================
        Unfortunately  a small,  but potentially confusing,  error  slipped 
    through  in  'the guide to bitmaps' on issue #8 of ICTARI  disk  (March 
    1994).   The error was in the 'pixels and planes in general' section of 
    part one (BITPLANE.TXT).

        Just  to re-cap:  in order to write to or read from  a  pixel,  the 
    block  (group of PLANE SIZE pixels) where the pixel is stored  must  be 
    determined.   As discussed previously (issue #8),  the Pth pixel is  in 
    block P DIV S on a machine with a plane size of S bits.

        A  pixel`s colour is determined by one bit in  every  plane,  where 
    planes count from 0 to N-1 (inclusive) on a machine with N planes:

                                 All  make  up  P15
                      +------------+------------+------------+
                      |            |            |            |
              B0a > B15a;  B0b > B15b;  B0c > B15c;  B0d > B15d;
              ~~~~~~~~~~   ~~~~~~~~~~   ~~~~~~~~~~   ~~~~~~~~~~
                Plane 0      Plane 1      Plane 2      Plane 3

        To clarify the error: on a machine with a plane size of S bits, the 
    POSITION  (of  the bit in each plane) for the Pth pixel  would  be  the 
>>  (P MOD S)th  bit,   if   the machine started counting  from  the  least  <<
    significant  bit (right end),  as illustrated  by listing all the  bits 
    (in hexadecimal) for the example above:                            ÿÿÿÿ

     FEDCBA9876543210  FEDCBA9876543210  FEDCBA9876543210  FEDCBA9876543210
     ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~
         Plane 0           Plane 1           Plane 2           Plane 3

        The machine actually starts counting from the most significant  bit 
    (left  end),  so the POSITION for the Pth pixel is the  (S-P MOD S-1)th 
    bit  in  each plane,  on a machine with a plane size  of  S  bits;  the 
    'positions order' is reversed to the 'natural bit order':
     ÿÿÿÿÿÿÿÿÿ
     0123456789ABCDEF  0123456789ABCDEF  0123456789ABCDEF  0123456789ABCDEF
     ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~
         Plane 0           Plane 1           Plane 2           Plane 3

        In  other words,  bit B in every plane holds part of the colour  of 
    pixel (S-B-1) in the respective block.


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