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1994-04-26
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57 lines
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: ¯¯¯¯
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':
¯¯¯¯¯¯¯¯¯
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.
---END---