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BITPLANE.TXT
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1989-07-29
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3KB
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68 lines
The ST`s Screens
================
Let`s start with something we know, the resolutions & colors of the
screens:
Screen | Resolution| Max Colors | => bits/pixel
-----------+-----------+------------+---------------
LOW | 320 * 200 | 16 | 4
MEDIUM | 640 * 200 | 4 | 2
HI (MONO) | 640 * 400 | 2 | 1
From this the screen length in bits can be calculated, and from that the
length in bytes can be calculated:
LOW = (320 * 200 * 4) / 8 = 32000 bytes
MED. = (640 * 200 * 2) / 8 = 32000 bytes
MONO = (640 * 400) / 8 = 32000 bytes
Bitplanes
=========
Starting with MEDIUM res, each pixel(P) needs two bits(B) as follows:
B2a B2b
\ /
P1, P2, P3 > P16; P17 > P32; P33 > P48; and so on...
/ \ / \
B1a B1b B16a B16b
These bits are not stored consecutively. Instead they are stored in
------------------------------------------
groups of 16, 16 being the ST`s WORD SIZE, as follows:
+-----Make up P2------+ +-Make up P17--+
| | | |
B1a,B2a,B3a > B16a; B1b,B2b > B16b; B17a > B32a; B17b > B32b; ...
~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~ ~~~~~~~~~~~ ~~~~~~~~~~~
Bitplane 1 Plane 2 Plane 1 Plane 2
In other words, a group of 16`s lower bits are all stored together,
followed by that groups higher bits - and these 'groups of bits' are called
BITPLANES!
Moving onto LOW res, and again starting with the pixels:
P1 > P16; P17 > P32; and so on...
For this case each pixel needs 4 bits(B), namely a > d, and thus 4
bitplanes are needed as follows:
All make up P16
+-------------+-------------+-------------+
| | | |
B1a > B16a; B1b > B16b; B1c > B16c; B1d > B16d;
B17a > B32a; B17b > B32b; B17c > B32c; B17d > B32d; ...
~~~~~~~~~~~ ~~~~~~~~~~~ ~~~~~~~~~~~ ~~~~~~~~~~~
Plane 1 Plane 2 Plane 3 Plane 4
In MONO, as there is only 1 bitplane (i.e. 1 bit/pixel), coding is a lot
less complex as 'P1>P16; P17>P32' is simply represented by 'B1>B16; B17>B32'.
---END---