Amiga.org
Amiga computer related discussion => General chat about Amiga topics => Topic started by: Pentad on September 06, 2018, 02:51:12 PM
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I came across this 1988 post from Hedley Davis on the A2024 monitor. This is the monitor he designed and was inspired by the look and feel of the NeXT machines at the time (crisp, high resolution gray scale monitors). Dave Haynie has talked bout them from time to time:
The monitor can display four grey levels at 1008x800. This is
because the monitor only has two bitplanes.
The brightness ratios of the four grey levels are moderately
adjustable by the user via the contrast control, however the 'best'
setting, with contrast all the way up, results in ratios of
0 : 0.25 : 0.5 : 1.0. Note that this is non-linear.
In the case of a 200 line display, or a deinterlaced 400 line
display, you get the same four solid colors, and an additional four
levels of brightness created by dithering two of the four solid colors
together on adjacent scan lines. The dithered colors come about because the
monitor always scans 800 lines, so each pair of displayed lines comes
from a single output line from the Amiga. 400 lines in, 800 lines out ;
one line in, two lines out.
Each of the four hundred input lines creates two monitor lines,
each of which can be any one of four intensities. The monitor uses the
R,G,B and I DIGITAL outputs from the amiga. R and B are always mapped to
the even lines as most and least significant bits. G and I are always
mapped to the odd lines as most and least significant bits.
This means each displayed line or pixel can be mapped to one of
sixteen values the monitor hardware can see. Unfortunately, the user
cannot really distinguish between things like "display full white on
only the 400 odd lines" and "display full white on only the 400 even
lines".
So lets build a table, and see what falls out:
RBGI Brightness Brightness Average
Even Lines Odd Lines both lines
0000 0 0 0 solid
0001 0 1/4 1/8 dithered
0010 0 1/2 2/8 dithered
0011 0 1 4/8 dithered
0100 1/4 0 1/8 dithered
0101 1/4 1/4 2/8 solid
0110 1/4 1/2 3/8 dithered
0111 1/4 1 5/8 dithered
1000 1/2 0 2/8 dithered
1001 1/2 1/4 3/8 dithered
1010 1/2 1/2 4/8 solid
1011 1/2 1 6/8 dithered
1100 1 0 4/8 dithered
1101 1 1/4 5/8 dithered
1110 1 1/2 6/8 dithered
1111 1 1 8/8 solid
Looking at the average column, you can see four solid colors
at 0/8, 2/8, 4/8, and 8/8 brightnesses. This is the same
as the aforementioned 0 : 0.25 : 0.5 : 1.0 ratios.
In addition, you can also create dithered colors at 1,2,3,4,5 &
6 eighths. Note that there is NO 7/8 value in the table.
So we concluded that you can get 8 discernable brightnesses in
deinterlacer mode. 0-6 eighths, and full white.
Of course, two of the solid colors have dithered equivelents
which 'look' different than the solid versions. Further, there
are several ways to create some of the dithered only levels.
These also look marginally different. However, there still
remains only 8 distinct brightnesses.
I hope this clears things up a tad.
Hedley