Path: utzoo!attcan!uunet!cbmvax!hedley
From: hedley@cbmvax.UUCP (Hedley Davis)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Hedley monitor, A2024
Message-ID: <4477@cbmvax.UUCP>
Date: 11 Aug 88 16:09:42 GMT
References: <880806105940.002@Janus.MRC.AdhocNet.CA>
Reply-To: hedley@cbmvax.UUCP (Hedley Davis)
Organization: Commodore Technology, West Chester, PA
Lines: 87

In article <880806105940.002@Janus.MRC.AdhocNet.CA> svermeulen%Janus.MRC.AdhocNet.CA@UNCAEDU.BITNET (Steve Vermeulen) writes:
>Regarding Hedley Monitors:
>
>Do you mean that you can think of ways of dithering to simulate 32 grey
>levels or have you received information from Commodore that says you can
>display 32 grey levels directly??
>
>
	The answer is that you can display four solid looking colors in
1008x800 mode. When the monitor is acting like a deinterlacer for normal
amiga displays, you can get eight colors, of which four are dithered.

	The rest of this note is the gory details of marginal interest.

Hedley




	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