Subject: Re: DHGR Video Riddle Solved!!!
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From: Holger Picker <pickerh@uni-muenster.de>
Newsgroups: comp.sys.apple2,comp.emulators.apple2
Date: Sun, 10 Sep 2000 20:49:39 +0200
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On 10 Sep 2000, David Wilson wrote:

> Date: 10 Sep 2000 11:24:02 +1100
> From: David Wilson <david@uow.edu.au>
> Newsgroups: comp.sys.apple2, comp.emulators.apple2
> Subject: Re: DHGR Video Riddle Solved!!!
> 
> Matt Jenkins <mdj@bilby.cs.rmit.edu.au> writes:
> >In comp.sys.apple2 Jeff Blakeney <CUTblakeney@home.com> wrote:
> >> By the way, I'm only aware of one DHR mode.  560x192.  The number of
> >> colours you get depends on whether you view it on a colour or
> >> monochrome monitor.  If viewed on a colour monitor, you will get 16
> >> colours but if you view it on a monochrome monitor, those 16 colours
> >> appear as different dot patterns and you can see all 560 horizontal
> >> dots.
> 
> >That's not correct. There is indeed *two* modes of DHR, 560x192 BW and
> >140x192 16 color. Whether a byte in the video area is interpreted as
> >color pixels or BW pixels is dependant on the high order bit of that byte.
> 
> But only if you have an RGB card. If you use composite video on a //e bit 7
> is ignored in DHR mode.
> --
> David Wilson  School of IT & CS, Uni of Wollongong, Australia
> 

Hello,

(Uh, this is my very first posting to a newsgroup. Please have patience with me and my poor English.)

If I understood this thread right, there seems to have been the question whether the high-bit in double-hiresmode influences the output of the screen, either monochrome, colour or something inbetween. To find proof of such a behaviour I've programmed my emulator in such a way that it tells me the value of the high-bit. Then I took a look at some well-known programmes whether they keep the high-bit on or off.
Here are my findings:

Text	= Characters (should typically use monochrome mode for higher resolution)
Graphic	= Pictures, shapes etc (should typically use coulor mode)
BW = black characters on white background
WB = white characters on black background

			Text		Graphic
Airheart		off		off
Battlechess		off		off
California Games	off		off
Dragon Wars		BW on		off
			WB off
Maniac Mansion		off		on
					every fourth column off(?)
Might&Magic 2		off		off
					every fourth column on(?)
Neuromancer		BW on		background on
			WB off		shapes off
Pirates			on		on
Rampage			on		on
Testdrive		off		off
					three bright columns on the car-select page
					have the high-bit set. Reason unknown.
Transylvania		off		on

Conclusion:
As you can see from this table, there doesn't seem to be any evidence of a systematic usage of the high-bit as a switch between different resolutions. Even if it was built into the hardware it was definitely not used by the programmers. 'Neuromancer' and 'Dragon Wars' show that the setting of the high-bit was rather influenced by the character set, not the resolution.
In addition, 'Neuromancer', 'Maniac Mansion', 'Might&Magic 2' and especially 'Testdrive' show that programmers even mixed both high-bit on and high-bit off within the same picture but without any relation to the actual content of the picture, only perhaps as a result of some programming algorithm (e.g. shapes).

Cheers

Holger