Re: Apple video question?

[steve@dosius.zzn.com (Dosius)]

Holger Picker <pickerh@uni-muenster.de> wrote in message news:<Pine.A41.4.44.0302202055160.86610-100000@zivunix.uni-muenster.de>...
> On 20 Feb 2003, Dosius wrote:
> 
> > Date: 20 Feb 2003 08:32:43 -0800
> > From: Dosius <steve@dosius.zzn.com>
> > Newsgroups: comp.sys.apple2
> > Subject: Re: Apple video question?
>  
> > (But the //e had something like the VBL - although AFAIK it worked
> > only on the //e and not even on the //c.  It was fake, however - there
> > was no real VBL. ;))
> Vertical Blank softswitch (AppleIIe, but not Apple//c): yes. Interrupt: no.
> You would have to write a waiting loop like
> blank1:	LDA $c019
> 	BPL blank1
> blank2:	LDA $c019
> 	BMI blank2
> 	...
> which was very time consuming. IIRC, "Karateka" used something like this for
> smooth scrolling (but tested for an AppleIIe before that). As someone
> mentioned before in a different thread, the C64 in comparison had the
> ability to program its video chip in such a way that it caused an interrupt
> signal as soon as the position of the cathode ray reached a certain, given y
> position.
> Uh, what do you mean by "fake"?

I mean that it wasn't an interrupt-driven thing like the C64.

> 
> > > Please note also the trick the programmers invented here: They used the
> > > scrolling loop as a delay loop for the sound generation. By combining sound
> > > and scrolling loop, they were able to reduce the amount of clock cycles a
> > > lot. (This technique was also used e.g. in "Boulder Dash".)
> > Sounds like Mel. ;) (see
> > http://catb.org/jargon/html/The-Story-of-Mel.html for info)
> You think that is weird? Well, you haven't seen nothing yet. :-)

!!!

> 
> > > (If you've got one of the following AppleII emulators for the PC, either
> > > ApplePC or Dapple, you can take a look at this by switching the emulator
> > > output to double view i.e. showing both hi-res pages at the same time.)
> > BTW - Dapple ][ - the original Dapple couldn't do double view. ;)
> Oops. Sorry. :-)

NP. ;)

> 
> > I also think you can do double-view green monochrome in Dapple ][.
> Yes, you can. Double-view works with all graphic modes. It is just not
> possible to use it together with scanlines.

Makes sense.

> 
> > > Another technique was dithering.
> > Didn't a lot of graphical text adventure games do this?
> Yes, but "Death in the Caribbean" is special in that way that it uses
> patterns that stretch over 4 lines instead of only two. There are far more
> different patterns found in this game than anywhere else (as far as my
> knowledge is concerned).

Oy.  Never seen anything *that* complex before.

> 
> Kind regards
> 
>   Holger

-uso.