Re: Apple video question?

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

Holger Picker <pickerh@uni-muenster.de> wrote in message news:<Pine.A41.4.44.0302201035540.124000-100000@zivunix.uni-muenster.de>...
> On Thu, 20 Feb 2003, Dan Ritter wrote:
> 
> > Date: Thu, 20 Feb 2003 09:05:20 GMT
> > From: Dan Ritter <dritter@angelfire.com>
> > Newsgroups: comp.sys.apple2
> > Subject: Apple video question?
> >
> > I was wondering how the Apple2 generated video?  Is it all software driven?
> > Because I remember playing some really good games on my old Apple.  The
> > video seemed very fast for software generated.
> >
> Hello,
> 
> yes, the graphic is all software driven. There is no hardware support for
> the programmer: no sprites (as found on the C64), hardware acceleration
> (like blitter), no raster interrupt etc. 

(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. ;))

> The speed of many games was indeed
> surprisingly fast, although the programmers hade to cope with a rather
> chaotic organisation of the video memory in addition. For example: For
> positioning a so-called shape (i.e. a spaceship or a monster) to all 280
> possible x coordinates you would need 14 different shapes. On the C64,
> however, which had a more "logically" structured video ram, you would only
> need 8 shapes for a resolution of 320 pixels. It was a demanding task to
> find out the correct byte of the video memory and within this the correct
> bit if you wanted to plot a single dot on the screen. (Most of the draw
> line function found in the AppleII rom deals with just that.) Nevertheless,
> the programmers were able to take advantage of this and combined the 14
> different shapes with 14 different looking images so that, while moving the
> shape around the screen, the shape was also animated.
> (BTW.: Things were getting even worse with double hi-res. Here the bitmap
> is split into two halves, and you need to switch between the two by poking at
> special memory softswitches. This is the reason why double hi-res was seldom
> used for action games. Programs like "Prince of Persia" or "Wings of Fury"
> used double hi-res to display the title screen, but the arcade graphic was
> still done by using the standard hi-res mode.)

DHGR is a nightmare.

<snip>
> 
> This very long copying loop had to be written twice: one for hi-res page 1
> and one for hi-res page 2.
> 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)

> Many games ("Karateka", "Drol", "Wings of Fury" etc.) also used double
> buffering, so the user could not see how the shapes were drawn to the
> screen. Even if the frame rate dropped very low (e.g. with flight/space
> simulators like "Flight Simulator II" or "Space Rogue"), the user would only
> see finished pictures, but never how they were drawn. (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. ;)  (I
also think you can do double-view green monochrome in Dapple ][, which
you can't do in ApplePC because it ties green monochrome and scanlines
together.)

> Another technique was dithering. Lots of games (especially graphic
> adventures) used various patterns to create the impression of having more
> than 6 colours on the screen. (A good example for this probably is "Death in
> the Caribbean".)

Didn't a lot of graphical text adventure games do this?

> All in all, I personally still admire the work that was done by the AppleII
> programmers. Very creative, very inventive.

Of course, when you have so little to work with.  O for those days
again. :D

> Kindest regards
> 
>   Holger

-uso.