overscan

[Idico]

I think that overscan is one of the finest feature of 8-bit Atari and, since it is built in feature, it is for free (i'm not counting additional cycles stollen by Antic of course) from the 6502 programmer point of view.

I find that feature espacially important when playing with the real hardware.

Many games use higher then normal (192/200 lines) screen but I only know two games (Cavernia and Invasion) which use wide screen.

Is there more ?

Do you like wide screen in games ass much as I do?

What about other 8-bit platforms? Are they capable of overscan?

(I know C64 is kind of capable. Is it possible to use it in scrolling games on C64?)

[TMR]

Do you like wide screen in games ass much as I do?

 

To be honest, no; some televisions clip off overscanned screens and having to rebuild all my software sprite multiply tables isn't my idea of a fun evening so i've never tried using it... =-)

 

What about other 8-bit platforms? Are they capable of overscan? (I know C64 is kind of capable. Is it possible to use it in scrolling games on C64?)

 

The C64 can scroll stuff through the side borders but it's very processor intensive and requires sprites in the border areas so isn't particularly good for games and has never been used to date. The Plus/4 can play with it's screen layout far more and is able to open the side borders for text or bitmap screen use, although again it's processor intensive and there aren't any overscanning games on the Plus/4 that i'm aware of.

 

The VIC20 and Amstrad CPC are both capable of being more flexible with screen dimensions than the Atari; both can fill the entire screen with an image, reduce their screen size or pretty much do whatever the programmer wants them to. The VIC hasn't got hardware smooth scrolling, so games like Dragonwing works by expanding the screen horizontally until it's bigger than the majority of televisions and then using the VIC's horizontal screen positioning feature to simulate it.

[Heaven/TQA]

"The VIC20 and Amstrad CPC are both capable of being more flexible with screen dimensions than the Atari; both can fill the entire screen with an image, reduce their screen size or pretty much do whatever the programmer wants them to"

 

well... atari can fill overscanned screen with everything you want... but i know what you mean with "more flexible" in terms of size on the vic 20... as you can set collum&row values more precisly than in the GTIA...

 

and yes i love overscan... as this is a feature all other platforms at that day (except amiga) werent capable to do without nasty tricks (esp. c64 and atari ST) and i love overscan demos on ST f.e. european demos by overlanders and friends...

[emkay]

 

 

and yes i love overscan... as this is a feature all other platforms at that day (except amiga) werent capable to do without nasty tricks (esp. c64 and atari ST) and i love overscan demos on ST f.e. european demos by overlanders and friends...

 

The truth is that the A8 is only capable of overscan in NTSC. To gain full PAL overscan some nasty tricks will be needed too .-)

 

The horizontal overscan is a cheap one when using scrolling. So at 40 bytes of width and active scrolling lines, there will be readen 48 Bytes even if borders are on or off.

The register for DMA and screenwidth can be set by DLI every screenline different, and with easy DL "commands" you can set every different possibility, ANTIC is able to do ... and ... straight timing for changing any registers is easy done by "sta wsync".

 

How much more easier could that be on other machines?

[TMR]

well... atari can fill overscanned screen with everything you want... but i know what you mean with "more flexible" in terms of size on the vic 20... as you can set collum&row values more precisly than in the GTIA...

 

You can also set horizontal or vertical positioning; i have beta code i didn't bother to release (Britelite/Dekadence beat me to it =-) of a VIC20 plasma that takes up the entire visible screen area and the only thing needing CPU time is the plasma itself because the VIC doesn't even have badlines! If you want something daft like 27 characters wide by 3 high at the bottom right of the visible screen that can be done too; four POKEs in all from BASIC will do the job, no need to play with the display list and no loss of CPU power to process the extra data required.

 

Try this into WinVICE's xVIC emulator;

POKE36864,6:POKE36865,14:POKE36866,156:POKE36867,200

[analmux]

 

 

and yes i love overscan... as this is a feature all other platforms at that day (except amiga) werent capable to do without nasty tricks (esp. c64 and atari ST) and i love overscan demos on ST f.e. european demos by overlanders and friends...

 

The truth is that the A8 is only capable of overscan in NTSC. To gain full PAL overscan some nasty tricks will be needed too .-)

 

The horizontal overscan is a cheap one when using scrolling. So at 40 bytes of width and active scrolling lines, there will be readen 48 Bytes even if borders are on or off.

The register for DMA and screenwidth can be set by DLI every screenline different, and with easy DL "commands" you can set every different possibility, ANTIC is able to do ... and ... straight timing for changing any registers is easy done by "sta wsync".

 

How much more easier could that be on other machines?

 

What do you mean???

 

Overscan is as easy as:

 

poke 559,35

 

nothing more. And it works fine on my PAL machine

[emkay]

 

What do you mean???

 

Overscan is as easy as:

 

poke 559,35

 

nothing more. And it works fine on my PAL machine  

 

Full width and 240 lines. In NTSC you have "full screen"

On PAL machines you have two black lines ... one on the top and one on the bottom of the screen.

For fullscreen on PAL you must have 256 lines (or more).

[Heaven/TQA]

ehm... in terms of demo scene overscan means displaying grafix in the "borders" where the "standard" gfx of the machine can't...

 

--> Atari ST has ugly green borders

--> c64 has ugly blue borders

--> vic 20 has more ugly blue borders

--> spectrum has borders

--> atari XL has borders (but not as big as the others...)

--> CPC i'll have to check...

--> BBC i'll have to check as well...

 

did i missed one 80s machine?

 

so a simple "poke 559,35" in atari basic is imho "overscan"...

[emkay]

ehm... in terms of demo scene overscan means displaying grafix in the "borders" where the "standard" gfx of the machine can't...

 

 

Yes.... but, when looking at the overscan-abilities on the AMIGA, every PAL AMIGA can "overscan" over the full screen. The A8 never had full PAL abilities. So the overscan is "limited" to 240 scanlines

 

 

--> Atari ST has ugly green borders

 

 

Wasn't it a "little" green desktop with white borders?

[Heaven/TQA]

damned... yes are right... haven't switched on my 1040ste since 5 years... and on steem i boot demos...

[Idico]

I've just saw Turrican for the Amigas and ST (unfortunatelly using emulators not the real thing).

I much preffer Amiga's version of this game over ST mainly due to the overscan.

Comparing the two I must say that playing ST version is like watching Amiga screen through the peephole (the same applies when comparing 8bit Atari's Black Lamp to the next relise of the same developers eg Cavernia)

I hate that so small number of ST games use overscan (probably due to the fact that it was discovered to late) but what I hate even more is that this feature was so often forgotten by 8bit programmers while it was achievable without any trics.

 

 

emkay said:

The horizontal overscan is a cheap one when using scrolling. So at 40 bytes of width and active scrolling lines, there will be readen 48 Bytes even if borders are on or off.

 

Given that I think all side scrolling games should utilize Atari's wide screen capabilities

It would be nice also to strech the screen horizontally but on my TV I am only able to display 239 lines and then screen starts to roll, so for safty and practical reasons (character modes) I think we should not exeed 232.

I've counted that on my TV streaching the screen (PAL machine) gives me about 25 per cent bigger screen ! Wow !

 

 

Atari was created with big things in mind so it needs big screen to show them

[TMR]

I've just saw Turrican for the Amigas and ST (unfortunatelly using emulators not the real thing).

I much preffer Amiga's version of this game over ST mainly due to the overscan.

 

The Turrican series aren't overscanned, at least not horizontally, they run at 320x256 if memory serves although i'd have to check; count off the tiles in the play area when they're loading and multiply by 16 pixels a tile, that'll give you a rough idea of screen sizes.

 

Given that I think all side scrolling games should utilize Atari's wide screen capabilities

 

There are reasons why they don't, anything dealing with software sprites is already doing a fairly involved job without having to manage a larger screen area; Cavernia doesn't deal with software sprites in the same sense that Black Lamp does, it's nowhere near as complex a game.

[Idico]

Cavernia doesn't deal with software sprites in the same sense that Black Lamp does, it's nowhere near as complex a game.

 

BL has for sure more CPU intensive techicques but it looks to me as an attempt to emulate C64. Quality of this game is impresive by Atari standards but not by C64 standards - except for number of colours of course (and yes I saw c64's BL joke but it proves nothing).

In my opinion Atari should stay Atari and to try to emulate C64 or NES style only if it is possible to do things better then in the original.

 

I'm going to start another thread to discuss what you all like about Atari and what Atarish means to you.

[+rdemming]

I hate that so small number of ST games use overscan (probably due to the fact that it was discovered to late)  

 

Left and right overscan is fairly difficult to use on an ST. To open the borders you need to synchronise your code with the electron beam. At the right electron beam positions you need to kick the video chip. So for each scanline you have code to open the left border, some other code or NOPs and at the end some code to open the right border. Since the timing is extremely critical you need to count the clockcyles of the inbetween borders instructions. Thus it is really difficult to interleave your normal code with the border opening code and works best with code that has a constant execution time. Code that has a variable execution time is extremely hard to interleave since you need code that counteracts the variable execution times. Thats why left and right overscan is almost only used in demos where the effects have a constant execution time. Interleaving game logic code is thus extremely hard.

 

I know of one 3D demo in horizontal overscan but the trick here is that only about half of the screen is in horizontal overscan so the 3d code is executed outside the ovescanned scanlines where no electronbeam synchronisation is necessary.

 

Opening the top and lower borders are much easier. Some games from mainly Thalion/Eclipse games used this (Lethal Xcess, Atomix).

 

Robert