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Old 14 April 2010, 13:54   #1
john4p
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Question Overscan

What exactly is overscan? How did it work for OCS-Amigas?

The usual PAL resolution for "fullscreen" OCS games is 320x256, but whilst playing Project X ('93 Edition) in WinUAE I saw the game draws 285 (*2) lines:



How is that possible? What's the maximum possible overscan mode (for playable scrolling games)?

Also the additional pixels don't seem to harm performance (apart from enemy sprites seeming to skip frames if there are too many) - so why didn't everyone use overscan?

Last edited by john4p; 14 April 2010 at 14:06.
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Old 14 April 2010, 13:58   #2
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http://en.wikipedia.org/wiki/Overscan
Max Amiga overscan : http://eab.abime.net/showthread.php?t=45691
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Old 14 April 2010, 14:05   #3
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Thanks.

So 576/2 = 288 (> 285)...
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Old 14 April 2010, 14:07   #4
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Quote:
Originally Posted by john4p View Post
So 576/2 = 288 (> 285)...
The wiki for A500 even says : Overscan mode enabled 700 x 600 resolution in PAL machines. (http://en.wikipedia.org/wiki/Amiga_500)
Guess there is a 'max used' resolution though
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Old 14 April 2010, 14:30   #5
Toni Wilen
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600 _visible_ lines is impossible in standard video modes.

Quote:
Also the additional pixels don't seem to harm performance (apart from enemy sprites seeming to skip frames if there are too many) - so why didn't everyone use overscan?
More pixels = more display DMA slots needed, DMA time needed increases linearly, less time for CPU/blitter, more memory needed..
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Old 14 April 2010, 14:31   #6
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@TCD: Okay, thanks.

I wonder why only Team 17 and PD-developers used overscan. Wikipedia says something like "only non-overscan-modes could be assured to be fully displayed by every display" - but I've never seen a display used with Amigas which couldn't fully display Team 17's games.

edit:
Quote:
Originally Posted by Toni Wilen View Post
More pixels = more display DMA slots needed, DMA time needed increases linearly, less time for CPU/blitter, more memory needed..
There's the answer - thank you!
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Old 14 April 2010, 14:36   #7
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Lazy developers, developers didn't care, direct ports from another platform, developing for lowest common denominator etc..
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Old 14 April 2010, 14:44   #8
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Well, while I'm not too fond of the gameplay of some of Team 17's games I have to say it seems like they were really dedicated Amiga developers. (Although they didn't seem to care much about parallax scrolling. )
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Old 14 April 2010, 14:46   #9
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Quote:
Originally Posted by john4p View Post
@TCD: Okay, thanks.

I wonder why only Team 17 and PD-developers used overscan. Wikipedia says something like "only non-overscan-modes could be assured to be fully displayed by every display" - but I've never seen a display used with Amigas which couldn't fully display Team 17's games.
You might want to try hooking up some tvs to the amiga. then youll notice the image not quite centred and lose the overscan area at least one side. Sure you can take the tv apart and adjust the pots, but its not a suitable solution for every tv .

A well designed game would make sure the entire game area is accessable on any display, and justuse the overscan for added effect, not loosing anything crucial if display doesnt fit. I remember settlers on on tv got shifted so far to the left, I couldnt see the msg icon. only able to click it as i knew roughly where it was.
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Old 14 April 2010, 14:54   #10
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Now that sounds annoying - didn't know about that. Guess I was lucky with my (RGB capable) TV back then.
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Old 14 April 2010, 15:36   #11
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A few threads that I had bookmarked and dealt with overscan, if you want to find additional references of games using it (besides Settlers mentioned by Mad-Matt):
http://eab.abime.net/showthread.php?t=20007
http://eab.abime.net/showthread.php?t=24381
http://eab.abime.net/showthread.php?t=47875&page=2
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Old 14 April 2010, 16:12   #12
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Thanks. Let me add that also Pinball Dreams & Fantasies used a slight overscan: 261 (or 262?) lines instead of 256.
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Old 14 April 2010, 19:10   #13
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I seem to remember Pacmania using overscan too, it certainly did on the Acorn Archimedes!
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Old 15 April 2010, 03:06   #14
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368x290?

I gave up trying to find the correct answer anyways.
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Old 15 April 2010, 07:40   #15
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Well...
Sandard 15kHz modes constist of ~15625 (pal standard) to ~15750 (ntsc standard) lines.

On PAL systems you usually use 50Hz.
15625 lines / 50 hz = 312,5 lines per run.

1 s = 1.000 ms = 1.000.000 µs

A somewhat standard value for the vertical flyback is around 1,4 ms, though thats slightly different from screen to screen.

1.000.000 µs / 50 Hz = 20.000 µs per run
20.000 µs / 312,5 lines = 64 µs per line.

1,4 ms = 1.400 µs / 64 µs per line = 21,875 lines (lets round to 22 lines).

312 per run - 22 lines vertical flyback = 290 lines left for visible screen.


P.S. Please note that it is possible to use more lines IF you increase the total line count in pal to 15750 lines and/or using less than 50Hz.

(15750 / 50 = 315; 20000µs / 315 = 63,5µs per line | 1.400 / 63,5 = ~22 lines | 315 - 22 = 293 lines visible)
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Old 15 April 2010, 07:48   #16
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Addendum:
As there is additional data transfered on the first lines (Video Text, Time of Day, 16/9 and 4/3 mode etc.) usually you can't see everything (only about 80%) on a TV.

(try hooking a Satelite tuner to a 1084 to see the video text data)
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Old 15 April 2010, 11:34   #17
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Note that standard PAL Amiga max visible lines is "only" 284 (if I remember correctly) because Denise only outputs background color during blanking lines (even if DMA or CPU writes to BPLxDAT registers) and it is Agnus that decides the line type (via horizontal strobe registers).

"Standard PAL Amiga" = no ECS Agnus programmable blanking/refresh rates used.
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Old 15 April 2010, 12:05   #18
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Max normal is 724x283 non-interlaced hi-res PAL, 566 if laced, at least it is/was in my A1200 Kick 3.0, Workbench 3.0.

Overscan Prefs uses hi-res non-laced for setting up the display, lo-res just halves the horizontal (ed: by changing the timing), lacing just doubles the lines (ed: by introducing flicker), nothing else in the system changes (ed: I think).

Last edited by meega; 15 April 2010 at 12:15.
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