HW free Overscan on the GEM desktop possible!

General Discussion, STOS.

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mrbombermillzy
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HW free Overscan on the GEM desktop possible!

Post by mrbombermillzy »

Found this interesting thread in the German Atari forums: https://forum.atari-home.de/index.php/t ... 900.0.html

It seems there's some preliminary STFM drivers for both a low and Hi-RES ST overscan for the GEM desktop which are enabled from the AUTO folder.

The ones available on the forum are for TOS 1.2/1.4? (and the German version at that), but it seems @thorsten.otto is on the case, trying to patch the hardcoded offsets to make them more language independent.

However, it seems that some help (with at least testing) is needed as the progress has apparently stalled, so I thought some folks on here might like to know about this and possibly help.

Would be fantastic to have this for our machines.
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JezC
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Re: HW free Overscan on the GEM desktop possible!

Post by JezC »

@mrbombermillzy Very interesting...would love to see this get somewhere close to working for other TOS versions.

I keep meaning to build an overscan board for some of my ST's but life seems to always find ways to defeat my planning!

If it's just helping with some testing then I can run a prg & report back ;)
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mrbombermillzy
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Re: HW free Overscan on the GEM desktop possible!

Post by mrbombermillzy »

Well, I've pinged Thorsten, and I've pointed the thread out here, so maybe interest can be rekindled.

There's also an article on an earlier version of the software here and details on how it works: https://www.stcarchiv.de/stc1992/04/screen-plus (in German).
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Re: HW free Overscan on the GEM desktop possible!

Post by troed »

(It's not possible to "open the lower border" in monochrome. I'm trying to remember what it is the demo really does, but in any case it would not work in GEM)

Otherwise, yes, I remember using a program myself back in the day that worked fine in GEM with lower border open. Disk access turns it off though.
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mrbombermillzy
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Re: HW free Overscan on the GEM desktop possible!

Post by mrbombermillzy »

My German language comprehension is not great (i.e. non existent!) and the translator I use isn't perfect, but it seems between Christian (czietz) and Thorsten, there was some progress made towards the end of the thread for monochrome bottom border removal.

From what I can gather, he was coding 'blind' on Hatari (as it cannot do mono overscan) so needs testers.
czietz
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Re: HW free Overscan on the GEM desktop possible!

Post by czietz »

mrbombermillzy wrote: Tue May 10, 2022 9:18 am but it seems between Christian (czietz) and Thorsten, there was some progress made towards the end of the thread for monochrome bottom border removal.
Not really, unfortunately. Thorsten's initial attempt for monochrome did not work: on an SM124 this happens:



... whereas a LCD monitor just says "no signal". I had identified one crucial thing Thorsten missed while recreating the code seen in the demo, but iirc he did not make any changes to his program afterwards.

But even then, I think @troed is right w.r.t. it not working in GEM: I think I understand what the demo does and I expect this to cause massive artifacts on the Desktop. (The demo hides them with a completely black background.)
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mrbombermillzy
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Re: HW free Overscan on the GEM desktop possible!

Post by mrbombermillzy »

@czietz Thanks for confirming Christian.

I would never argue the finer points of ST overscan effects with @troed. ;)

However, I do try to keep an open mind in case future knowledge can change older limitations that couldn't be overcome back in the day.

It seems like this one is still unconquerable then. :(
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Re: HW free Overscan on the GEM desktop possible!

Post by troed »

mrbombermillzy wrote: Tue May 10, 2022 7:22 pm However, I do try to keep an open mind in case future knowledge can change older limitations that couldn't be overcome back in the day.
It's boring now when Ijor has dumped the chips ;) The exact same position (= GLUE code) you _would_ enable lower border is unfortunately the same position that enables hsync. Thus, you can't perform a sync-clean opening**.

In the same vein it's not possible to open the top border (caveat, I have something I've been thinking of but I give it a slim chance of being sync clean) or the left border*. The right border is possible and I have a test program that does it.

*) Well, you can "preload" the Shifter so that it starts displaying pixels earlier than usual. If not this demo than there is another mono demo that does precisely that.
**) Well ... I have a theory that you can do something completely different to "border opening", but would still cause graphics to end up there. Let's see when I find time to explore.
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Re: HW free Overscan on the GEM desktop possible!

Post by czietz »

Patching the "Mono-No-Border Screen" demo (https://demozoo.org/productions/73157/) to run with an inverted screen, i.e., white background, explains what I meant:

IMG_4958.jpg
IMG_4958.jpg (85.76 KiB) Viewed 1777 times

As you can see: Yes, it manages to display the scroll text below what would normally be the lower border of the monochrome screen. But it doing so, it massively messes with HSYNC, causing one very long line and, thus, very visible artifacts. You can see the analog electronics in the deflection circuitry of my poor SM124 slowly settling again during the next lines. I guess this is what troed is referring to when he says "you can't perform a sync-clean opening" [in monochrome]. The demo just hides all that in the black background; but you can't use it with arbitrary programs.
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Re: HW free Overscan on the GEM desktop possible!

Post by troed »

czietz wrote: Tue May 10, 2022 8:29 pmit massively messes with HSYNC, causing one very long line
Yeah, exactly :)
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