| Posted By
Charles
on 2021-12-07
18:43:26
|  Weird screen artefacts
Hi Team,
Have you ever noticed weird flashing artefacts / unexpected colors when displaying only slightly different color pixels next to each other and fine-scrolling the screen (both in X and Y dirs)? I am developing a small shooter game but this drives me mad. On emulators it all look correct, but on the real Plus/4 the bug appears consistently. Using S-video (with a decent HDMI ocnverter) or composite out. May be something with my TVs or the Plus/4 itself? All games/demos play quite nicely though using the same HW configuration so I suspect some more generic issue.
Any help is appreciated,
Karoly
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Posted By
siz
on 2021-12-08
03:34:12
| Re: Weird screen artefacts
I use a RetroTink2X to convert S-Video to HDMI and these artifacts are present too. I guess it's a side effect of upscaling and/or deinterlacing. I'll check it again tonight and will take a look if it's present with a C64 as well.
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Posted By
MIK
on 2021-12-08
04:16:31
 | Re: Weird screen artefacts
Yeah just like a real C64 there are some odd interference lines going down the screen on C16/Plus4 at the edge of each character with some lighter screen colours like Grey or Blue making it look even worse. And yep they show up the worst with S-Video or BNC. Composite Video tends to be a little better with the lines but you get pixel colour bleeding with some colours which is nasty to look at so we try to avoid using it.
RF is the only one where these lines all but disappear on both C64 and C16/Plus4 as it has an overall darker picture, or a loss of quality because the sound is on the same cable. Sound is also louder on RF and it also adds more bass to the tone.
Setting up a screen so the picture looks it's best on a real C16/Plus4...
Contrast and colour are easy to fine tune to your taste but what I do is use the Novaload turbo loader to set the brightness.
The main screen colour of Novaload is like a dark charcoal grey, if you see black your screen is too dark so up the brightness until you can just about see that it is a dark charcoal grey. Get this right and those screen lines won't look as bad with the majority of colours. Just make sure black looks black and the Novaload screen with text on it is a dark charcoal grey.
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Posted By
Charles
on 2021-12-08
15:12:28
| Re: Weird screen artefacts
Hi Guys,
Thanks for your response. I was afraid this is not something as simple as a faulty capacitor, indeed greyish-blue artefacts, very annoying. Would these appear also on an original Commodore monitor? I have never owned one.. I will test RF, as composite is also terrible indeed.. I wonder what can be behind these artefacts, a technician advised one day that Plus/4's video output is not even fully compliant to S-video standard (unlike C64 - but I have doubts as he was quite prejudiced  ). Did you investigate under which conditions this bug appears? Weird enough but it looks to me the backround color (FF15) is more prone to cause it than ff16, ff17 or foreground. Dunno yet why..
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Posted By
MIK
on 2021-12-08
18:12:02
| Re: Weird screen artefacts
Unfortunately Commodore monitors also show the lines if using the Luma/CVBS Chroma lead, from memory if the screen is a light blue the artefact shows as a darker blue. I guess it's shocking to see in any form but you do get used to it if your playing games.
And yeah it's not just a C16/Plus4 issue the C64 suffers with it using the same video leads, maybe not as bad but it's also there.
And indeed I recall reading once that Ted machines don't support S-Video yet it works.
If you have a look at some of the images under the "Let's Play (2021 #3)" topic your just about see I have an old Commodore Monitor but annoyingly I resize the picture which hides the lines. That said Luca posted a Jetbrix picture and you can see the lines on his screen on a black background which I believe is a commodore monitor.
Any screen shots of me showing a big TV is S-Video, but again images have been resized. 
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Posted By
Charles
on 2021-12-09
12:46:06
| Re: Weird screen artefacts
I will need to investigate the limitations further, my current work in progress is supposed to be a game some day, but with these glitches it would be barely enjoyable I wanted to use ground and grass terrain but the amount of artefacts is overkill. If I use "cleaner" terrain patterns it might work, but I was wondering if we can benefit from colors close to each other, making the terrain look more natural.
This is in YAPE:
Wanted to show the same on TV, but cannot double-post or add another image here :)
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Posted By
MIK
on 2021-12-09
15:27:39
| Re: Weird screen artefacts
Neat pattern, kinda reminds me of a snooker table cloth. Has a cool smooth look to it.
Maybe MicroProse Soccer on C64 could help with ideas for grass? Or something on the Amiga like Sensible Soccer, Cannon Fodder or Sensible Golf. Or games that have a desert looking ground you could change to green to see how it looks.
Maybe there is a proper way to add extra pictures after you have posted but what I do...
I start a reply to a different topic and drop my new picture into the text box to get the html link..., then copy that html link into the post I want to edit.
Just don't post that dummy reply you used to upload the new picture. 
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Posted By
Charles
on 2021-12-10
16:51:48
| Re: Weird screen artefacts
Hi MIK,
Thanks for the tips, I will have a look at them later.
This is how the "grass" looks on TV:
Blue, purple, red pixels... Quite colorful, I would not even mind if it was steady
And this is the idea, screenshot from a charset editor on PC: (C64 colors so different shades obviously)
What I realize now that even the YAPE screenshot (prev post) has more colors than expected but at least it's not flashing. I really wonder why it's like that, do you have any idea by chance..?
Many Thanks,
Karoly
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Posted By
Luca
on 2021-12-10
17:04:34
 | Re: Weird screen artefacts
Mmmh... Maybe I'm missing the point here, I'm almost sure of that. Are you guys talking about the usual metamersim which occurs in the CRT tv sets and monitors, and which is usually kept in count every time we draw graphics on 8bit machines since the beginnings? I mean, the same effect which has his demo dedication in Metamerism, and you can see in techdemos like Colmix3?
I didn't posted previously on this thread because I was almost sure that I didn't understand the real point...
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Posted By
MIK
on 2021-12-10
18:20:30
| Re: Weird screen artefacts
@Charles
Your first picture with the blue is different. I like the second picture, nice and clean looking. Hopefully you will find something that is perfect for what your after.
@Luca
You posted the perfect example. The darker black ghost lines running down the screen spoiling what should just be a perfect black background. I've only ever seen them on C64, C16 & Plus/4. You don't really see them using the RF lead but of course there is a loss in picture quality via RF.
These strange artefacts are not seen on ZX Spectrum, Amstrad CPC or 8bit games consoles. It's like they run down the edge of each character block going across the screen. With light grey and blue colors it can look much worse.
Hopefully you can see these interference lines in your photo below.
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Posted By
Charles
on 2021-12-12
08:52:42
| Re: Weird screen artefacts
Thank you both for your help.
@Luca,
I might have been lame 30 years ago, but definitively did not count with metamerism in my proggys Though I think this might be something different or not the full story. Colmix3 shows diff color lines to generate diff (fake) perception of colors (from a distance?), but in my case the actual individual pixels interfere with each other seemingly, making valid different coloured pixels on the screen. Or this might be the metamerism how it's perceived and how it's also emulated by Yape? Metamerism demo is awesome, not sure where to look for actual metamerism in it but purely awesome.
In the end the real problem is what I showed how it looks like on the TV, vastly different salient blue/purple colors, and they are changing / flashing as you do fine scroll in either X or Y direction. The result is horrendous. This might be due to what MIK is suggesting - or combined with metamerism? - , but they indeed occur around character edges. I will need to get an RF cable and play with diff colors to see if I can reduce the artefacts. Ideally a proggy should look nice on both emulators and a real +4 machine - using modern TV sets I've just not seen this before and was stunned by the extent of it - so initilally I was suspecting my HW setup.
@MIK, what do you mean by my first pic with the blue is different? It's taken of the same pattern, maybe slightly different screen area, but definitely the same pattern.
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Posted By
MIK
on 2021-12-13
17:45:40
| Re: Weird screen artefacts
The English language can be stupid at times as a single word can have multiple meanings and when it's text you don't hear how it's being said. Different can also mean "interesting" if it's said when your looking at something, and interesting in a good way.
Different can also mean "surprising" if you look at something and say it, like "That's different", not seen before.
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