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Forums > C64 Pixeling > dithering question
2023-03-01 11:11
NoiseEHC

Registered: Feb 2005
Posts: 51
dithering question

Hi!

I have been just looking at recently released MULTI graphics, and what I noticed is that the majority of images use horizontal line dithering. For example:
Always Look at the Bright Side of Life

While in the past most images used checkerboard dithering, for example Hein's graphics in Dutch Breeze. But current example:
Birdman

(Note both images contain both type of dithering, just one is much more dominant.)

And very few use diffuse (random) dithering for some reason.

So what is the reason of this difference? Is it only the preference of the artist, or is there an advantage of horizontal line dithering on a TV? I do not have a real C64 anymore, so I cannot check it unfortunately.

Another question: why are there so few diffuse dithering out there? Is this because this is much harder to do, or is there some visible disadvantage?

Thanks
 
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2023-03-03 12:23
chatGPZ

Registered: Dec 2001
Posts: 11148
Quote:
As far as I see, VICE (GtkVICE 3.6.1) already emulates something like that, it would be good to know from a VICE developer if that is the same thing we are talking about here

First, use a recent VICE, not the one from last year. In recent VICE the colors used are from new measurements (made by Tobias) that accurately represent the differences between odd and even lines, and the differences between old/new VIC. Do NOT use any external palettes - because then those things are plain wrong. Do NOT disable double size, because then some aspects of the CRT emulation do not work right. Basically leave everything at their default settings. Preferably use the latest development build from https://github.com/VICE-Team/svn-mirror/releases - then you will also get the recently fixed default for gamma correction (which was not neutral before). And if the colors are too vibrant for your taste, dial down brightness/saturation/contrast all to 100%. It does not make a lot of sense to use a color picker to pick individual pixel colors from the resulting output either :) If you want to use a palette for your gimp experiments, you might want to use the "colodore" palette, which is more accurate than "pepto". (Or ask Tobias for the calculated averaged RGB palette based on his measurements, that will be even more accurate)

Now for what it emulates and how good:
- The vertical color mixing ("PAL" mixing) is very accurate and reproduces all known side effects. That includes "hannover bars", and the differences between monitors that use a delay line only for one color component ("u only delayline" setting). The later is the case for example in the 1084 monitor. As said above, you must NOT use any external palette (or disable double size), or those things will not work right.
- The horizontal color mixing ("color bleeding") is not emulated well, it is basically just some blur. In particular non linear effects like "black bleed" is not reproduced at all, nor is chroma/luma crosstalk (the famous green/red pattern you can see when doing a checkerboard pattern in hires). I believe the current state is closest to what a C64C (new VIC) produces when the modulator has been removed, and the s-video signal is used.

Last not least here are some test programs: https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/crt..
2023-03-09 12:48
NoiseEHC

Registered: Feb 2005
Posts: 51
Thank you for taking the time for this informative response!

Quoting Groepaz
First, use a recent VICE, not the one from last year. In recent VICE the colors used are from new measurements (made by Tobias) that accurately represent the differences between odd and even lines, and the differences between old/new VIC. Do NOT use any external palettes - because then those things are plain wrong. Do NOT disable double size, because then some aspects of the CRT emulation do not work right. Basically leave everything at their default settings. Preferably use the latest development build from https://github.com/VICE-Team/svn-mirror/releases - then you will also get the recently fixed default for gamma correction (which was not neutral before).


Yeah, in the meantime I moved to the latest release, but now I started using GTK3VICE-3.7.1-win64-r43382. The "change" to the "pepto" palette was only my misunderstanding of the settings dialog. I thought that "internal" means that you choose from the internal palettes using the combo box, and "external" means that you have to load the palette with the folder button. The newer UI is not that confusing though. (The best would be to have "internal" the first item in the combo box and remove the radio buttons, but in r43382 it is good enough I think.)

Quoting Groepaz
And if the colors are too vibrant for your taste, dial down brightness/saturation/contrast all to 100%.


I like vibrant colors. My concern was if the colors are not the same as in a C64 monitor, but if these are measured then that is awesome.


Quoting Groepaz
Now for what it emulates and how good:
- The vertical color mixing ("PAL" mixing) is very accurate and reproduces all known side effects. That includes "hannover bars", and the differences between monitors that use a delay line only for one color component ("u only delayline" setting). The later is the case for example in the 1084 monitor. As said above, you must NOT use any external palette (or disable double size), or those things will not work right.
- The horizontal color mixing ("color bleeding") is not emulated well, it is basically just some blur. In particular non linear effects like "black bleed" is not reproduced at all, nor is chroma/luma crosstalk (the famous green/red pattern you can see when doing a checkerboard pattern in hires). I believe the current state is closest to what a C64C (new VIC) produces when the modulator has been removed, and the s-video signal is used.


And here is my problem. In VICE I can create very interesting effects, but I am not that sure now if the display represents reality or not. Or maybe reality has changed in the past to look more like VICE without CRT emulation? :)

Here is a picture from last year's Transmission64 (I asked them and they are recording separate luma/chroma):
https://youtu.be/WwHTp5tI9mY?t=509
The prg file can be found here:
Boimyboi

For example the light red / dark green mix on the neck of the frog looks quite similar both in VICE and on YouTube. But on YouTube the odd/even lines do not look different color at all, while VICE emulates the cheap PAL chroma phase mirroring effect. How can it be? The other difference is that there is some color mixing on YouTube, but it is much less than in VICE. Which one is real?

I am interested in these details because I would like to create some cool effects in my next demo, but it would be kinda pointless if none of the compos would support the real C64 output... :)

Currently I am not 100% convinced if the VICE CRT emulation is the real thing, as I have no access to a real C64 and that way it is quite hard to check it. I will try to find some scene people in New Zealand, as I saw that there were several groups here, and will try to borrow a real C64 and monitor to check my assumptions. In case compos do not look real I can still release my demo just on csdb I guess.
2023-03-09 12:51
Jetboy

Registered: Jul 2006
Posts: 228
Quote: Hi!

I have been just looking at recently released MULTI graphics, and what I noticed is that the majority of images use horizontal line dithering. For example:
Always Look at the Bright Side of Life

While in the past most images used checkerboard dithering, for example Hein's graphics in Dutch Breeze. But current example:
Birdman

(Note both images contain both type of dithering, just one is much more dominant.)

And very few use diffuse (random) dithering for some reason.

So what is the reason of this difference? Is it only the preference of the artist, or is there an advantage of horizontal line dithering on a TV? I do not have a real C64 anymore, so I cannot check it unfortunately.

Another question: why are there so few diffuse dithering out there? Is this because this is much harder to do, or is there some visible disadvantage?

Thanks


> And very few use diffuse (random) dithering for some reason.

The reason is quite simple. It is used when converting ready image, by device that does not care to do a lot of calculations, but for human doing it while painting would be fucking inconvenient and hard to automate. People are lazy and tend to use solutions that are less work for them :) Unless they try to prove a point :) Well, most of the scene prods are doing that though. "So you say it is impossible? Please hold my beer..."
2023-03-09 15:11
chatGPZ

Registered: Dec 2001
Posts: 11148
Quote:

For example the light red / dark green mix on the neck of the frog looks quite similar both in VICE and on YouTube. But on YouTube the odd/even lines do not look different color at all, while VICE emulates the cheap PAL chroma phase mirroring effect. How can it be? The other difference is that there is some color mixing on YouTube, but it is much less than in VICE. Which one is real?

Never judge those things from a recording - and much less from youtube video. That adds so many things to the mix, all of which may introduce additional "smoothing". On my setup (new C64/1701 monitor) eg the odd/even lines difference is even more visible than in default VICE settings, eg in solid violet areas.

Quote:
I am interested in these details because I would like to create some cool effects in my next demo, but it would be kinda pointless if none of the compos would support the real C64 output... :)


You probably can not rely on the details of those things, they will not be reproduced the same on different c64+crt setups, and some things just wont be reproduced correctly at all once you add a projector or recording into the mix. I'd always at least check with a couple different c64+crt combinations - And then you will likely come to the conclusion that you can only rely on the principle of PAL mixing, but not on certain details.

Regarding emulation this is a similar problem as SID filters. There is no "right" or "wrong" when it comes to certain things, and all the emulator can try to do is matching "a" C64. But it will never match "all" C64s. And it will always be different to "your" C64 =)
2023-05-04 12:55
NoiseEHC

Registered: Feb 2005
Posts: 51
Quoting Groepaz
Quote:

On my setup (new C64/1701 monitor) eg the odd/even lines difference is even more visible than in default VICE settings, eg in solid violet areas.


Today I finally got an analog TV (Panasonic TX-29S95Z) which has an S-Video port - that is why I choose this one, getting a Commodore monitor here is night impossible. I have created a Luma/Chroma cable, and the results with a C64C are below (unfiltered is when my cheapo phone camera focused on the TV pixels):
https://i.ibb.co/Thf4hqy/c64-filtered.jpg
https://i.ibb.co/dt7n7k2/c64-unfiltered.jpg

There is absolutely no difference between even/odd lines. How can it be? Does it happen only through composite connection? I do not think so, as your 1701 should be connected with a Luma/Chroma cable...
2023-05-04 13:18
NoiseEHC

Registered: Feb 2005
Posts: 51
What is even more interesting is that if I connect with RF, then there is still no even/odd line difference, but there is a very visible dot crawl (https://en.wikipedia.org/wiki/Dot_crawl).
https://i.ibb.co/G2Vb8Gs/c64-rf-filtered.jpg

This dot crawl was one of the reasons that I did not use my big LCD TV. (The other is that it lost the sync of the C64 repeatedly, so I thought that the problem is with the C64 signal and LCD TV DSP compatibility.)

So now I think that the dot crawl might be simply that the DSP ICs in modern TVs interpret the wrong Chroma phase as a pixel shift every even line? Can it be? If so then how could I check this?
2023-05-04 13:54
chatGPZ

Registered: Dec 2001
Posts: 11148
This TV seems to be quite modern and probably does more image processing than what a plain old PAL TV does... one interesting feature is that you can apparently independently select the color system (and it has both "old" and "new" NTSC) and the audio carrier frequency (this is usually tied to the color system in a certain region). My guess would be that it simply evens out the difference between two adjacent lines completely, perhaps by mixing 3 instead of 2 lines, or something like that. You could play with the "sharpness" parameter and see if that makes it visible in one of the extremes. There is also a "channel colour set" parameter which is not really explained in detail.
2023-05-04 14:18
NoiseEHC

Registered: Feb 2005
Posts: 51
Can you please post a photo how your image looks like? (It can be uploaded to imgbb.com without creating an account there...)

The image is just inverse (Control+9 RVS ON) SPACE characters with the colors changes by Control+1..8 then Commodore+1..8

When I was young, we were poor so I never had a color monitor, coded on a mono one so unfortunately have no idea how it should look like. Two pictures with and without visible TV pixels would be the best. Thanks!!!
2023-05-04 14:27
chatGPZ

Registered: Dec 2001
Posts: 11148
I'll do... but cant now, there is a bit of chaos around me and the setup is out of order right now :) Perhaps someone else can help out? :)
2023-05-08 08:12
NoiseEHC

Registered: Feb 2005
Posts: 51
After paying a ridiculous amount of money to buy it, then driving 16 hours to fetch it, I am a proud owner of an original Commodore 1802 monitor.

Still no even/odd lines, LOL:
https://i.ibb.co/QcMQZBf/commodore-1802.jpg

I will borrow another C64 and will test with that, maybe I have a special unit, who knows?
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