very interesting that both the left of the backpack and both the top of the legs use the same dark blue / brown dithering, but the backpack looks greyish in PAL emulation while the legs look purplish. ... Why?

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?

One thing worth mentioning is that checkerboard dithering in hires is a definite no-no*. It's very close to the colour carrier in both NTSC and PAL (a touch higher than NTSC, a touch lower than PAL). Either way, if you view it using a composite video or RF cable you get spurious colour banding.

* it's fine in multicolour

Not supporting people using RF is no different than not supporting people loading from cassette.

And the reality is that most people judge an image based the screenshot anyway...

I mean, it's more like "only supporting people using s-video or buggy emulators" (admittedly at the moment the latter is most of them)

The reason I ask these questions because I am working on an editor which is a plugin for GIMP. The plan is to draw pictures using my pen tablet, which are converted into 16 color dithered images in real-time. Controlling colors/sprites will be done using some extra layers in the source image. Obviously it will work only if the dithering is good in the first place.

And at least s-video still exhibits black bleed

Ok, fair points. Perhaps in the future it will just be like songs specifying which SID chip is preferred :)

I haven't experimented alot with different patterns, but I think it'd be nice if you'd let the user select the pattern to use while painting. I suppose the choice depends on what kind of effect or material needs to be created.

Color bleeding happens horizontally, while the PAL mixing happens vertically. And yes, VICE (and every other emu) should emulate it - however, this is something to be implemented in shaders, its way too costly to do in software (if you want to do it correctly)

Yeah, that is the plan to have different dithering settings, most likely per layer, or per character.

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%.

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.

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... :)

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.