Best way is to get an example (just like i.e. Jingle Balls or Fly Her to the Moon) and study it.

What thB said is good for this. I know that it has something to do with the FX folder.

Actually, fx.xml.bin and resources.xml.bin in the properties folder (will be in merge folder in a goomod). Making custom particles is fun! I did it for one level in WOG 2, which of course hasn't been released yet, but the level turned out really cool. If used correctly, new particle systems can add a great deal of fun to a new ball/level. I look forward to see what you can come up with!

Teach me how?

Seriously, the best way to learn is to go into XDBoy's mod "Fly Her to the Moon" and start changing stuff in the merge/properties folder. It's just as easy as creating new ball types, only a little more cryptic because you're using .xml.xsl stuff instead of just plain .xml's. It's a little hard to explain on a forum, though.

Use the goo, Luke. In other words, just do what MOM4Evr said, okay?

my name's Luke, or are you referring to Luke goowalker?

Uh, I thought it was SKYwalker. Anyway, inwog hasn't responded to any of these posts, so I'd say we stop on this forum topic until he has something else to ask. OK?

i was trying to be funny! sigh....

Nobody has documented the particle file format as far as I'm aware. However it should be fairly intuitive if you're familiar with the concept of particle generators. Take a look at the game's existing fx.xml to get an idea of what you can achieve.

And as far as adding them to your mods, as someone else mentioned, check out merge\properties\fx.xml.xsl in Jingle Balls - the <particleeffect> element is what gets added, you can copy and ignore the rest of the file.

Within your level, <particles> is the element to add to scene.xml for both particleeffect and ambientparticleeffect (the pos attribute is ignored for the latter).

-davidc

Ah... so THAT'S the difference between the two...
Hmm, I don't think that particleeffect and ambientparticleeffect are interchangeable, because I think that I used an ambientparticleeffect on one ball, and WOG crashed. I'll have to keep experimenting with it...

No, a ball probably can't have an ambient particle effect. But a scene can have either, and you insert both using the same <particles> element.

Is there a "World Of Goo Particles Editor"

Nope, not yet.

...
The particles are in \images\fx\