Megan Fox
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Gotchas with Unreal's Fracture system
🕑 Added 2023-10-07 00:06:17 +0000 UTCI hit a bunch of issues with this initially, so thought I'd run folks through the process, and a couple of hurdles I hit.
1.) Don't start with a big whole-mesh shatter
When you shatter, it's going to create some creepy things, which you'll want to prune. But. If you just shatter from the start, the weird bits will be invisible, and it'll come through as weird little shards that are harder to find.
Here, you can see some random shards. Finding these by stepping through every little piece would be super annoying, and just removing them might leave holes in the mesh.
2.) You'll want to both prune and merge shards you find, depending on the sort
By contrast, you can see here that I've sliced the barrel into logical bits, which'll keep the hoops from breaking weird (yay!). This all came from a single planar cut there above the top band of the bucket, btw. Bit weird, because notice how a single cut generated 6 pieces? You'll see this depending on how the mesh was put together. Still, MOST of these groups are actually useful! So we can use them.
But you can also see a weird bit, boo! Still, this is why we do small breaks at first. We isolate out the problems. Why did this make a giant plane? I have no idea. The bottom of this barrel is a one-sided polygon, so that probably didn't help, but like. Who knows. Fracture isn't the most stable thing. Treat it with kid gloves if you want good result. Hence, this going-slow approach.
Two things you can do here, either prune it, which deletes it, or merge it, which is often the better idea. These shards aren't creating data usually, they're often just really awkward isolated vertices that'll look fine when you merge them back in with the group they came from.
In our case, hey, let's try merging the two groups, because sometimes this can fix shards, and-
No. No that did not work. Ok, so we prune it. No big!
End result? Hey, reasonable pieces. We can work with this.
3.) Smaller chunks mean you can do more with shatter shape
Now that we've condensed the little bits down, either by merging or pruning, when we shatter these smaller chunks, we'll get better results. Fewer shards, easier to find the shards when they crop up. So let's take a sec to isolate the 3 pieces so that the two bands break off in solid pieces.
BTW, this process generated more shards I had to prune (go slowly!), and you also have to move the results "up" because you want all of these to be top-level.
You might have also noticed the bands separated, but after breaking the wood down, I really wanted the wood "behind" the bands to be part of the bands, so again, more merging required.
Now then, separate wood pieces. But we're not doing a random shatter yet. First, we can do PLANKS, with the radial tool.
Doing this kind of thing on a larger mesh would mean the bottom and top would get sliced up like a pizza. Not a great look, and it creates tons of tiny little shards that physically don't behave well. But this? This works great. So we preview by setting the Explode amount to .5 so we can see the pieces aaaand,
you see the shards, right? Now I have to select each of these and re-merge them with the plank they're obviously part of, so the resultant mesh isn't garbage when it shatters. They only look separate because of the Explode setting, as I merge them they snap right together. Again. This is why we do small steps. I ALSO have to isolate the bottom of the barrel, so it doesn't turn into pizza slices.
Anyways, THERE. Fixed.
4.) Save the random shatter pass for the very last step in a group
Now, once we've isolated our major parts, broken them up structurally - in our case radially for planks - NOW, we can build the next tier of shatter down, wherein we do a more random break. Now it's safe. This operation generally won't create shards, so now is when you can break out the big random shredder.
See why we didn't do this at the start? Imagine shard-hunting now. There probably still are some! But we've at least minimized them!
5.) If your mesh collides oddly pre-smash, look for tiny shards.
A lot of times there will be a tiny spec somewhere in your geometry collection that gets generated, and you'll never see it unless you step through each group, but bumps the physics enough that the barrel won't sit flat on the ground anymore. So if you get weird cases like that? Go spec hunting. This looks good though.
Anyways, happy smashing!
EDIT: OH! Last minute edit. For fixing the shards that appear in your final shatter? Try the TinyGeo tool. Waaaay at the bottom. Only run it on your groups that you want to merge down, because it may make uh. Interesting, choices. Still! It'll likely save you a ton of time.
