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View Full Version : Simulate a bullet going through a skull



JJJenkins
04-09-2012, 09:32 AM
Does anyone have any tips on how best to achieve a realistic simulation of a bullet going through a skull? I have the skull shattering when the object goes through the skull just fine, but I'm after as realistic a look as possible and right now the 'bullet' kind of just blows the whole back of the skull away into big chunks. How do I get smaller pieces? Are there any material settings I should use for the bullet and and the skull to make it look correct?

Also, the docs say I should treat the maya units as 1 meter instead of 1 cm even though it says cm in the prefs. I read the average skull is about 10 inches tall. So this would mean my skull would need to be .25 units tall. When I made it that size the default materials did not behave predictably. I tried the crystal_weak preset and the object was way too strong. I then increased the size of the skull to 1 maya unit which is 1 meter. The material settings seemed to behave much better at this size. Any thoughts on this?

Thanks,

Justin

vik
04-09-2012, 04:37 PM
Hi Justin,

I have a couple of thoughts on that:
1) You probably want to increase the tet density at the bullet exit area so that you get increased fracture detail.
2) Use autocage to construct the tet mesh, you will get more natural looking fracture than if you just make the surface mesh into a bare DMM object.
3) Stick to the guidelines for Maya units. The scale of things matters to DMM since it is physically simulating the properties of the materials.
4) I would suggest looking at real bone breaking. Reference videos are probably pretty available on the internet. If not, check out that ape scene from "2001: A Space Odyssey", they break a lot of bone there. Adjusting the physical properties based on your intended result is a key to getting a good sim. Start out with a relatively low-tet scene and increase the tet density as you get the results you want. Beyond some number of tets you will find that you won't have to adjust the material properties as much.

Experiment with the rigid physical material properties in the existing DMM material settings catalog. I would start with concrete. Bone has some give in it, so you will need to slightly lower young's modulus to get the flex right. I would not mess with the other settings too much, but it doesn't hurt to tweak them.

-Vik

JJJenkins
04-11-2012, 04:46 AM
Thanks for the suggestions! Really appreciate that. I have a couple of questions regarding your thoughts:

1) So you think I should stick with a skull that is only .25 Maya units tall even though the default material settings don't behave correctly when it's at this size? You think I should just tweak them to get the look I want? I was thinking that if this is all physically accurate, if I made my skull the correct 'real-world' size then the default material settings should behave correctly at that size. But when I apply the crystal_weak preset the skull is so strong it barely even cracks when a bullet made out of 'concrete_strong' shoots through it. This made me think that my skull is probably too small at that height.

2) I'm not familiar with the autocage to create the tet mesh. I used that technique in tutorial 2 where you create a lower poly cage around your high-poly object and turn that cage into a bare DMM object. Then I added my high-poly object to that per the instructions in that tutorial. To use that autocage method, do I apply the autocage to my high poly object itself, or to the low poly cage and then add the high poly object back to the low poly cage?

**Edit**

OK, so I see 2 different options for using the autocage method:

1) Create DMM object with Autocage
2) Create Bare DMM object from Polymesh with Autocage

Since my skull is pretty high poly, would it be better to still create the low poly cage around it and use Autocage on that? Or should I just use the high poly skull itself with autocage? I'm leaning towards using the low poly cage. Then I can go through like you suggested and add in more splits around the bullet enter and exit areas on the cage and not waste a ton of detail where I don't need it. Does that sound like a good approach?

Thanks again for the help!

-Justin