Can the game engine handle small objects?

Hi all. There’s probably a simple way round this but, as I understand it, 1 Blender unit is meant to be roughly equivalent to 1m in the real world. So if you drop a cube that’s 1 BU high, it’ll behave like a box that’s 1m high. Collision works fine too. However, if you want to simulate a die (which is going to be between 10mm and 20mm - roughly 0.02 BU), it does behave like a small object, but the collision bounds are way outside the cube itself and the die rolls within what appears to be a spherical force field.

There just doesn’t seem to be a way of getting the collision bounds right when working on such a small scale. I’ve even tried scaling up the mass to compensate, but that doesn’t seem to make a difference.

Admittedly, I’m not that familiar with the GE, but it does seem to be quite happy until you drop below 1BU. I’m sure I’m missing something obvious though. Can anyone help?

BTW, I’m using Blender 2.57

Yeah, the problem is that you haven’t set the physics bounding object type in the Physics menu. Make sure you have “Blender Game” selected at the top of the Blender Window, and then go to the physics tab (the ball bouncing off of a wall, I think). There should be a Collision Bounds section there - enable it, and select a bound type that matches your object - it sounds like a box would do well for your idea.

However, Bullet doesn’t do small objects very well - you might have to increase the physics substep setting in the World tab, and / or make the collision bounds larger (parent the non-collision enabled die to a larger, invisible physics-enabled box) to have them work correctly.

The BU=M thing is the default physics settings, but if your whole scale is smaller (IE if you are making a board game, and you don’t need any physics objects larger than, say, a few inches) you can just bump the gravity way up, and the rest of the settings that rely on scale are subtle enough not to matter. That way you can make your die half a BU or so, and it’ll still look like it’s small.

Thanks guys. I was wondering if bumping up the gravity would do (and it probably will for present purposes) but I can’t help thinking about the scenario where you’ll be using large and small objects together, so changing the gravity would have adverse effects on one or the other.

I was already in Blender Game and set up a plane as a static object, and the die as a rigid body. I’m actually using a die model which has bevelled edges, so I set the collision bounds to convex hull, which works fine for the large objects. I did try dropping the collision radius too (as I could see it was way too big in the 3d view) but that didn’t seem to work. I hsven’t tried the physics substep though, so maybe that’ll do something. I like the sound of parenting the die to a larger box, but I’m not sure I totally understand what you mean, SolarLune. Won’t it just make the smaller die behave like a large box? Can you elaborate at all?

Again, thanks to all who’ve replied. It’s nice to know there are folks out there who are prepared to help noobs such as myself.

Well, yeah, it would just perform like a larger box. You could still try it and see how it feels - just set the die to be “No Collision”, and parent it to a larger, invisible bounding box. Then, set Rigid Body on that box, and see if you like it. However, I would also recommend making the gravity larger and just compensating with the difference in physics.

In your case, for better precision, I’d consider 1BU as 1cm and work from there, meaning scale your scene up! the minimum bound radius is 0.01, at tat time it’s very badly calculated, so scaling up by just a factor of 2 can save you a lot of trouble. If your object is a rigid body and has triangle mesh as bound, the radius wont be an issue that much… I think!

Scaling the scene and increasing the gravity should produce plausible results already.
But you should increase the mass of your objects too, that the motion moment (is that the right word?) fits the scene size. I do not even know what unit the mass field has. So you need to play with it anyway ;).

Interessting. made a test with a 1cm (0.01bu) big die. Thats way to small cos I can’t set weight to less then 10g.

But with a little tweaking it works ok. Used box bound and added a small margin to round of the corners. Then added another mesh parented to the die that is without physics and a little bigger and with bevels. Did the same trick with the floor to lessen the risk to fall through… but don’t seams to make that much of a difference. Had to turn physics steps up to 5 to get it stable. +some other tweaks. Blend included.

die.blend (485 KB)