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Dark Physics & Dark A.I. & Dark Dynamix / Mass, gravity : object weight

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UDun
18
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Joined: 28th Dec 2005
Location: France
Posted: 31st Aug 2006 16:56
Hello,

I try to simulate a realistic scene with a stack of boxes that falls down. The boxes must be heavy when they fall so the drop must be quick. But I didn't succeed. I changed the mass, a material... but the boxes seem to be very light.

I can change the gravity (phy set gravity) and it's working but all the scene is concerned but I only want that for the boxes.

I find this command "phy set rigid body gravity" but I can't understand it because I can't find it in the reference guide.

Here is a small example :



Can someone show me how to simulate a heavy box ?

Thanks.

Coding is dangerous for health ...
ESP
17
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Joined: 2nd Aug 2006
Location: London, England, U.K.
Posted: 31st Aug 2006 17:18
Hi UDun/All

I have been going made with this for weeks now.

I just up the gravity...



Robin
JerBil
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Location: Somewhere along the Z axis...
Posted: 31st Aug 2006 17:39 Edited at: 31st Aug 2006 17:40
If you don't use sync rate 60, it will fall much faster.

Ad Astra Per Asper
ESP
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Location: London, England, U.K.
Posted: 31st Aug 2006 17:47
Hi JerBil,

It seems to make no difference here.

Robin

P.S. Still awaiting my PPU, but that should make no difference???
JerBil
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Location: Somewhere along the Z axis...
Posted: 31st Aug 2006 17:58
Maybe it's the difference in our video cards or something, but it's
at least 5x faster here without sync rate 60.

Ad Astra Per Asper
UDun
18
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Location: France
Posted: 31st Aug 2006 18:17
Thanks for your answers.

I understand but if I change the sync rate or the gravity for example, all the scene will be affected. It's like I want to create a scene with heavy boxes and normal boxes. I only want to change the weight of some boxes.

Coding is dangerous for health ...
imekon
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Posted: 31st Aug 2006 18:26
Quote: "If you don't use sync rate 60, it will fall much faster."


Doesn't that become system dependant, as in the speed varies from machine to machine?



"Reality Bites"
Math89
20
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Joined: 23rd Jan 2004
Location: UK
Posted: 31st Aug 2006 18:43
I'm almost sure you can set a time factor to the 'phy update' command (to make a timer based physics).
And I think 1 db-unit = 1 meter for dark physics so, in your game, if 100 units = 1 meter, you have to multiply the gravity by 100.
JerBil
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Location: Somewhere along the Z axis...
Posted: 31st Aug 2006 21:56
UDun, set different boxes to different masses. That is the same as changing their weight.


@imekon:
You are right, it is system dependent, but sync rate throttles the program. What we need is control without the need for sync rate.

Ad Astra Per Asper
UDun
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Location: France
Posted: 31st Aug 2006 22:33 Edited at: 1st Sep 2006 13:40
Yes but I already changed the mass but it seems to do nothing. The velocity of the box is not modified. The mass only seems to change the force applied against other rigid bodies (joints...).

With a mass=1


With a mass=100000 (the box goes through the floor)


Coding is dangerous for health ...
Chenak
21
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Joined: 13th Sep 2002
Location: United Kingdom
Posted: 1st Sep 2006 17:23
I get the same results here, i think weight in general is bugged. Everything seems to fall at the same speed and they bounce the same way. It looks aweful. Something very heavy should not bounce in the air like a basket ball.
jenkmeister
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Posted: 1st Sep 2006 17:58
Here's an example i quickly put together. The box certainly bounces... It's mass is set at 500. I tried playing with some materials as well but I'm not sure they are really having much of an impact.

My gut reason that the box bounces so high is because it doesn't lose any energy to sound, heat or any other kinetic energy transfer. Nothing is breaking apart on impact, which would certainly lessen the block bouncing up so high.

BealziBob
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Location: The Grim North (UK)
Posted: 1st Sep 2006 20:24 Edited at: 1st Sep 2006 21:12
Ok UDun, you are getting confused by a very common misconception. Just to clear it up for you:

Quote: "Any 2 bodies exposed to gravity will fall at exactly the same rate regardless of their mass."


This theory was proposed by Galilao who (according to legend) attempted to prove it by dropping various items from the top of the leaning tower of Piza. This was proven in a very famous experiment on the moon, where a hammer and a feather were dropped together, and they hit the ground at the same time.

The reason they appear to fall at different rates on earth has nothing to do with gravity but everything to do with air resistance (atmospheric friction). I suppose you could emulate this by applying some sort of up force on objects which you consider should "float more". But basically it boils down to an object’s buoyancy in the air.

Remember gravity is expressed as an acceleration (metres per second per second m/s/s) so is applied to all objects in the same way, mass does not come in to it.

http://www1.jsc.nasa.gov/er/seh/feather.avi

[/smartarse off]


Just a casual observer.
UDun
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Location: France
Posted: 1st Sep 2006 22:03
Thanks for your answers.

Yes, BealziBob, I now remember my physics lessons. Of course, you're right and I have forgotten these confused concepts. It seems so logical that the mass influences the fall but in fact, it's wrong.

But now I wonder how we can simulate this with dark physics. I found this command : "phy set rigid body gravity" but we have no explanations in the reference guide. If we can apply a different gravity to each object, this can be a solution, I think.

Coding is dangerous for health ...
jenkmeister
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Posted: 2nd Sep 2006 00:49
I think the answer lies in the "restitution" command. Been playing with this today. Really low values like 0.05 on both objects appear to reduce the bounce and make it look heavy.

BealziBob
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Posted: 2nd Sep 2006 10:34 Edited at: 2nd Sep 2006 11:52
A simple way to fake air resistance would be to use the Set rigid body linear damping command.



The effect of the damping factor depends on how strong your gravity is.


Just a casual observer.
UDun
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Posted: 3rd Sep 2006 14:49
Great

It's exactly what I want. I'll use this command now to simulate air resistance.

Thanks for your help

Coding is dangerous for health ...
Bmad6
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Location: Virginia, United States
Posted: 3rd Sep 2006 22:52
Quote: "A simple way to fake air resistance would be to use the Set rigid body linear damping command."


Yes, I think this is probably what you want, because the lighter objects will be slowed by air resistance more than the heavy objects, so that the heavy objects will look heavier - then, just get them to bounce less, and it should look pretty realistic...

Harvard's Law, as Applied to Computers:
Under the most rigorously controlled conditions of pressure, temperature, volume, humidity and other variables, the computer will do as it d*mn well pleases.
UDun
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Posted: 5th Sep 2006 14:03
Yes, you're right. I can simulate light and heavy objects with this command. It's really what I want.

Coding is dangerous for health ...

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