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 Post subject: Objects feel "floaty"Posted: Mon Apr 25, 2011 2:13 pm

Joined: Mon Apr 25, 2011 1:53 pm
Posts: 3
Good morning,

Have a quick question, I went through the hello world example given on the wiki, and then proceeded to add an OpenGL sphere and rendered it bouncing off the plane. After the initial "COOL" factor died down a bit, I began to notice the sphere seemed to be a bit floaty. Anyone have ideas as to what could cause this? Thanks!

FYI, the gravity is set to -10, I know not true gravity constant, but it should be close enough to get a realistic effect.

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 Post subject: Re: Objects feel "floaty"Posted: Mon Apr 25, 2011 5:53 pm

Joined: Mon Dec 20, 2010 7:13 pm
Posts: 22
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 Post subject: Re: Objects feel "floaty"Posted: Mon Apr 25, 2011 6:11 pm

Joined: Mon Apr 25, 2011 1:53 pm
Posts: 3
Thanks for the reply, will take a look at this when I get a chance. Just as a sidenote, the hello world tutorial has a rigid body with a radius of 1, and I am making a gluSphere with radius 1, both of which should fall into the .5 - 10 range for bullet.

Am I misunderstanding this?

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 Post subject: Re: Objects feel "floaty"Posted: Wed May 11, 2011 1:12 am

Joined: Wed May 14, 2008 5:58 am
Posts: 26
Your mind has an internal expectation of what gravity should be based on what your mind interprets as the (spatial and time) scale of the world. The "floaty" feeling is a psychological effect; your mind is telling you what you're seeing is inconsistent in some way. Even if the world is internally consistent, it might not be consistent with your perception of it (if that makes any sense )

Just change gravity (increase it) until things don't feel floaty anymore. Trust your gut over the numbers.

This is relevant.

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 Post subject: Re: Objects feel "floaty"Posted: Wed May 18, 2011 12:08 pm

Joined: Sun Jan 07, 2007 4:29 pm
Posts: 55
Location: Oxford, England
The reason why physics tech demos tend to feel "floaty" is that they use objects that are much bigger than you'd find in real life. The kind of real objects one is used to see falling (coffee cups, for example) around are typically only a few cm up to maybe 30cm in size. However, physics engines, when using gravity of 9.81m/s^2 and dt = 1/30 will often be jittery with objects this small, so one tends to populate the demo with larger objects.

An object dropped at rest in earth gravity will fall about 5m in one second (distance = 0.5 * accel * t^2). If you're playing with a demo you often don't have any indication of the size of objects, so if the scene contains an sphere that drops its own diameter in one second under gravity, this could be interpreted as:

- a 5m sphere dropping its own body length in normal gravity
- a 50cm sphere dropping its own body length in 1/10th normal gravity
- a 5cm sphere dropping its own body length in 1/100th normal gravity

(etc)

I think what happens is your brain thinks that it's more likely that the object is smaller than was actually authored, and therefore gravity is less than expected.

However, if you use realistic object sizes, and provide good size references in the environment, the floaty sensation will (hopefully) disappear.

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