-"Sunna or Sol, Germanic solar deity,
associated with, or personification of the Sun"

-"In Greek mythology the sun was personified as Helios,
Homer often calls him simply Titan or Hyperion, while Hesiod separates him as a son of the Titans Hyperion and Theia, and brother of the goddesses Selene (Moon) and Eos (Dawn).

Imagined as a god crowned with the shining aureole of the sun.
As time passed, he was increasingly identified with the god of light Apollo. The equivalent of Helios in Roman mythology was Sol, specifically Sol Invictus."

-"Ra is an ancient Egyptian sun god.
Ra changed greatly over time and in one form or another, much later he was said to represent the sun at all times of the day. The chief cult centre of Ra was first based in Heliopolis, ancient Inunu (City of the Sun).

Ra is most commonly pronounced 'rah', it is more likely, however, that it should be pronounced as 'rei'; hence the alternative spelling Re rather than Ra. The meaning of Ra's name is uncertain, but it is thought if not a word for 'sun' it may be a variant of or linked to 'creative'."

why Bullet demos feel like happening on the moon?
(Open Source - BULLET PHYSICS LIBRARY, Physics Symulation Forum)

Aug 21, 2008
hi,

eeem.. is it just me or is there something wrong with gravity in Bullet...

it just feels like everything is happening on the moon, and i mean for both effects - gravity seems too weak and the lack of air resistance, so as a result its frequent to see objects spin way too much and too fast... and dare i say - fall too slow?

whats wrong with my eyes?

Aug 21, 2008
hi,

i see what you mean,
but do you confirm that there is some "moon-like" or "seem-too-light" feeling in the demos?

basically i want to make sure its not some weird bug with my graphics card or anything else but how it was intended to be, it just seem so obviously wrong ..and easily fixable i would think

edit:
to clarify,
i see you "acknowledge" moon-like behaviour, im just not sure if you're suggesting that it only *appears to some people*, and if we look more carefully we can see its actually ok,
OR
you're saying -"yes, its like on the moon, but *all animations look like that* when there are no textures or other ways to reference sizes and distance relations"?

cheers

Aug 21, 2008
i disagree passionately,
more i look at it and more i think about it, i realize that scale has nothing to do with this..

..i mean, if its not VERY, VERY obvious what "visual errors" im talking about then i must assume something is wrong with my computer... it would be just too crazy if everyone actually saw this anomaly and not realize huge difference with real life behaviour

>>"Bigger objects that are further from the ground will only take the same time to hit the ground if you increase gravity appropriately."

but size is not a factor when it comes to gravity without simulating air-friction, therefore you must be wrong whatever is you want to say there

>>"If you study the maths, you'll see why.."
- im afraid that is where the origin of the error is,
how objects bounce it has, it would seem in most of the other cases but balls/spheres, quite a bit to do with material properties and less so with Newton physics/mathematics of objects action/reaction and more so with inner workings of energy transfer and interaction on object structure - atomic properties, molecular bonds and other "inner" material stuff that dictates how object will bounce OR not bounce at all

so in Bullet,
bricks, no mater what the size, appear as if they were made out of paper filled with hot air.. at least on my computer and in my mind ..but i may be crazy and this computer may be a dead cat in fact, who am i to say?

Aug 21, 2008
...like when they film a movie with miniatures and then speed it up to look more realistic... or is that completely opposite to this?

tho,
i agree that Bullet library is great and that it can be adjusted easily to produce desired results..

so, as this is a matter of opinion,
i still think Bullet demos need to speed up free fall and slow down bouncing and spinning

sparkprime,

>>"You're confusing the issue of rigid body dynamics with deformation."
- no, my opinion is simply that objects should come to rest sooner...

>>"A large building collapsing is not a rigid body."
- ok, that explains everything

>>"the demos are in a vacuum therefore there is no air resistance"
- hey, thats what i said -no air resistance- so what you said was not making sense as you were relating "Bigger objects" and "increase gravity"

>>Only offline engineering applications tend to model such things. This is not what bullet is for. It is very inefficient and adds very little to realism."
- i dont agree, many things instead of "modeled" can be approximated... like this approximation of free fall that we are talking about

>>"As long as the step size and solver iterations are sufficient, bricks will behave like uniformly-dense non-deformable indestructable cuboids of matter falling and colliding in a vaccuum."
- ah.. cuboids, they're truly super

really helpful answer would come in some form similar to:
1.) it only *appears to some people*, and if you look more carefully you'll see that its actually ok
2.) yes, its like on the moon, but *all animations look like that* when there are no textures or other ways to reference sizes and distance relations"?
3.) everything is perfect.. your cat is slow, turn it off and go to sleep

Aug 22, 2008
>>One thing I do agree about is that they would look better if they were on a more intuitive scale, e.g. actual barrels sizes.
- well, that would prove it to me.. if thats simple matter for you, please scale down one of these Bullet demos so i can compare before and after and see for myself how that illusion came to be

...anyway,
whats up with new bunny-with-wheels demo?
is that some kind of hot-air balloon... that is not earth gravity, right? when it jumps, it literary *floats*, not falls - do you not agree?

Aug 22, 2008
>>"1. It is possible that your computer returns weird timing results (I saw this very occasionally a while back with one or two people having my flight sim run too slow/fast - I think most likely on laptops where there's CPU throttling or something going on)."
- you know... that is my best guess actually,
tho chances are slim - im using Bullet on two laptops and 3 OS, 2xLinux and Xp... for couple of months now, they both set to full throttle, everything works perfect.. Bullet library works perfect for me (after setting a few parameters over or there) and its actually just the Bullet demos with this slight, weird, lightweight feeling

it was the first thing i noted about Bullet, it struck me straight away with 1st demo, but for the longest time i couldnt even point my finger at it, i didnt know what felt wrong and i never thought about it as i went on developing my own code that worked as i thought it should..

its just when i downloaded new Bullet 2.70 and saw all the demos all over again, especially new bunny-car, when i said - hey, this is really, really, terribly, horribly, without-a-doubt wrong.. dont get me wrong, i love the demos, they're supercool

>>"2. Why don't you just test what you see. Set gravity to 10, drop an object from a height of 100, and time (with a stopwatch) how long it takes to hit a ground plane at a height of 0 (time should be sqrt(2 * 100 / 10) = 4.47 seconds. Or to check the timing just modify one of the demos so it prints out a message every 10 seconds (and check it really comes out every 10 seconds). These tests will be much more decisive than your intuition..."
- maybe you're right,
it didnt occur to me to do such a basic test as everything seem to be fine on 2 computers, used Bullet with glut timer and in SDL app with SLD_timer, having "expected" results on variable time intervals or fixed 1/60, 1/30, 1/100..

what really, really bugs me tho,
is that everyone seem to know what iam talking about, but everyone seem to "tolerate" it... just as i did, its real easy to just wave hand at that.. until bunny rolled in that is

i dont believe "scale" argument and no one has commented on that bunny-with-tires demo which without a doubt floats in mid-air very unrealistically and is most obvious example of what im talking about.. unless thats very, very cool simulation of the hot-air baloon thermodinamics ..this thing should not be so foggy and matter of opinion.. i think

why do bullet demos slow down (on slow computers) instead of just droping framerates?

Aug 22, 2008
>>"Again, I strongly encourage you to create your own demo with objects that you can predict the movement of. I notice the behavior you mention, and I do tolerate it, in the demos, because I do not see the behavior in my own applications with objects that are sized like objects with real world counterparts."
- yes, sorry i should have been more clear about it,
Bullet works great for me too and i dont see any of the problems in my demos, they are too without any "scale", but look fine - its just Bullet demos, barely noticeable, but bunny-car definitely very obvious "low-gravity" effect, i think vehicle even flies faster upwards as it spins faster... it surely did overcome "escape velocity" at some point and i could see a floor disappear behind far clipping plane underneath

>>"..since the last frame. This would result in the type of display you mention."
- wait a second,
..so you know the cause and solution to the problem?

i suppose we only need to change stepSimulation arguments to something like (dT, 10, dT), is that it? ...and my computer is indeed too slow?

well, that sounds reasonable, going to try...

thought,
cant see a good reason why handle computer animation like that, didnt we all learn from those DOS games, huh? speed of computer should not dictate speed of animation, but only number of times these animation frames/screens can be rendered in a second, right?

cheers

Genesis, Monday

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abaraba1@yahoo.com
http://www.oocities.org/ze_aks/myos.htm