Avatar Height And The Collision Model: A Question

[Perrie Juran]

I am posting this question here in this sub-forum because I am figuring the experts on my questions would be mainly looking  here.

On a regular basis there are many threads about Avatar Height in the Forums.  As many people know, the average height of Avatars as measured in the SL viewers is taller than average real life people.  We hear people complain that if they make themselves a RL height as measured in the viewer, for instance a woman makes her Avatar measure 5'2, they get accused of looking like a child because they then appear shorter than the average Avatar in SL.

There are a small but vocal number who would like to see everyone use a RL measurement for their Avatars. 

As best as I understand, the original height fot the Avatars was set by Andrew Linden.  He stated:

 

"As I recall, the max and min heights (head to toe) possible are somewhere around 2.95 and 1.25 meters, respectively. I determined those numbers emperically back in late beta.

The historical reason the "middle of the sliders" avatar is taller than the average human is because our last non-modifiable avatar back in pre-alpha just happened to be that tall (a guy we called "Primitar". Then we changed how the avatars looked without changing the physical representation of the avatar on the server, and their default size was scaled to match the collision model."

http://forums-archive.secondlife.com/120/4d/7240/1.html#post54400

One downside to everyone 'shrinking' their Avatars of course is that most content is built scaled to accommodate tall Ava's.  The smaller you make your Avatar, the more you are dwarfed by things like furniture, etc.

But one of the things Andrew said in his post regarding Avatars was that "their default size was scaled to match the collision model."  Which leads me to my question.

I am not sure I really understand what is meant by the phrase, "collision model."  Would everyone making smaller Avatars effect more than just content.  Would it effect the physics engine, the way Avatars respond to physics in SL?  Would there be other side effects?

Thank you in advance for your replies.

[Abigail Merlin]

I suspect in 2005 or before the avatar colision box was a set size but for years now the colision box scales with the avatar hight, it still does not match the avtar itself tho, the x and y size is fixed for all avatars and z reflect the avatar in standing position, if you crawl laydown or squat it can happen that you look like you can craw under something but still get stopped because the colision bounding box is to high, als your head can disapear into a wall because the avatar sticks out the bounding box in that direction.

A litle trivia, animations and bounding box are linked to the pelvis.

[Madelaine McMasters]

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Abigail Merlin wrote:

I suspect in 2005 or before the avatar colision box was a set size but for years now the colision box scales with the avatar hight, it still does not match the avtar itself tho, the x and y size is fixed for all avatars and z reflect the avatar in standing position, if you crawl laydown or squat it can happen that you look like you can craw under something but still get stopped because the colision bounding box is to high, als your head can disapear into a wall because the avatar sticks out the bounding box in that direction.

A litle trivia, animations and bounding box are linked to the pelvis.

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Yep, I like to probe the bounding box with snowballs! ;-)

While SL furniture may be scaled to "nominal" SL avatar height, I think architecture is scaled to account for camera tracking. My RL height is 5'2" and my RL doorways are all 6'8" (1.3x my height). My SL height is 6'4" (in high heels ;-) and the doorway to my cozy lighthouse is 11'6" (1.8x my height). The ceilings in my RL home are eight feet high (1.55x my RL height), in the lighthouse, they're 17 feet high (2.72 times my SL height).

You could not cam into a properly scaled SL home, so we all live with absurd proportions for SL architecture. I think this is because of choices made long ago about camera following position.

[Helium Loon]

There are two major things to remember here....

 

First, since the collision model (aka, bounding box), does not animate.  It does not bend or deform.  So in order for architecture to allow for those avatars which are set at maximum height (2.95 meters!), the doorway HAS to be big enough for that bounding volume to pass through.  While those who set their height that large are relatively few, there is the potential, and if the doorway is NOT large enough, it becomes a blockage to them.  The bounding box can't 'stoop down' or 'crawl' through a doorway that is too short/narrow.  So, in addition to the cam problem, there is an accessibility problem.  Thus if the doors are scaled up, the floors themselves have to scale up as well.  Which means windows and most everything has to scale up proportionately.

 

Second, in a virtual space, units are just that.  Units.  They are arbitrary.  In SL, we use meters, and we assume they correspond with RL meters.  But they could just as easily be inches, angstroms, furlongs.......anything.  Everything is relative to the base units chosen.  In the real world, we have concrete points of reference that we use to define our systems of measurement (aka, units).  We agree with others to use them for our calculations and measurements, so that we can agree on quantities and dimensions.  But in a virtual world, there IS no concrete point of reference, only numbers.  So, everything is relative to whatever units they chose to use.

 

So if your avatar is 2.3 meters tall in SL, that's fine.  Those meters may not be the same as RL meters.  We build and design based on that, but it would be more appropriate to measure things based on a standard default avatar size......so if the average avatar was 2.3 former meters in height, They would now be 1 'height unit' tall.  And a standard cube would rez at 0.21739 'height units' tall.  It's all relative.

 

[Madelaine McMasters]

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Helium Loon wrote:

There are two major things to remember here....

 

First, since the collision model (aka, bounding box), does not animate.  It does not bend or deform.  So in order for architecture to allow for those avatars which are set at maximum height (2.95 meters!), the doorway HAS to be big enough for that bounding volume to pass through.  While those who set their height that large are relatively few, there is the potential, and if the doorway is NOT large enough, it becomes a blockage to them.  The bounding box can't 'stoop down' or 'crawl' through a doorway that is too short/narrow.  So, in addition to the cam problem, there is an accessibility problem.  Thus if the doors are scaled up, the floors themselves have to scale up as well.  Which means windows and most everything has to scale up proportionately.

 

Second, in a virtual space, units are just that.  Units.  They are arbitrary.  In SL, we use meters, and we assume they correspond with RL meters.  But they could just as easily be inches, angstroms, furlongs.......anything.  Everything is relative to the base units chosen.  In the real world, we have concrete points of reference that we use to define our systems of measurement (aka, units).  We agree with others to use them for our calculations and measurements, so that we can agree on quantities and dimensions.  But in a virtual world, there IS no concrete point of reference, only numbers.  So, everything is relative to whatever units they chose to use.

 

So if your avatar is 2.3 meters tall in SL, that's fine.  Those meters may not be the same as RL meters.  We build and design based on that, but it would be more appropriate to measure things based on a standard default avatar size......so if the average avatar was 2.3 former meters in height, They would now be 1 'height unit' tall.  And a standard cube would rez at 0.21739 'height units' tall.  It's all relative.

 

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I agree about the ambivalence of units, which is why I described the architecture as a ratio to my height. SL's architecture is simply proportionally much larger than RL architecture. Nevertheless, we seem to enjoy it just fine.

I just popped in world to probe my avatar bounding box with a tiny sphere. I get the impression it's actually a bounding ovoid. I moved the sphere near my head until it started to push me, then lowered it towards the ground. It continue to push me until around pelvis height, then stopped. At ground level I moved the sphere towards me again until it pushed, then raised it. Again, it continued to push me until reaching pelvis height. The top of the ovoid was about at the top of my head, well under 2.95meters. So Abigail's claim that the box started scaling in 2005 seems true. Making the box an ovoid also makes sense as that maps better to the head, though it's a bit more difficult computationally.

 

[Jennifer Boyle]

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Madelaine McMasters wrote:

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...  While SL furniture may be scaled to "nominal" SL avatar height, I think architecture is scaled to account for camera tracking....  I think this is because of choices made long ago about camera following position....

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I had been interested in changing my default camera position to just on front of my face because I thought it might provide a more immersive experience.  Recently, a poster on these forums made me aware that I could make the default camera position whatever I like with the CameraOffsetRearView debug setting; I also found it necessary to adjust the CameraAngle setting.  I am still getting used to this, but I think I am going to like it.  It feels more like I am in the scene instead of watching it. BTW, I'm 5'!!".

[Ceera Murakami]

In theory, a smaller avatar is probably slightly kinder on avatar physics calculations than a taller one, but it really doesn't matter much there.

An interesting tangent here: 

As others mentioned, the bounding box of an avatar does not animate. It follows the hips of the avatar in the same vertical position that the avatar would be in if standing upright, regardless of how the avatar is actually positioned.

This makes designing for certain avatar types a challenge.

I made a "doggy crate" for a quadruped canine avatar. One of those wire mesh cages about a meter tall and 3/4 of a meter wide and 1.5 Meters long, with a door on the end. I had to design it with a phantom top, and with a door that went right to the top edge of the end, or else the doggy couldn't walk into the crate! He would get stuck as his hips got close to the roof of the crate. 

Try designing a to-scale home for Tiny avatars. Even though the deformer in the avatar makes them maybe 0.7 M tall, they still have a bounding box that is as tall as their undeformed height! To make a usable door that looks right for the tiny avatar, I had to make the wall above the door and the door frame above the door phantom. Low ceilings might even require the roof and ceiling to be phantom as well!

[Perrie Juran]

sorry i haven't been back to this thread sooner, but thanks all for your replies.

the issues with the bounding box are interesting.

[Helium Loon]

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Madelaine McMasters wrote:

I agree about the ambivalence of units, which is why I described the architecture as a ratio to my height. SL's architecture is simply proportionally much larger than RL architecture. Nevertheless, we seem to enjoy it just fine.

I just popped in world to probe my avatar bounding box with a tiny sphere. I get the impression it's actually a bounding ovoid. I moved the sphere near my head until it started to push me, then lowered it towards the ground. It continue to push me until around pelvis height, then stopped. At ground level I moved the sphere towards me again until it pushed, then raised it. Again, it continued to push me until reaching pelvis height. The top of the ovoid was about at the top of my head, well under 2.95meters. So Abigail's claim that the box started scaling in 2005 seems true. Making the box an ovoid also makes sense as that maps better to the head, though it's a bit more difficult computationally.

 

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That actually isn't surprising it's an ovoid shape....specifically, probably a scaled sphere......which is actually computationally easier than a box shape to calculate intersection tests with (which is what the physics engine has to do), since it can simply be represented as a set of radii and a central point.  And ray-sphere and object-sphere intersection tests are highly optimized and computationally much simpler than just about any other shape.

 

And while the bounding box scales to the 'height' of the avatar (based on the shape worn) to some degree, it doesn't handle a lot of deformations done with odd shapes (llike Tinys) so the actual max and min size of the bounding shape's extents is pretty fixed.  Even designing architecturally for the 80% median range sizes, means scale is still skewed a LOT.......human height ranges for that 80% median scale only cover a range of about 18 inches (5 foot 6 inches, +/- 9 inches) but in SL that range goes a LOT further.  The median height in SL runs at 2.1 meters, or around 6 foot 11 inches, with a variance (at 80% median range) of +/- 68cm, or about 27 inches!  That's TRIPLE the potential difference compared to reality.  That means scaling up of architectural details is correspondingly larger.

 

Furthermore, as has been mentioned already, camera position is offset above the avatar....resulting in even MORE height adjustment for architecture, since we have to accomodate the camera position as well.  Fortunately, if you are already scaling based on the avatar size ranges, you are pretty safe, except for the very tallest avatars.  The very smallest avatars (not counting tinys) will seem to be dwarfed by comparison to the architecture, though.

 

 

[Cerise Sorbet]

The avatar's collision shape is not a box or sphere. Since Havok 4 it is a strangely shaped custom mesh. There is a nice picture of it on the llCastRay talk page.

The avatar is not really rooted at the pelvis, but it is usually near that for basic animations. You can see the real position if you attach an object to the recently exposed "avatar center" point, or viewer the red crosshair with Develop>Avatar>Display Agent Target.

Some recent viewers are a little broken and display all the agent target crosshairs as blue, but on a typical standing human avatar it is the upper of the two that don't move around with animation.