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Add pick parenting markdeep doc

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<meta charset="utf-8">
**Pick Parenting**
# What is a pick?
A pick calculates spatial information about the world on on the client.
Picks are immutable to allow efficient use by scripts. There are currently four types of picks:
- Ray pick - Finds the first intersected object along a straight path
- Parabola pick - Finds the first intersected object along a parabolic path
- Stylus pick - Finds the distance between a point and a list of whitelisted objects
- Collision pick - Uses a volume to check for collisions in the physics engine
With the exception of collision picks, all picks use the visible appearance of the object to check for collisions.
# What is parenting?
Parenting allows an object's position, orientation, and scale (where applicable) to be calculated to another object. This collection of state defines the transform of the object.
Pick parenting allows the pick's transform to be calculated relative to another object, without creating a new pick.
# What can be parented to what?
Child type: | Entity | My Avatar | Other Avatar | 3D Overlay | Pick^1 | Pointer^1
---------------|----------|-------------|----------------|--------------|----------|---------------
Entity | yes | no | no | yes | yes | yes
My Avatar | yes | no | no | yes | yes | yes
Other Avatar | yes | no | no | yes | yes | yes
3D Overlay | yes | no | no | yes | yes | yes
Pick | no | no | no | no | yes | yes
Pointer | no | no | no | no | no | no
The Mouse | no | no | no | no | yes | yes
- ^1 Excluding stylus, which can only be parented to My Avatar's hands
# How pick parenting works
This section describes what happens when picks are parented to things.
Since pointers use picks to calculate their results, the rules below
also apply when pointers are parented to things.
## Parent transform exposed
The way a pick is moved/rescaled depends on its parent. In particular,
parenting to a pick actually parents to the pick's result, which
has no notion of scale. A pick can also be transformed relative to
a joint on a model, such as an avatar's hand.
Parent type: | Entity/Avatar | 3D Overlay | Pick
--------------------------|-----------------|--------------|--------
Has position | yes | yes | yes
Has orientation | yes | yes | no
Has scale | yes | yes | no
Can have joints | yes | no | no
## Entity or overlay parent
Parenting a pick to an entity or overlay works similarly to
parenting entities to other things. However, unlike entities,
picks with scale will rescale their dimensions when
their parent rescales.
************************************************************************
* May have a joint, otherwise parent to the parent's origin *
* | *
* scale ^ +--+------+ ^ --+ orientation *
* determined / / | /| |orientation \ offset *
* from / / v / | | v *
* dimensions / / o /..|...................... ^ *
* +-> +---------+ | ------------. . / *
* | | | / position +----> */ <----+ *
* | | | / offset child *
* | | |/ ^ transform*
* v +---------+ / <---+ *
* . +-> dimensions *
* . . . . . . . . . . . . . . | relative *
* . . . v to parent *
************************************************************************
!!! WARNING
Skew is not supported for collision picks. Scaling a parent entity or
overlay non-uniformly can lead to inaccurate shapes for the child
collision pick.
## Avatar parent
A pick parented to an avatar behaves like a wearable. It will maintain
its position relative to some point on the avatar's body. If the pick
has scale (currently only collision picks), then the pick will rescale
when the avatar rescales.
****************************************************************
* Avatar .---. hand ^ -. *
* | | | joint / | *
* '-----> | | \ /orientation | *
* '-+-' +-> o | *
* | / ^ position |relative *
* +---------' +----- |avatar *
* | |scale *
* | if not a joint, |factor *
* | parent to the |(default: 1) *
* | avatar's origin | *
* |\ | |uniform scale *
* | \ | | ^ *
* | \ | | +-> *
* | o \ <-+ -' v *
****************************************************************
***************************************************************
* orientation offset *
* <-----+ *
* ^ \ *
* \ *
* \ child *
* Avatar .---. hand .......... * <-- transform *
* | | | joint . ^ | *
* '-----> | | \ . / v *
* '-+-' +-> o ---------+ ^ *
* | / position +-> scale *
* +---------' offset v relative *
* | to avatar *
***************************************************************
## Pick parent
Picks can also be parented to other picks.
When this is done, the child pick is actually parented to
the parent pick's result, not the pick itself.
A pick parented to another pick will have its position changed,
but not its orientation or scale.
### Ray pick parent
**********************************
* position *
* o *
* \ \ orientation *
* \ \ *
* \ v . . . . *
* \ . *
* \ . result transform *
* * <-- no scale *
* . no orientation *
* . *
**********************************
### Parabola pick parent
**************************************************************
* .------. acceleration *
* + + | ^ *
* speed x ^ / \ . | | *
* orientation / / \ . v | *
* / / * y axis to rotate *
* | . ^ acceleration with *
* o . | *
* position result transform *
* no scale *
* no orientation *
**************************************************************
### Stylus pick parent
********************************************
* *
* *
* . *
* . *
* . *
* .---. avatar * *
* | | hand / ^. *
* | | \ / \ . *
* '-+-' +-> o \ *
* | / result transform *
* +---------' no scale *
* | no orientation *
********************************************
### Collision pick parent
********************************************
* *
* .---. *
* | | <-- collision pick *
* | | *
* | *<-+--- result transform at *
* | | collision pick position *
* | | no scale *
* . . . '---' . . no orientation *
* *
********************************************
# Effect of scale on picks
Scale affects the position and shape of picks, which in turn affects the pick result.
Scale currently does not affect the max distance of a pick.
## Ray and stylus pick scaling
Rescaling the parent of a ray pick or stylus pick can result in a translation of the pick.
***************************************
* before after *
* *
* pick *
* pick +--------------+ ^ *
* ^ | |/ *
* +-----+/ | o *
* | o | | *
* | | | | *
* +-----+ | | *
* parent | | *
* +--------------+ *
* parent *
* *
***************************************
## Parabola pick scaling
***************************************************************************
* before after after *
* (scaleWithParent (scaleWithParent *
* is false) is true) *
* *
* .----. *
* + + *
* .--. / \ *
* + + / \ *
* .--. / \ / \ *
* + + /pick \ /pick \ *
* / \ +-------o +-------o *
* /pick \ | | | | *
* +---o | | | | *
* | | | | | | *
* +---+ +-------+ +-------+ *
* parent parent parent *
***************************************************************************
## Collision pick scaling
Collision picks use the full transform of their parent (position, orientation, and scale/dimensions).
When first created, a collision pick's transform is defined in world space.
As the parent rescales, the collision pick rescales proportionally.
The collision pick's threshold also rescales. The change is proportional to the largest
dimension of the parent. So, if the largest dimension of the parent was 3.0 and is now 6.0,
the threshold doubles.
************************************************************
* *
* before after *
* *
* pick *
* +------------+ *
* | | *
* | ........ | *
* | . . | *
* | . . | *
* | . . | *
* pick | . . | *
* +-----+ | ........ +___ *
* |.....| | | theshold *
* |. .| theshold +------------+--- *
* |.....|___ | *
* +-----+--- <-+ *
* +----------+ *
* +---+ / \ *
* +-----+ +--------------+ *
* parent parent *
* *
************************************************************
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