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Clarify some things about native origins #1071

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Jun 2, 2020
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Clarify some things about native origins #1071

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Specify the full nature of native and effectve origins
Manishearth Jun 2, 2020
0b1b51a
Copying native origins is not worthwile since they will be updated la…
Manishearth Jun 2, 2020
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8 changes: 6 additions & 2 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -1098,7 +1098,11 @@ An {{XRSpace}} represents a virtual coordinate system with an origin that corres

Each {{XRSpace}} has a <dfn for="XRSpace">session</dfn> which is set to the {{XRSession}} that created the {{XRSpace}}.

Each {{XRSpace}} has a <dfn for="XRSpace">native origin</dfn> that is tracked by the [=XRSession/XR device=]'s underlying tracking system, and an <dfn for="XRSpace">effective origin</dfn>, which is the basis of the {{XRSpace}}'s <dfn for="XRSpace">coordinate system</dfn>. The transform from the effective space to the [=native origin=]'s space is defined by an <dfn for="XRSpace">origin offset</dfn>, which is an {{XRRigidTransform}} initially set to an [=identity transform=].
Each {{XRSpace}} has a <dfn for="XRSpace">native origin</dfn> which is a position and orientation in space. The {{XRSpace}}'s [=XRSpace/native origin=] may be updated by the [=XRSession/XR device=]'s underlying tracking system, and different {{XRSpaces}} may define different semantics as to how their [=native origins=] are tracked and updated.
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"a position and orientation in space" - which space, & more importantly, at what time? Having a time-dependent property on objects that live outside of any particular time is surprising to me. This seems to make native origins not very useful since UAs will always have to query the XR systems about poses in a time-indexed manner (as per discussions in immersive-web/anchors#37, getPose() can be invoked on a historical frame), unless we specify exactly when the underlying system is supposed to update the native origins.

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"Real world space".

Yeah, this is supposed to be a quantity you can query for different times -- "natve origin at time T".


Each {{XRSpace}} has an <dfn for="XRSpace">effective origin</dfn>, which is the basis of the {{XRSpace}}'s <dfn for="XRSpace">coordinate system</dfn>.

The transform from the effective space to the [=native origin=]'s space is defined by an <dfn for="XRSpace">origin offset</dfn>, which is an {{XRRigidTransform}} initially set to an [=identity transform=]. In other words, the [=effective origin=] can be obtained by [=multiply transforms|multiplying=] [=origin offset=] and the [=native origin=].

The [=effective origin=] of an {{XRSpace}} can only be observed in the coordinate system of another {{XRSpace}} as an {{XRPose}}, returned by an {{XRFrame}}'s {{XRFrame/getPose()}} method. The spatial relationship between {{XRSpace}}s MAY change between {{XRFrame}}s.

Expand Down Expand Up @@ -1223,7 +1227,7 @@ The <dfn method for="XRReferenceSpace">getOffsetReferenceSpace(|originOffset|)</
<dt> Else
<dd> Let |offsetSpace| be a [=new=] {{XRReferenceSpace}} in the [=relevant realm=] of |base|.
</dl>
1. Set |offsetSpace|'s [=native origin=] to |base|'s [=native origin=].
1. Set |offsetSpace|'s [=XRReferenceSpace/type=] to |base|'s [=XRReferenceSpace/type=].
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The changes higher up look fine, and mesh with my recollection of the intent behind these terms. I feel like this may have lost something, though. Shouldn't we be copying both the native origin and the type?

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The native origin is already defined for all XRReferenceSpaces above, it's not necessary to copy it, because we already define what it is in all cases

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In that case LGTM!

1. Set |offsetSpace|'s [=origin offset=] to the result of [=multiply transforms|multiplying=] |base|'s [=origin offset=] by |originOffset| in the [=relevant realm=] of |base|.
1. Return |offsetSpace|.

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