The open source OpenXR runtime
doc: Add spaces design document
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doc/design-spaces.md
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doc/design-spaces.md
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# Spaces in Monado design {#design-spaces}
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<!--
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Copyright 2022-2023, Collabora, Ltd. and the Monado contributors
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SPDX-License-Identifier: BSL-1.0
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-->
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## Goals
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* Hold a super set of the semantic spaces that OpenXR requires, local, stage,
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unbounded, etc...
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* Allow devices to be attached to other devices.
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* Generating events when a space is moved, dealing with the requirement that
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the move only happens after a certain time in the future.
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* Make the internal graph be simple and non-cyclic.
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* No bi-directional links.
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* Always have a root object.
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## Overall design
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The `xrt_space` object is fairly straight forward, it's a space. Some have
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semantic meaning like local, stage, unbounded. The `xrt_space_overseer` object
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is where most of the interesting things happens. By having a separate object
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where functionality is implemented we can handle things like: different sessions
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with different spaces, have a object that space events comes from. It also makes
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it easier for the IPC layer to implement it.
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## Issues & questions
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* Where do space events come from?
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* **UNRESOLVED:** Probably will come from the space overseer.
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* Do we map all spaces one to one, like the each `XrSpace` with an offset gets
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backed with a `xrt_space`. There is going to be a limit on spaces from the
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point of view of the IPC layer, do we create some of them locally?
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* **RESOLVED:** While you can create a offset `xrt_space`, the offsets from
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from the `XrSpace`s are not expressed as an `xrt_space` this is to reduce
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the number of spaces created.
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* For the IPC gap do we mirror the entire space graph between the app side and
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the service side. Or is it enough to only expose the spaces/nodes a app needs
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and keep all of the links on the service side only.
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* **RESOLVED:** The graph is kept opaque from the application (and all other
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parts of Monado, an space overseer has greater freedom in how to configure
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the space graph).