The case for a "SceneGraph"

Robert Carr robert.carr at
Wed Jul 24 23:28:40 UTC 2013

Trying to add my two cents. I agree we need this data structure and set of
interfaces that is outlined (roughly) in the original email. It is true
that in interface presented to the compositor, this component takes on a
scene graph role...I think that is not the problem we are trying to solve
at the moment though.

It seems to me our most presenting problem is synchronization issues due to
distribution of state. Essentially several parts of the system (shell,
surfaces, input) each maintain some internal model which corresponds to a
view of the surface stack. The lack of an authoritative source on state
makes synchronization difficult. Likewise, it makes it unclear how to
implement some really common patterns for the shell (i.e. simple
notify-interfere patterns), and I think has lead to the sort of
'anti-pattern' where the shell has to subclass particular factory
interfaces to view certain state.

So, I think we need to rework the mir interfaces around some sort of multi
index data structure (perhaps with some sort of transactional support) to
fulfill these requirements. The difficult challenge is the design of the
interfaces between this data store, and other mir components/the shell.
Given this I expect the data structure implementation to be trivial.

As for the Qt scene graph.

I think it is easy to say "mir will provide these interfaces, and the Qt
scene graph will implement them". It's difficult for me to understand what
this means though. Are we discussing using the Qt scene graph for
rendering? or only as a data store? Either way what sort of approach can we
use to integrate it with the compositor and/or  renderer (accounting for
features like, hardware overlays, multimonitor, and composition bypass). In
doing so are we expecting performance gains? We should consider that the
usage profile optimized for in open scene graph is likely quite different
than the usage profile exposed by Mir. It would be surprising to me if a
custom renderer tailored to Mir were not more performant.

Another potential benefit thats discussed is ease of interopability with
QML. I think maybe we are forgetting the whole picture here though. If the
idea is that different parts of the mir system communicate through this set
of interfaces implemented by the Qt scene graph, then how can we really
allow QML to skip these interfaces and deal directly with the scene graph
without losing our architecture for synchronization?

I don't mean to raise FUD but it is unclear enough to me that I would not
know the first step if assigned it as a task (My first question would be,
does ms::Surface implement QQuickItem? If not, how do we avoid two copies
of the SurfaceStack). I also think that, ease of applying graphical
effects, is not really something holding us up at the moment, or something
likely to be a significant drain on development resources any time soon.

So anyway, I think we should focus on the data structures and interfaces
around the surface store in Mir before spiking on using the Qt scene graph.
I'll try and write a second email soon on thoughts im starting to develop
about this surface store.

On Thu, Jul 18, 2013 at 3:27 AM, Alan Griffiths <
alan.griffiths at> wrote:

>  Hi All,
> at a recent hangout I took the action to produce some notes on what a
> "SceneGraph" means for Mir.
> We've had frequent discussions along the lines of "this information
> belongs in the SceneGraph, but as we don't have one we'll stick it
> <somewhere>". This, so far mythical, SceneGraph fulfills the "model" role
> in our core MVC pattern (see design/Architecture.dia in the source tree)
> and incorporates functionality currently present in a number of parts of
> the system.
> The present SurfaceStack implementation is a primitive SceneGraph that was
> "the simplest thing that might work" when it was added, but we've worked
> around its limitations for long enough to know that it is simplistic and
> fails to support the way we want the system to grow. The SurfaceController
> was initially intended as the Controller, but I think it is now clear that
> the Controller should be a part of the shell.
> So, what do we want from a SceneGraph?
> Looking at the architecture diagram and the code (which are in remarkable
> accord) there are a number of clients to the SceneGraph:
> *The compositor:*
> This needs access to the buffers to be composited and corresponding meta
> information about the way they are to be rendered. It doesn't happen now
> (the compositor has logic to determine which buffers to render), but the
> SceneGraph should be supplying exactly those buffers and meta information
> that the compositor needs.
> Requirement: the SceneGraph need to be able to send the buffers (and
> compositing metadata) that are visible on a specified output to a receiver
> within the compositor.
> An implication of this is that the SceneGraph needs to be able to model
> the output devices and know which surfaces are associated with each output.
> The current implementation has some simple logic that informs the
> compositor that changes have happened. We could make this more useful with
> a cleverer SceneGraph - e.g. identifying what has changed for which output.
> Side note: We currently implement visibility as a flag on each surface,
> but it might equally be a association within the SceneGraph - what is clear
> is that it should be part of the way that the SceneGraph passes buffers for
> rendering, not part of the metadata.
> *Input:*
> Input needs to determine where to route events based upon the input aware
> areas presented by some surfaces. The code here is undergoing a migration
> from an android based input stack to something more integrated into Mir.
> There are two scenarios: keyboard events (which go to the current input
> focus) and events that specify a point for which a corresponding input
> target has to be found.
> Currently the shell conspire with input to keep track of the input target.
> (The shell, in its role as Controller, should control the focus, but the
> focus information should be in the model - i.e. the SceneGraph.)
> I'm not sure of the exact state of the mechanism for position based input
> events currently but, to avoid race conditions when the SceneGraph changes,
> it should be the SceneGraph that identifies candidate input targets based
> on position.
> *The Shell:*
> As the Controller the shell should be mediating all updates to the
> SceneGraph - it gets to decide the policy for surface placement, ordering,
> focus changes, etc. This is best done by taking the raw request and posting
> the corresponding updates to the SceneGraph.
> From the point of view of the SceneGraph we don't need to distinguish
> between the Mir "shell" code and the Unity shell code (the requirements on
> the SceneGraph should be the same). In practical terms a lot of
> interactions should stay on the C++ side and these probably best fit in the
> Mir codebase.
> Some examples:
> Surface creation: the shell needs to know when sessions create surfaces,
> and have control over where they are placed. Currently the Mir
> ApplicationSession receives the create_surface from frontend and has
> control of the request passed to the SurfaceController: we have two objects
> where we need one.
> Session creation: this is currently modelled in shell.
> Surface association: the shell (and possibly other clients) needs to be
> able to navigate between a session and its associated surfaces. The current
> SceneGraph has no concept of "session" - this relationship is currently
> held within ApplicationSession: this gives ApplicationSession a mixture of
> Controller and Model responsibilities.
> The SceneGraph should have a concept of "session" as there are operations
> it should perform on all the surfaces associated with the session.
> DepthID, z-order, etc. The stuff I've seen implemented around these seem
> doesn't leave me with a clear perception of the problem being solved -
> maybe someone else can fill out some details. There clearly is a need for
> an ordering to the buffers presented for rendering on each output but, so
> far, I don't see the need for the complexity we have.
> *Summary:*
> We need an improved Model and Controller for our SceneGraph - and that the
> model should support at least three types of node: sessions, surfaces and
> outputs (and understand the relationship between them.)
> The representation of outputs in the SceneGraph implies that there is more
> interaction with the display configuration than at present. (N.b. any
> updates to the display configuration should be mediated by the
> Controller/shell.)
> We're currently distributing knowledge of our "scene" around compositor,
> shell, and input. That is going to cause synchronization issues - we should
> move all this knowledge into one place.
> The above is just a first, high-level, stab at outlining the requirements
> - we clearly need to agree the scope and then drill down into a lot more
> detail as the next piece of work.
> Alan
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