NB: The usual blog disclaimer for this site applies to posts around HoloLens. I am not on the HoloLens team. I have no details on HoloLens other than what is on the public web and so what I post here is just from my own experience experimenting with pieces that are publicly available and you should always check out the official developer site for the product documentation.

Backdrop – Shared Holographic Experiences (or “Previously….”)

Recently, I seem to have adopted this topic of shared holographic experiences and I’ve written quite a few posts that relate to it and I keep returning to it as I find it really interesting although most of what I’ve posted has definitely been experimental rather than any kind of finished/polished solution.

One set of posts began quite a while ago with this post;

Windows 10, UWP, HoloLens & A Simple Two-Way Socket Library

where I experimented with writing my own comms library between two HoloLens devices on a local network with the initial network discovery being handled by Bluetooth and with no server or cloud involved.

That had limits though and I moved on to using the sharing service from the HoloToolkit-Unity culminating (so far) in this post;

Hitchhiking the HoloToolkit-Unity, Leg 13–Continuing with Shared Experiences

although I did recently go off on another journey to see if I could build a shared holographic experience on top of the AllJoyn protocol in this post;

Experiments with Shared Holographic Experiences and AllJoyn (Spoiler Alert- this one does not end well)

I should really have got this out of my system by now but I’m returning to it again in this post for another basic experiment.

That recent AllJoyn experiment had a couple of advantages including;

• Performing automatic device discovery (i.e. letting AllJoyn handle the discovery)
• Not requiring a cloud connection
• Easy programming model (using the UWP tooling)

but the disadvantages came in that I ended up having to introduce some kind of ‘server’ app when I didn’t really intend to plus there was pretty bad performance when it came to passing around what are often large world anchor buffers.

That left me wanting to try out a few other options and I spent a bit of time looking at Unity networking (or UNET) but didn’t progress it too far because I couldn’t get the discovery mechanisms (based on UDP multicasting) to work nicely for me across a single HoloLens device and the HoloLens emulator and so I let that drop although, again, it looks to offer a server-less solution with a single device being able to operate as both ‘client’ and ‘host’ and the programming model seemed pretty easy.

Photon Unity Networking

Putting that to one side for the moment, I turned my attention to “Photon Unity Networking” (or PUN) to see if I could make use of that to build out the basics of a shared holographic experience and this post is a write up of my first experiment there.

PUN seems to involve a server which can either be run locally or in the cloud and Photon provide a hosted version of it and I figured that had to be the easiest starting point and so I went with that although, as you’ll see later, it brought with it a limitation that I could have avoided if I’d decided to host the server myself.

Getting started with cloud-hosted PUN is easy. I went for the free version of this cloud hosted model which seems to offer me up to 20 concurrent users and it was very easy to;

1. Sign up for the service
2. Use the portal to create an application as my first app and get an ID that can be fed into the SDK
3. Download the SDK pieces from the Unity asset store and bring them into a Unity project

and so from there I thought it would be fun to see if I could get some basic experiment with shared holograms up and running on PUN and that’s what the rest of this post is about.

The Code

The code that I’m referring to here is all hosted on Github and it’s very basic in that all that it does (or tries to do) is to let the user use 3 voice commands;

• “create”
• “open debug log”
• “close debug log”

and the keyword “create” creates a cube which should be visible across all the devices that are running the app and in the same place in the same physical location.

That’s it I haven’t yet added the ability to move, manipulate holograms or show the user’s head positions as I’ve done in earlier posts. Perhaps I’ll return to that later.

But the code is hosted here;

Code on Github

and I’m going to refer to classes from it through the rest of the post.

It’s important to realise that the code is supplied without the Photon Application ID (you’d need to get your own) and without the storage access keys for my Azure storage account (you’d need to get your own).

The Blank Project

I think it’s fair to say that Photon has quite a lot of functionality that I’m not even going to attempt to make use of around lobbies and matchmaking – I really just wanted the simplest solution that I could make use of and so I started a new Unity project and added 4 sets of code to it straight off the bat as shown below;

Those pieces are;

1. The HoloToolkit-Unity
2. The Mixed Reality Design Labs
3. The Photon Unity Networking Scripts
4. A StorageServices library

I’ll return to the 4th one later in the post but I’m hoping that the other 3 are well understood and, if not, you can find reference to them on this blog site in many places;

Posts about Mixed Reality

I made sure that my Unity project was set up for Holographic development using the HoloToolkit menu options to set up the basic scene settings, project settings;

and specifically that my app had the capability to access both the microphone (for voice commands) and spatial perception (for world anchoring).

From there, I created a scene with very little in it other than a single empty Root object along with the HoloLens prefab from the Mixed Reality Design Labs (highlighted orange below) which provides the basics of getting that library into my project;

and I’m now “ready to go” in the sense of trying to make use of PUN to get a hologram shared across devices. Here’s the steps I undertook.

Configuring PUN

PUN makes it pretty easy to specify the details of your networking setup including your app key in that they have an option to use a configuration file which can be edited in the Unity editor and so I went via that route.

I didn’t change too much of the setup here other than to add my application id, specify TCP (more on that later) and a region of EU and then specify that I didn’t want to auto-join a lobby or enable stats as I’m hoping to avoid lobbies.

Making a Server Connection

I needed to make a connection to the server and PUN makes that pretty simple.

There’s a model in PUN of deriving your class from a PunBehaviour which then has a set of overrides that you can use to run code as/when certain networking events happen like a server connection or a player joining the game. I wrapped up the tiny bit of code needed to make a server connection based on a configuration file into a simple component that I called PhotonConnector which essentially takes the override-model of PUN and turns it into an event based model that suited me better. Here’s that class;

The PhotonConnector Class

I dropped an instance of this component onto my Root object;

So, that’s hopefully my code connected to the PUN cloud server.

Making/Joining a Room

Like many multiplayer game libraries, PUN deals with the notion of a bounded set of users inside of a “room” (joined from a “lobby”) and I wanted to keep this as simple as possible for my experiment here and so I tried to bypass lobbies in as much as possible and tried to avoid building UI for the user to select a room.

Instead, I just wanted to hard-wire my app such that it would attempt to join (or create if necessary) a room given a room name and so I wrote a simple component which would attempt to either create or join a room given the room name;

The PhotoRoomJoiner Class

and so this component is prepared to look for the PhotonConnector, wait for it to connect to the network before then attempting to join/create a room on the server. Once done, like the PhotonConnector it fires an event to signify that it has completed.

I dropped an instance of this component onto my Root object leaving the room name setting as “Default Room”;

and by this point I was starting to realise that I was lacking any way of visualising Debug.Log calls on my device and that was starting to be a limiting factor…

Visualising Debug Output

I’ve written a few ugly solutions to displaying debug output on the HoloLens and I wanted to avoid writing yet another one and so I finally woke up and realised that I could make use of the DebugLog prefab from the Mixed Reality Design Labs;

and I left its configuration entirely alone but now I can see all my Debug.Log output by simply saying “open debug log” inside of my application which is a “very useful thing indeed” given how little I paid for it!

One World Anchor Per App or Per Hologram?

In order to have holograms appear in a consistent position across devices, those devices are going to have to agree on a common coordinate system and that’s done by;

• Creating an object at some position on one device
• Applying a world anchor to that object to lock it in position in the real world
• Obtaining (‘exporting’) the blob representing that world anchor
• Sending the blob over the network to other devices
• On those additional devices
• Receiving the blob over the network
• Creating the same type of object
• Importing the world anchor blob onto the device
• Applying (‘locking’) the newly created object with the imported world anchor blob so as to position it in the same position in the physical world as the original

It’s a multi-step process and, naturally, there’s many things that can go wrong along the way.

One of the first decisions to make is whether to apply a world anchor to every hologram shared or to perhaps apply one world anchor across the whole scene and parent all holograms from it. The former is likely to have great accuracy but the latter is a lot less expensive in terms of how many bytes need to be shipped around the network.

For this experiment, I decided to go with a halfway house. The guidance suggests that;

“A good rule of thumb is to ensure that anything you render based on a distant spatial anchor’s coordinate system is within about 3 meters of its origin”

and so I decided to go with that and to essentially create and share a new world anchor any time a hologram is created more than 3m from an existing world anchor.

In order to do that, I need to track where world anchors have been placed and I do that locally on the device.

Rather than use a hologram itself as a world anchor, I create an empty object as the world anchor and then any hologram within 3m of that anchor would be parented from that anchor.

Tracking World Anchor Positions

In order to keep track of the world anchors that a device has created or which it has received from other devices I have each device maintain a simple list of world anchors with a GUID-based naming scheme to ensure that I can refer to these world-anchors across devices. It’s a fairly simple thing and it’s listed here;

The AnchorPositionList Class

Importing/Exporting World Anchors

The business of importing or exporting world anchors takes quite a few steps and I’ve previously written code which wraps this up into a (relatively) simple single method call where I can hand a GameObject over to a method which will;

• For export
• Add a WorldAnchor component to the GameObject
• Wait for that WorldAnchor component to flag that it isLocated in the world
• Export the data for that WorldAnchor using the WorldAnchorTransferBatch
• Return the byte[] array exported
• For import
• Take a byte[] array and import it using the WorldAnchorTransferBatch
• Apply the LockObject call to the GameObject

That code is all wrapped up in a class I called SpatialAnchorHelpers …

The SpatialAnchorHelpers class

One thing I’d add about this class is that it is very much “UWP” specific in that I made no attempt to make this code particularly usable from the Unity Editor and to avoid getting tied up in lots of asynchronous callbacks I just wrote code with async/await which Unity can’t make sense of but, for me, makes for much more readable code.

This code also needs to “wait” for the isLocated flag on a WorldAnchor component to signal ‘true’ and so I needed to make an awaitable version of this and I used this pretty ugly class that I’ve used before;

The PredicateLoopWatcher class

I’m not too proud of that and it perhaps needs a rethink but it’s “kind of working” for me for now although if you look at it you’ll realise that there’s a strong chance that it might loop forever and so some kind of timeout might be a good idea!

Using async/await without a suitable SynchronizationContext can mean that code can easily end up on the wrong thread for interacting with Unity’s UI objects and so I added a Dispatcher component which I try to use to help with marshalling code back onto Unity’s UI thread;

The Dispatcher Class

and so that’s part of the scripts I wrote here too and I just added an instance of it to my root script so that I’d be able to get hold of it;

Passing World Anchor Blobs Around the Network

For even the simplest, most basic solution like this one there comes a time when one device needs to ‘notify’ another device that either;

• a new world anchor has been created
• a new hologram has been created relative to an existing world anchor

and so there’s a need for some kind of ‘network notification’ which carries some data with it. The major decision though is how much data and initially what I was hoping to achieve here was for the notification to carry all of the data.

To put that into plainer English, I was hoping to use PUN’s RPC feature to enable me to send out an RPC from one device to another saying

“Hey, there’s a new world anchor called {GUID} and here’s the 1-10MB of data representing it”

Now, I must admit that I suspected that this would cause me problems (like it did when I tried it with AllJoyn) and it did

Firstly, the default protocol for PUN is UDP and, naturally, it’s not a great idea to try and send MB over UDP this way and so I switched the protocol for my app to be TCP via the configuration screen that I screenshotted earlier.

Making an RPC method in PUN is simple, I just need to make sure that there’s a PhotonView component on my GameObject and then I can just add an [PunRPC] attribute and make sure that the parameters can be serialized by PUN or by my custom code if necessary.

Invoking the RPC method is also simple – you grab hold of the PhotonView component and use the RPC() method on it and there’s a target parameter on there which was really interesting to me.

In my scenario, I only really need two RPCs, something like;

• NewWorldAnchorAdded( anchor identifier, anchor byte array )
• NewHologramAdded( anchor identifier, hologram position relative to anchor )

Given that I was hoping to pass the entire world anchor blob over the RPC call, I didn’t want that mirrored back to the originating client by the server because that client already had that blob and so it would be wasteful.

Consequently, I used the Targets.OthersBuffered option to try and send the RPC to all the other devices in the room.

The other nice aspect around this option is the Buffered part in the sense that the server will keep the RPC details around and deliver it (and others) to new clients as they join the room.

Perfect.

It didn’t work for me though because, although PUN doesn’t place size limits on parameters to an RPC call, the cloud-hosted version of PUN does and the server bounced my RPCs straight back at me and after a little online discussion I was pointed to this article which flags that the server limit is 0.5MB for a parameter.

So, using RPCs for these large blobs wasn’t going to work much like it didn’t really work very nicely for me when I looked at doing something similar over AllJoyn.

What next? Use a blob store…

Putting Blobs in…a Blob Store!

I decided that I’d stick with the RPC mechanism for signalling the details of new world anchors and new holograms but I wouldn’t try and pass all of the bytes of the blob representing the world anchor across that boundary.

Instead, given that I’d already assumed a cloud connection to the PUN server I’d use the Azure cloud to store the blobs for my world anchors.

The next question is then how to best make use of Azure blob storage from Unity without having to hand-crank a bunch of code and set up HTTP headers etc. myself.

Fortunately, my colleague Dave has done some work around calling into Azure app services and blob storage from Unity and he has a blog post around it here;

Unity 3D and Azure Blog Storage

which points to a github repo over here;

Unity3DAzure on Github

and so I lifted this code into my project and wrote my own little BlobStorageHelper class around it so as to make it relatively easy to use in my scenario;

The AzureBlobStorageHelper class

There’s not a lot to it on top of what Dave already wrote – I just wrap it up for my use and add a little bit of code to download a blob directly from blob storage.

Naturally, to set this up I needed an Azure storage account (I already had one) and I just made a container within it (named ‘sharedholograms’) and made sure that it allowed public reads and authenticated writes and I copied out the access key such that the code would be able to make use of it.

I can then set up an instance of this component on my root game object;

so it’s available any time I want it from that script.

Back to RPCs

With my issue around what to do with large byte array parameters out of the way, I could return to my RPCs being as simple as their final signatures ended up being;

  [PunRPC]
  void WorldAnchorCreatedRemotely(string sessionId, string anchorId)
  {
  }
  
  [PunRPC]
  void CubeCreatedRemotely(string sessionId, string anchorId, Vector3 relativePosition)
  {

  }

because the name of the blob on the blob store can be derived from the anchorId and so it’s enough just to distribute that id.

However, what’s this sessionId parameter? This goes back to the earlier idea that I would dispatch my RPC calls using the Targets.OtherBuffered flag to notify all devices apart from the current one that something had changed.

However, what I seemed to find was that if DeviceA created one world anchor and three holograms and then quit/rejoined the server it didn’t seem to receive those 4 buffered RPCs from the server which would tell it to recreate those objects.

I’m unsure how PUN makes the distinction of “Other” but I decided that perhaps the best idea was to switch OtherBuffered to AllBuffered and then just my own mechanism to ignore RPCs which originated on the current device. Because I’m no longer sending large byte arrays over the network this didn’t feel like a particularly wasteful thing to do and so I stuck with it but it could do with a little more investigation on my part.

The other thing that I played with here was the way in which the room is originally created by my PhotoRoomJoiner component in that, initially, I wasn’t setting the RoomOptions.CleanUpCacheOnLeave which I think means that the buffered RPCs left by a player would disappear when they left the room.

However, I still seemed to find that even when I asked the room to keep around RPCs for a player that left the room the OtherBuffered option didn’t seem to deliver those RPCs back to that player when they connected again and hence me sticking with the AllBuffered option for the moment. Again, it needs more investigation.

Those big blob buffers though still cause me another problem…

Ordering of RPCs

I saw this one coming Now that the upload/download of the blob representing a world anchor is done asynchronously through the cloud in a manner that’s outside the bounds of the RPCs being delivered by Photon it’s fairly easy to see a sequence of events where an RPC is delivered to create a hologram relative to a world anchor that has not yet been downloaded to the device – it’s a race and it’s pretty much certain to happen and especially if a device connected to a room with buffered RPCs containing a sequence of anchors and holograms.

Consequently, I simply keep a little lookaside list of holograms that a client has been asked to create when the world anchor that they are parented off has not yet been created. The assumption is that the world anchor will show up at some point in the future and this list can be consulted to check for all the pending holograms that then need to be created.

The AnchorCubeList Class

Bringing it All Together

All of these components are ultimately brought together by a simple “co-ordinating” script on my (almost) empty GameObject named Root that has been in the scene all along;

The only component that I haven’t mentioned there is the use of a KeywordManager from the HoloToolkit-Unity which sends the voice keyword “create” through to a function on my Root script which kicks off the whole process of creating a world anchor if necessary before creating a hologram (cube) 3m along the user’s gaze vector.

That Root script is longer than I’d like it to be at the moment so I could tidy that up a little but here it is in its completeness;

The Root Class

Testing and Carrying On…

I’ve left it to the end of the blog post to admit that I haven’t tested this much at the time of writing – it’s a bit of an experiment and so don’t expect too much from it

One of the reasons for that is that I’m currently working with one HoloLens and the emulator and so importing/exporting of world anchors can be a bit of a challenge as it’s hard to know in the emulator whether things are working correctly or not and it’s much easier to test with multiple devices for that reason.

I’ll try that out in the coming days/weeks and will update the post or add to another post. I’d also like to add a little more into the code to make it possible to manipulate the holograms, show the user’s position as an avatar and so on as I’ve done in other posts around this topic so I’ll create a branch and keep working on that.

Beyond that, it might be “nice” to take away the dependency on PUN here and just build out a solution using nothing but standard pieces from Azure like service bus + blob storage as I don’t think that’d be a long way from what I’ve got here – that might be another avenue for a future post…