Pi Internet-controlled Rotating and Streaming Video Prototype!

After lots of work, the prototype has been successfully operated in all desired configurations.

This post is a wrap-up of all that went into completing this milestone.

Premise

I wanted to see if I could make a remote control movable real-time video streamer out of the Pi, a Webcam, and a Servo.

Bonus points added if it works over the Internet with no onerous setup.

Bonus points added if I don't have to program anything to do with video encode/decode/transcode.

The gist of how I wanted to do it:

• Get a Raspberry Pi
• Put a Webcam on top of a Servo
• Have the Pi operate the Webcam (USB)
• Have the Pi operate the Servo (GPIO)
• Run some Pi software which can read from the Webcam and stream via WebRTC
• Write some Pi software to operate the Servo
• Write some software to expose both the WebRTC and Servo control exposed on the internet
• Write some software to make use of the two interfaces and combine them into a single interface

Defined so broadly, it's hard to see how this wouldn't work.  Challenges came from the details at each stage.

I will cover each point below in varying degrees of detail.

Get a Raspberry Pi

I went with a Raspberry Pi 2.  This turned out to be a good choice since the software for WebRTC only works for Raspberry Pi 2.

Also got a WiFi dongle.

Put a Webcam on top of a Servo

Ha, just use some twist-ties and rubber bands.

Have the Pi operate the Webcam (USB)

The statement is vague, but the basic meaning is to let the Linux distro support the details of interaction with the USB Webcam.

USB isn't a prerequisite per-se, but it's common and what I had on hand.

I happened to be using a Logitech QuickCam Pro 9000, which worked well for this purpose.

I'm not super familiar with Linux Kernel support for video devices, but I've come to understand there is a class of video devices which adhere to the UVC standard, which stands for "USB Video Class."

Anyway, plug that into the Pi and you'll see that it's recognized straight away.  Here is the /var/log/messages entry.

Dec  3 19:12:49 raspberrypi kernel: [348152.387472] usb 1-1.4: new high-speed USB device number 5 using dwc_otg
Dec  3 19:12:49 raspberrypi kernel: [348152.614881] usb 1-1.4: New USB device found, idVendor=046d, idProduct=0990
Dec  3 19:12:49 raspberrypi kernel: [348152.614907] usb 1-1.4: New USB device strings: Mfr=0, Product=0, SerialNumber=2
Dec  3 19:12:49 raspberrypi kernel: [348152.614925] usb 1-1.4: SerialNumber: 4BA96858
Dec  3 19:12:49 raspberrypi kernel: [348152.853207] media: Linux media interface: v0.10
Dec  3 19:12:49 raspberrypi kernel: [348152.881669] Linux video capture interface: v2.00
Dec  3 19:12:50 raspberrypi kernel: [348153.320477] usb 1-1.4: Warning! Unlikely big volume range (=3072), cval->res is probably wrong.
Dec  3 19:12:50 raspberrypi kernel: [348153.320512] usb 1-1.4: [5] FU [Mic Capture Volume] ch = 1, val = 4608/7680/1
Dec  3 19:12:50 raspberrypi kernel: [348153.321508] usbcore: registered new interface driver snd-usb-audio
Dec  3 19:12:50 raspberrypi kernel: [348153.327973] usbcore: registered new interface driver uvcvideo
Dec  3 19:12:50 raspberrypi kernel: [348153.328001] USB Video Class driver (1.1.1)

I hadn't known at the time, but my webcam shows up on the list of UVC supported devices which can be found here:  http://www.ideasonboard.org/uvc/

Have the Pi operate the Servo (GPIO)

There is so much written about how to make a servo move online it's not useful to re-write it here.

I did make a post about some issues I ran into early on since I'm an amateur regarding this kind of thing.  Post here: (link).

Basically, watch out for power and feedback issues.

Run some Pi software which can read from the Webcam and stream via WebRTC

This was actually one of the first things I researched when starting on this project.  I posted some thoughts on it at the time here (link).

All told, UV4L (Userspace Video 4 Linux) is what I went with, and some early successes in my work on this demonstrated it had a high likelihood of working in all desired configurations.

I cared a lot about this part since working with video is likely very difficult, and definitely something I don't know how to do.  Nor do I want to do it.

Most of the majorly-difficult issues are solved if there is something can do this task.

Specifically, UV4L solves the difficulties in:

• Reading video from the webcam and doing anything whatsoever with it.
• Encoding the video stream to something appropriate for the end-consumer.
• NAT traversal.
• Transmitting video while adapting to congestion, latency, buffering, etc.

The first point is the specific implementation relating to UV4L, which does support the UVC driver for webcams, thankfully.

The last three points are attributes of whatever can 'do' WebRTC, which UV4L can.


Notably, the UV4L process supports Signaling via websockets and JSON-formatted message passing.

However, due to the purposefully-undefined Signaling requirement for setting up WebRTC, I was forced to sniff out the specific message structures defined by UV4L.

In short, I hid the details of UV4L behind a translating proxy I wrote such that I could replace UV4L for something else if I want to later.  And other reasons related to the RDVP Server which I talk about later.

Also I wasn't 100% happy with the interface presented by UV4L anyway so bye bye to that also.


You can find lots of details about UV4L starting with the announcement about WebRTC support on their site (link).

Note that their site says only Raspberry Pi 2 is supported for now.  Perhaps this will change.

Write some Pi software to operate the Servo

I basically wrote a piece of software which accepts commands to move a servo.  And then it does it.

The software has no idea there is a webcam strapped to the top of the servo.

I had a post about this here: (link)

Write some software to expose both the WebRTC and Servo control exposed on the internet

From a prior post (link) I noted the high-level architecture of the overall setup I was aiming for.

It mostly discussed the function of what I called the Rendez-vous Point (RDVP) Server.

The RDVP Server is a central place where different services and clients can register themselves for the purpose of having a connection set up between them.

This satisfies most of the "control from the internet" requirement.  Since both the controller and controlee will "connect out" to a place on the internet, NAT issues are basically eliminated.

In the diagram below:

• The left-hand-side is the Raspberry Pi.
• The upper-right-hand-side is the RDVP Server, which needs to be anywhere accessible to both the Pi and the Controller.  If you want internet control, it had better be on the internet.
• The lower-right-hand-side is the Client (controller).  It's a Browser in this diagram, but could be anything really.

The "translating proxy" I discussed in the UV4L section is labeled as the WSBridge, which also acts as a male-to-male proxy.

Meaning I can ensure it reaches out to the RDVP server to make itself known.  Once connected, it can relay messages back to UV4L for the purposes of setting up a WebRTC session.

Write some software to make use of the two interfaces and combine them into a single interface

As noted in a prior post (link), I chose WebSockets as the mechanism for communication between server-side processes.

I did this with the forethought that I'd ultimately have a Browser involved in the action, and browsers speak WebSockets well, so why not be uniform.

A security dimension of WebSockets on Browsers is that at the time of writing, Browsers only want to open WebSockets to the host which served up the page you're on.

So, that means the RDVP Server needs to also serve up a web site which contains the code to connect back to the RDVP Server to control the Servo and UV4L.

Not so hard.  In fact, the WebSockets library I used was Tornado for Python, which has lots of code already written for serving up webpages.

So, I wrote some simple HTML/Javascript to serve up from my RDVP server, changed the Server to serve it up, and pointed a Browser at it.

The moment of success

Pressing the 'connect' button leads to javascript connecting two WebSockets to the RDVP server.

Speaking RDVP language, they asked their messages to be relayed to the endpoints on the Pi servicing the UV4L and Servo controllers.

From there, the webpage sets up a WebRTC session with nearly no interaction from the user other than to accept that the camera is about to be used.  Once the remote video stream is acquired, it is dropped into a video tag on the page and the rest is handled by the Browser.

Additionally, the buttons 0, 10, 20, ..., 100 become active.  Click them, and a message is sent to the Servo controller indicating that it should move to that percent of its range-of-motion.

This is a screenshot from the laptop I was working at.

A few notes on network conditions relating to WebRTC and NATs

Key in getting video to stream properly is NAT traversal.  So I wanted to be sure that wherever the Pi was, I'd be able to run the Browser somewhere else.

The NAT traversal is handled by WebRTC libs I never have to touch, but can configure.  I instruct those libs to use the google STUN servers to identify both endpoints' locations for WebRTC handshaking.

TURN (TCP relaying of traffic) is never used.  It is always peer-to-peer by constraint.

With that said, I note that the moment-of-success was:

• Run a RDVP Server on the Internet
• Run the Pi in my apt on the LAN WiFi
• Run my laptop tethered against my cell phone (so outside the network of both other systems)
• Created a peer-to-peer (non TURN) WebRTC connection between the Pi and my external laptop.

I wanted to try several configurations for where the Pi and Browser were in relation to one another's networks.

Always keeping the RDVP Server on the internet, I have subsequently placed the Pi and Browser:

• On the same LAN (in my apt)
• Apt LAN and Internet (Browser tethered to cell phone, maybe no NATing going on)
• Apt LAN to different LAN over the internet (so two computers in different LANs having to do NAT traversal)

Issues:

• Framerate is slow.  I think it may be a combination of:
• Congested 2.4GHz WiFi.
• UV4L CPU utilization.
• No attempt yet made to configure the Webcam to operate differently.

• The servo jerks around a lot, I think due to the GPIO pins being software-based, and the PWM not hitting its timing targets.  Seems much worse than normal, I think it's being affected by UV4L.
• I'd like to solve this with RPIO, but that's not supported for Raspberry Pi 2 (yet).  Perhaps another lib will work for the time being.

• The webcam on top of the servo is not particularly stable and keeps tipping over!

WebRTC Prototype Works!

It works!

Tested successfully on:

• The local LAN between two browsers on the same laptop
• The local LAN between phone and laptop
• The internet, phone on cell data, laptop on LAN

The RDVP Server acted as middleman and all worked as hoped.

Only STUN servers were used (the google ones).  No TURN.  Therefore direct peer-to-peer transfer while busting through the LAN NAT.

A special testing webpage was created (served up by the RDVP Server) which allowed step-by-step negotiation.

The step-by-step page was necessary just to work out how the WebRTC handshaking actually worked, since a number of examples found online were difficult to follow.

Video quality was good.  I requested and sent both audio and video streams, however I muted the audio on both ends (in the video tag) so I wouldn't hear any feedback, etc.  Something to test more in the future.

The video stream selected from the laptop defaulted to the primary lid camera, not the USB camera I also have plugged in.  For the phone, it defaulted to the front-facing camera.  Also something to test more in the future.

The webpage is far from a finished product.  It will, however, stay in existence to test out additional features of WebRTC and as a testbed while more featureful libraries are built.

Here are some screenshots.

This is a screenshot of the phone, front facing the screen.  The left side is what my front-facing camera saw, which is my second laptop screen displaying the laptop-side webpage.  The right is what my laptop camera saw (which is me aiming the front of my phone at my screen).

This is also a screenshot of the phone.  This time taking a self-portrait with my laptop in the background.

Lastly, here is a screen grab of the chrome webrtc-internals page for the session, on the laptop, keeping stats about received data.  I think this one was for an audio track of the stream, given the low bandwidth (and audioOutputLevel stat).

Overall, several megabits of data were being sent out by the laptop to the phone.

The frame rate received on the phone from the laptop wasn't as good as the frame rate in the other direction.

Next up, now that I understand WebRTC handshaking a bit better, time to sniff out the message-passing format that UV4L uses, and set up a connection.

Network Control Architecture

The first prototype of the Rendez-vous Point (RDVP) Server and Client was completed today.

This is a network communication scheme that aims to answer some of the basic questions that come up about communication between different devices.  Namely how do they find one another and communicate?

Objectives include:

• Minimal or no configuration
• Maximize discovery of services and capabilities
• Common protocol
• Easily extensible
• Interface available to wide variety of devices
• Agnostic as to whether operating on local network or internet

Some of the first uses intended include:

• Discovery mechanism for purposes of WebRTC
• Signalling mechanism for purposes of WebRTC
• Remote control of Pi locally or over internet

The basic principles are:

• There is a centralized server (the RDVP Server)
• Everything connects to it
• Password protected
• Primary functions include:
• Act as a transparent bridge between clients who want to communicate directly.
• Allow Admin control
• Present web interface
• Enforce tightly-defined roles
• Three tightly-defined roles for systems connecting in:
• Admin Client
• Speaks RDVP protocol (JSON over WebSockets)
• Can monitor and control activities and state of Server
• Likely spawned from web interface, but also possibly from other system such as those implemented in Python in the first prototype client.
• Server Client
• States ID and waits for messages to be sent to it, which it will respond to as-appropriate.
• No messages will be sent to the Server Client unless a Client Client indicates a desire to connect to it, and the RDVP Server bridges them.
• Client Client
• States ID and intended Server Client to connect to.
• Connection to Server Client will immediately be bridged or this client will be rejected and closed.

The expected use of the above is:

• Server Clients might be anything "dumb" (like a Pi program) which is waiting to be controlled but doesn't know by who.  Simply announce itself and wait.  This also makes internet communication and control possible since the RDVP Server can easily live on the internet.
• Client Clients are expected to be initiated from more knowledgeable control mechanisms.  This might be from a web interface, or other purpose-built mechanism.
• Admin Clients are meant for systems which want to discover Server Clients to connect to, or otherwise observe state.

Having a central server which brokers communication also allows for snooping of communication as well as control over systems connecting in.

Future projects should be able to make use of this scheme as well.

A very specific first-use-case will be attempting to initiate WebRTC with the UV4L server.

This UV4L server is a bit opaque but does have a webserver which also responds to WebSocket connections leading to a WebRTC connection.

I don't want the webpage, I just want it to give up its video stream.  In the diagram which follows, there is a bridge between the UV4L server and the RDVP server.  This is essentially a male-to-male connection which itself is a bridge, making connection to the UV4L server possible remotely.

Here is a rough architecture diagram:

Pi SSH Disconnects and Power Management

Solved an ongoing issue with the Pi, and that is I often come back to my computer with all of my SSH terminals disconnected.

I would log back in again, and check that the uptime was still days whereas I had been away for much less than that.  So I concluded the Pi hadn't rebooted, thereby disconnecting the terminals.

It always seemed to be after a somewhat lengthy time away, but sometimes not all that long (like a few hours).

I thought first it was possibly idle connections being cut.  I played around with some settings but it didn't really do much.

After some reading online, I decided to try looking into power management on the WiFi adapter.  This was the solution.  The stackexchange discussion which led me to the solution is here (link).

The adapter I use is the recommended adapter noted in a prior post (link).

The commands required are seen below.

$ iwconfig
wlan0     IEEE 802.11bgn  ESSID:"DV_2.4Ghz"
          Mode:Managed  Frequency:2.412 GHz  Access Point: 00:81:D8:D5:46:3C
          Bit Rate=5.5 Mb/s   Tx-Power=1496 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=70/70  Signal level=-34 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

$ sudo iwconfig wlan0 power off

$ iwconfig
wlan0     IEEE 802.11bgn  ESSID:"DV_2.4Ghz"
          Mode:Managed  Frequency:2.412 GHz  Access Point: 00:81:D8:D5:46:3C
          Bit Rate=5.5 Mb/s   Tx-Power=1496 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality=70/70  Signal level=-33 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

After having run this command, the issue has not come up again.

A persistent on-boot configuration will likely be required.  I doubt this command stays in effect across reboots.

Network Servo Control Demo

Now that my WebSockets libraries are working better, I decided to try to use them in a program which can control a Servo.

To make the demo more interesting, I strapped a webcam to the top of the servo so I could use it like a DIY security cam.

The servo controlling software opens a WebSocket port to listen for incoming messages.  For now, the messages are simple -- A number, which represents the percent of range of motion of the servo.

So if you send 50, that means 50% of the range of motion, and the program will instruct the servo to move there.

To send messages to the servo controller, I use a super simple program written to bridge stdio to WebSocket connection, basically a WebSocket 'telnet'.  This is my wsEndpoint program.

Feeding into the wsEndpoint is stdio, but now controlled by a 'tk' gui program showing the numbers 1-100.  Click the number, it prints the number to stdout, which is relayed over the WebSocket connection, and the servo moves.

In the screenshot above you can see a washed-out square image on the screen, that is the grid of numbers.

The webcam in this demo is hooked up to my laptop, not the Pi.  No WebRTC involved here.

Network Control of the Pi (WebSockets) 2

Some success with WebSockets, finally.

I shied away from using asyncio support for WebSockets for several reasons:

• I don't know much Python programming and don't understand the examples
• It seems to be a much lower-level implementation than I want (maybe mistaken)
• I don't want to require Python 3.4, which is required for asyncio.  I want to be able to use Python 2.7 in the event that I get some less-capable or less-modern devices involved.

I tried several libraries, and in the end had difficulty getting them to work properly on Pi.

In the end I fell back to use Tornado libraries, which is among many things, lets you implement and use:

• An event manager
• WebServer libs
• WebSocket client libs

With these I've been able to build a basic set of libraries which allow me to write very short Python programs which can:

• Listen on arbitrary WebSocket addresses (eg ws://127.0.01:4000/).
• Connect to any arbitrary WebSocket adddress.
• Handle console input while doing any of the above.
• Do all of the above via an Event Loop, no blocking required.

Difficulties came up understanding how various events fire in association with WebSocket connections working, etc, and how that fit into the interface I wanted to build.

An example being - What if your program tries to connect outbound (async) and wants to cancel the attempt before finding out whether it worked.  Not supported (from what I can tell).  I had to implement a wrapper to fake it.

Network Control of the Pi (WebSockets)

I don't know what the specific architecture will be for control of the Pi over the network.

However, it does appear likely at the moment that:

• Python will be involved
• WebRTC will be involved (in some cases)
• Browsers will be involved

This leads me to consider WebSockets, which is bi-directional message passing, supported by Browsers, and also now naively by Python and other languages.

Python is in scope due to it being the language of choice on the Pi, and is covered by a number of libraries which claim WebSocket support.

WebRTC is in scope due to the requirement for "Signalling," which is the discovery and transfer of media/network details.  This can easily be done over WebSockets, and is seemingly commonly done that way.

Browsers are in scope since any control systems put together would be much nicer with a point-and-click interface, and Browsers are perfect for this.

So, I have deiced to learn how to implement, in Python, some libraries which can act as the middleware between my Pi and the internet (probably a relay of some sort for WebRTC Signalling).

Also any process-to-process communication on the Pi can be done over WebSockets because why not.  Keep it all simple and hopefully make the testing easier as well.