Pico projectors (not overly detailed Wikipedia page; a nice “simples” guide here) have been around for a few years now and - as the name suggests - offer projection in a small form factor over relatively short throw distances. They are great for on-the-hoof projection but, as yet, remain relatively expensive new. I plumped a couple of years ago for the Acer C112, although Acer’s range continues to evolve. The C112 is WVGA (same as DVD) with 70 ANSI lumens brightness - this isn’t exactly stunning and you need a darkened room to see it clearly, but it still works well. What’s great is that these projectors (and it’s sibling the C110) are selling for less than £100 which makes them a bit of a steal. The updated C205 comes in at a sprightly 200 lumens which makes it a more appealing alternative or the K137 which is 700 lumens but quite a bit pricier.
In order to reduce size and costs the C112 is what’s call Display over USB (DoUSB) - that is to say the projector is recognised as a display device and JPGs sent over the USB connection to the device. This is quite different from a traditional VGA connector and requires you to mirror the display that it’s connected to. When you first connect the C112 to a Windows machine it recognises it as a mass storage device which allows you to install a Windows mirror driver/application from the device itself. This switches the connection in to display-mode and then mirror the current screen. The C112 also has a relatively simple onboard playback system that allows you to play various multimedia files. Supported formats are shown here including the all important mp4 (although my experimentation suggests that it *doesnt* support h264 as that page suggests, rather only mpeg4 simple profile) and mp3 which is great, plus usual support for JPG. Presentations are a little trickier, but you can get the Acer MPtG Converter which may be on the internal memory or from the Acer website. This actually works on your host PC to convert PDF and PPT files - it actually temporarily reduces the screen resolution of the computer, “plays” the presentation and records it to a series of audio, video and JPEG files that the projector can playback - files can get very large when multimedia is involved and this presents a problem with significant slowdown of playback on the projector itself (probably due to a restriction with loading the presentation in to internal memory). Splitting the presentation up in to smaller “chunks” and then converting them seems to work well though and offers a reasonable (lightweight) workaraound.
Unfortunately playback (screen mirroring) functionality broke with Windows 8 and so anyone using the C112 was cut adrift. Given the relative simplicity of the device, the protocol has been reverse engineered and a Linux driver developed - this uses the standard libusb to access the device then various libav libraries to mirror the display and push it to the projector in display mode. Take a look at the “hacking doc” to see how to compile it on Linux. I have Linux Mint 17 installed on my laptop - when compiling there was a problem with calls to some of the libav libraries. The developer, Antonio, is super responsive (on IRC #am7xxx over at Freenode) and realised that Mint 17 uses an older set of libraries. One diff patch later allowed it to compile and it worked perfectly.
The driver works particularly well with the C112 and is excellent for displaying presentations or DVDs, although currently only “video” is sent over the USB connection, no audio so you’d need to use the computer to play the sound (not that that’s a particular problem). It can be cross-compiled (for Windows or Android) although this is experimental at the moment. However a separate fork has a working version for Windows 8 which utilises Zadig, a port of libusb for Windows. Note that this is now no longer open source, although the last version (only works on C110) is available here
All in all an extremely capable and cost-effective solution and definitely a space to watch as devices get smaller and cheaper.
