Building a lightstage

April 30, 2016 - plants / research / shadows

A Lightstage is a system which lets you completely control illumination in a particular space, and capture images from multiple views. They’re used for high resolution graphics capture and computer vision, and they’re fairly rare. I don’t think there are many non-commercial ones in the UK, and they’re research kit (which means you can’t really just go out and buy one, you’ve got to actually build it). Usually, Lightstages are used for facial feature capture, but I’m kinda interested to use them with plants. With the support of the National Plant Phenomics Centre, here in Aberystwyth, and and Aberystwyth University Research Fund grant (URF) I’ve been slowly putting one together.

The key ingredient of a Lightstage is a frame which can hold the lights and the cameras equidistant from the target object. We’ve gone for a geodesic dome construction. Here’s a time-lapse video of Geoff from Geodomes.com building ours (a 2 metre 3v dome made out of rigid struts covered in non-reflectant paint). He has a bit of help from Alassane Seck, who did a PhD here in Aberystwyth on Lightstage imaging.

Once we’d got the dome, the next job was to think about mounting lights on the dome. There are a couple of different approaches we can take, but the essential features are that some of the lights are polarised and some of the cameras also have polarising filters. This means we can separate out specular reflections (light that bounces straight off) and diffuse reflections (light that interacts more with the surface of the object). Pete Scully‘s been working on the light placement, doing a lot of work in simulation. Here’s an early simulated placement: dots are lights, boxes are cameras.

The dome was housed in the Physical Sciences building but it’s recently moved. This puts us in a room which is actually light-tight, a key consideration for reducing interference in the controlled lighting situation. Here’s an arty shot of the top of the dome in its new home.

Since the move of room (very recently) things have really picked up. We’ve got a light-proof space, and we’ve got an intern from France (Robin Dousse) working with us too. Andy Starr‘s been working on the electronics and construction from the outset, and during breaks in teaching has really driven the project forwards. Here’s a shot of Robin, Pete and Andy by the dome:

Robin’s been working on programming our LED controllers. We’ve a fairly complicated master-slave controller system, as running 100 or so ultra-bright lights is not trivial. We’re aiming for a pair (one polarised, one not) at each vertex. Here’s a 12 second video of some flashing LEDs. It’s going to look a lot more impressive than this once it’s actually going, but hey, this is actual lights lighting up on our actual dome so I am very pleased.

We’ve now also, finally, got cameras on the dome. We’re not 100% certain about positioning, but we’re getting there. Andy’s been working on the camera triggers. Soon we’ll have something which flashes, takes pictures, and gives us the data we want.

› tags: lightstage / plants / research / shadows / vision /

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