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iPhone RTI

14 May, 2012

I have been spending quite a lot of time lately working with Reflectance Transformation Imaging (RTI). It is a computational photography technique developed by Tom Malzbender at HP labs. It is based upon photographing an object from a fixed camera position, in each image the lightsource is moved. These images are then compiled into an interactive image within which the light can be virtually moved. As well as light movement the RTI allows the user to control a range other other variables and to render the surface using a range of algorithms which alter the surface appearance of the objects in the scene. In archaeology this technique has great potential as a means of dissemination (this beats a normal photo) but also as a means of interpreting things which are not normally visible to the naked eye. It is this technology which first inspired our (Nicole and I) project, Re-Reading the British Memorial. Much more can be found on the website of Mark Mudge and Carla Schroer’s organisation Cultural Heritage Imaging. They have popularised this technique for cultural heritage in the USA and have created an abundance of materials and advice all available at no cost. Amazing.

One of the things which makes RTI capture so fantastic is that you need very little equipment to get recording. As if to prove this point, Graeme noticed this morning that the IOS 5 version of the interface for the iPhone has a focus and exposure lock. This means that the iPhone camera can capture RTIs! Here is how we did it, with preliminary results.

So the first challenge was to attach the phone to a tripod, this was easily solved with a few elastic bands. Next and perhaps most challengingly we had to figure out how to adjust the exposure control so that the vast majority of the light in our images came from our mobile light source instead of the ambient light in the room. There are no explicit settings for this on the iPhone so you need to trick it. We put the light source right by the object we were capturing to slightly over-expose the image. We waited for the autofocus to adjust to the high light levels, then I turned the autofocus/exposure lock on by holding my finger on the screen for a few seconds. When you take the light source away you have a totally dark image which becomes beautifully exposed when your light is switched on.  Perfect.

Next, you capture as normal. We had no way of triggering a flash from the iPhone so we used a constant light source (a deconstructed desk lamp). We didn’t have a remote control either so we had to just very gently tap the shoot button and hope that the camera didn’t move. The highly sensitive controls on the iphone made this possible but I have noticed that a remote control is available so probably is worth the investment. In this case we were lucky and the camera didn’t move.

The result was extremely impressive, the iPhone camera is  only three megapixels so this is not the highest resolution option but it certainly has the power to reveal things which are invisible to the naked eye. More importantly it places the technology needed to make RTIs into the hands of anybody with an iPhone a desk lamp and a reflective sphere.

I am sure this is possible with other mobile devices too. Get your phone out and have a look!

Lit from the front:

Lilt from the front

Raking Light:

Raking light

Specular Enhancement:

Specular enhancement

Diffuse Gain:

Diffuse gain

— Gareth

I have added this post to my own blog also: Here I write mostly about my own research into Roman painted statues and digital representation, so take a look there if you are interested in following this work.


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