Tuesday, 22 April 2014

Arc-Team tries Large Scale Reflectance Transformation Imaging (RTI)

With the data, collected during our mission presented recently in the post „@MAP“the Arc-Team Mobile Mapping Platform, we've tried for the first time to apply a method called Reflectance Transformation Imaging (RTI) on landscape:

Aerial Photo of the project area taken from Arc-Teams Drone

RTI is a computational photographic method that captures a subject’s surface shape and color and enables the interactive re-lighting of the subject from any direction. RTI also permits the mathematical enhancement of the subject’s surface shape and color attributes. The enhancement functions of RTI reveal surface information that is not disclosed under direct empirical examination of the physical object. (...) RTI images are created from information derived from multiple digital photographs of a subject shot from a stationary camera position. In each photograph, light is projected from a different known, or knowable, direction. This process produces a series of images of the same subject with varying highlights and shadows. Lighting information from the images is mathematically synthesized to generate a mathematical model of the surface, enabling a user to re-light the RTI image interactively and examine its surface on a screen.“ (http://culturalheritageimaging.org/Technologies/RTI/)

We've used the processing software and viewer of Cultural Heritage Imaging, their RTIBuilder software is made available under the Gnu General Public License ver. 3.

RTI is usually used for objects of small or medium size beause of the difficulty or impossibility to illuminate whole structures or even areas / landscapes.

At this point GIS comes to our aid:

Starting from a DTM it's easily possible to create shadow reliefs with GRASSGIS' module r.shaded.relief
The highlight of the module in our case is the capability to modify the altitude of the sun in degrees above the horizon and the azimuth of the sun in degrees to the east of north. 

In this way we could produce artificially the needed data for our RTI-landscape attempt. 
The next step was to export from GRASS a set of 60 images with different lighting positions creating an imaginary light dome around the object:

At this point we reached the first bottelneck of our approach:

Usually, you include at least one reflective sphere in each shot. 

The reflection of the light source on the spheres enables the processing software to calculate the light direction for that image. 

So we had to create and copy in every image a fake sphere with the reflection corresponding to the sunlight direction choosen in GRASS.

It was a stiff piece of work!

At the end everything was ready for processing the images in RTIBuilder. The single steps in the software are very easy to execute and well described on the ProcessingGuide

We've just had some problems with the size of our images (8200x6500 pixels), which the software couldn't process, but maybe it was because of the age of our hardware...

After reducing the image-size everthing worked fine...

At the end, after installing also RTIViewer, we've held in our hands an interactive scene of an archaeological site of nearly 10.000m2 which is almost invisible from the ground.

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