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Making PTMs

 

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PTMs are a simple representation and file format that allow lighting direction and material properties to be controlled interactively. This is especially useful for seeing subtle shape detail on the surface of objects.  PTMs are produced from a set of images taken with varying lighting direction. There are two general approaches to making PTMs:

1. Highlight-based PTMsPlace one or two red or black balls (snooker balls work well) next to the object you are photographing. Take multiple pictures of the object and ball(s) with a digital camera set on a tripod, triggered by a handheld flash. Move the handheld flash to various positions keeping it at fixed distance to the object. Allow the flash to trigger the camera, taking multiple images with different lighting directions. Put all the images into a directory, and choose that directory within the HighlighPTMbuilder tool. This tool will automatically find the ball(s) in the images, recover the light direction and build a PTM from this dataset.

2. Manually built a .lp (light position) file : With a text editor that tells the PTMfitter where the image files and lighting directions are. You can use any number of aids to collect the images under know lighting direction, some methods we and others have employed are shown below. 

Our original dome was made out of hot-melt glue and wooden dowels in the shape of a subdivided icosahedron. A tripod holds a camera looking down from directly above and a handheld lamp is sequentially moved to each triangular face  to collect 40 images. Images were taken at night to avoid ambient illumination.

  

Although somewhat complex, this automated dome collects 50 images under varying lighting without manual intervention. It incorporates 50 custom-made flash boards under hardware control. A digital camera mounted on top is controlled by a laptop, where images are immediately downloaded to after they are taken. Construction by Bill Ambrisco and Eric Montgomery.


This ‘light-arm’ was designed and constructed by Bill Ambrisco and uses 12 commercial photographic flashes on a manually rotated arm. A separate tripod holds the digital camera, avoiding mechanical shake. It views the object or scene through the races that provide the pivot for the arm. This device was used by the National Gallery in London to collect PTMs.





 

This portable device was constructed for John Yoshida at  California Department of Justice for collecting PTMs of footprints. Mike Cavallo did the electrical design and software. The system uses flash bulbs as the illumination source and the arm is manually rotated to vary incident lighting direction in one axis.

 

Wouter Verhesen built this very inexpensive PTM setup in the Netherlands. A used point-and-shoot digital camera  was modified to extend the flash unit. It is moved to various cutouts in a Styrofoam hemisphere to vary lighting direction.

 

 

 

Cultural Heritage Imaging has built a PTM dome that allows careful control of light source color spectra in addition to lighting direction. Light is filtered at the illumination source and brought into the dome with fiber optic cables. This permits elimination of unnecessary and/or damaging light wavelengths, which makes imaging of light sensitive materials possible. For example, the CHI dome has been used to image fragile wax and lead seals attached with string to medieval documents, and oil paintings. Multi-spectral imaging has also been done using this equipment.






Cultural Heritage Imaging has acquired PTM images for using a the simple photographic assembly shown to the left. A digital camera is fixed to provide an overhead view of an object. The object rests on a template that directs the photographer to place a light source at controlled position in x,y,z. Although the acquisition sequence takes some time, it requires minimal hardware.
 

We have built a real-time system that flashes 8 high intensity, white, L.E.D.s (Luxeon Vstars) at 500 f/second while a synchronized high speed video camera captures  frames and feeds the images to a GPU. Normals vectors are computed at 60 f/sec and used to display objects with transformed reflectance functions  in real time. The EGSR paper is available here

 

Paul Debevec’s group at USC has also built a series of ‘light stages’ for collecting images and video of actors under varying lighting.

 
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