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Image-based relighting methods
require multiple images of a static object under varying, known lighting
directions. A simple way of collecting these images and light directions is to
place a shiny black or red sphere in the scene before capturing a number of
images illuminated by an external light source (such as photographic flash) from
various directions. A simple approach is to set a digital camera on a tripod and
set a handheld flash to trigger the camera. Move the handheld flash to various
positions, keeping it a fixed distance to the object being photographed. If
these images are placed in a directory, and that directory specified in the
PTMbuilder application, highlights appearing in the ball will be automatically
found for each image. The software will then compute light directions from these
highlights and construct a
PTM
from the images automatically.
This
PTMbuilder software can be freely downloaded (URL for
PTM Builder Software License) and was jointly developed by teams at
HP Labs and at University of Minho, Portugal (Joao Barbosa, Joao Sobral and
Alberto Proenca). At present, the software runs on Windows or Linux, but Apple
systems are currently not supported. Once downloaded and unzipped, the process
is started by double-clicking on the LPtracker.jar file. The program starts by
prompting you to specify a directory containing images with varying lighting
directions, each containing a shiny red or black ball.
Suitable reflective spheres are
available from various suppliers, two sources are
here and
here. An example of highlight based captured is discussed in the VAST 2006 paper
available from
Tom Malzbender’s
home page.
*image
credit: Cultural Heritage Imaging, 2007.
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