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.
PTMs are usually
produced with the PTM fitter program which is downloadable from
http://www.hpl.hp.com/research/ptm/downloads/agreement.html
. To use the PTMfitter, one first needs to construct a .lp file.
This is easily done with a text editor such as Wordpad. The .lp
file just tells the fitter where the files are located on your
computer and what the lighting direction is for each image. Here
is a sample .lp file:
50
C:\Leaves512\91-001.jpg -0.015760 0.196076 0.980462
C:\Leaves512\91-002.jpg 0.181637 0.075514 0.980462
C:\Leaves512\91-003.jpg 0.127865 -0.149482 0.980462
C:\Leaves512\91-004.jpg -0.102547 -0.167864 0.980462
C:\Leaves512\91-005.jpg -0.191321 0.045724 0.980462
C:\Leaves512\91-006.jpg -0.269702 0.314284 0.910212
.
.
.
C:\Leaves512\91-049.jpg -0.950980 -0.309253 -0.000204
C:\Leaves512\91-050.jpg -0.587692 0.809084 -0.000204
The first line
contains the number of images the fitter will read. For each
image, the image filename is given (either .jpg, .tga or .ppm),
then the x,y and z coordinates of a vector pointing at the light
for that image. As one is looking at through the camera, the x
axis is off to the right, the y axis is towards the top, and the
z axis points at the camera from the center of the image. So, a
light positioned where the camera is would have direction vector
(0,0,1). Note that these vectors are normalized, which just
means they have length 1. To normalize any vector (a,b,c) find
its length = sqrt(a*a + b*b + c*c), and divide all three
components by this length.
To run the fitter
program double click on it. It will ask you several questions,
here are suggested answers for ones that may be unclear:
Enter desired
fitting format: 1
Enter basis: 0
That’s it, if
everything went well you should have a PTM that you can use the
PTMviewer to look at.
Printable version
