Example DIS set, Mark's
bookshelf 1
This is documentation and
notes for the creation and display of a particular set of photographic
images for the purpose of demonstration and illustration of the DIS
concept.
Photographic details and
geometry
The example photographs were acquired
using a Canon Rebel XT using a 50 mm lens (Leitz Wetzlar Summilux-R
1:1.4/50, with a Leica/R for EOS Canon adapter), at F/8, 1/5 sec, with
ISA = 800 (I forgot to reset this to a lower sensitivity, like
gazillions of other people). Lighting was all diffuse daylight from a
nearby glass door on a clear afternoon, but with little direct
sunlight, inside or out.
The image set consists of a set of 10
photographs taken from 10 viewpoints in a linear horizontal row. The
optic axes of all images are roughly parallel. Here are the images,
from left to right (in reading order, of course):
All raw images in a zipped file: Marks_bookshelf_1_DIS_raw_images.zip
(21 MB)
The spacing between viewpoints is about
2", and to reduce the perspective distortion and other geometry errors
between shots, I used a tripod that I slid along a steel rule taped to
the floor (the front two feet of the tripod along the rule edge, and a
pencil mark extension). [Sorry I didn't use metric units, as the rule I
used happened to be marked in inches.] This is a context photo, with
the tripod at the first photo location:
This is a plan view, of the photo
geometry, approximately to scale.
[[ geometry illustration ]]
The angular field of view with this
camera and lens is about [[θ = ??° =
??rad.]]. In most cases a wider angular field of view is
desireable for DIS; the range of angles from which a subject point will
viewable with DIS is about this angular field of view. I have a 17-55
mm zoom lens, but the spherical ("barrel") distortion is significant at
short
focal lengths, and I didn't want to go to the trouble here correcting
each image for this distortion. StereoMaker, the software used below
for coregistration, does include a good tool for barrel distortion
correction. In general, a wider field of view will
also require more images/overlap for an equivalent "smooth coverage" of
image space -- since you are closer to the subject, the perspective
change relative to the camera shift is higher.
The fractional overlap at the subject
distance, Sd (chosen to be at spines of the books), with this geometry
(d = 2", Sd
~= 38") is about [?? = ??%]].
Each native pixel subtends about
[.??mm]] at this subject distance.
Mutual image coregistration
In order to view the images in allignment, they need
to be coregisetered. This involves
warping each image such that the overlapping regions are projected onto
the same imaginary at a
fixed subject distance (in this case, near the book spines), and
finding the
relative offsets between each image at the subject distance. I
took some care to avoid acquisition position and and rotation errors,
and
approximately alligned the optic axes using the steel rule and tripod
on a flat floor. But there were still significant error involved, and I
was interested in how accurate
this method was. Ideally I'd like to be able to "eyeball" the
aquisition of image sets like this, in which case automated
image coregistration will definitely be necessary.
StereoView could be used to provide this
coregistration information, but I chose to use the StereoMaker tools.
Here's a brief description of the steps I used.
[[ document coregistration procedure ]]
[[ coregistration parameters and relative position text ]]
All coregistered images in zipped file: Marks_bookshelf_1_DIS_corrected_images.zip
(19 MB)
DIS demo
See Dense
Image Space (DIS) description
for a conceptual description of how these images can be arranged such
that they can be viewed in the context of a DIS. Here I describe a slow
and dirty method for visualizing a coregistered DIS set using
existing software (Space) that was not designed for this task. This is
not an easy or slick demonstration. It requires following obscure
instructions and imagining how it might look and feel with software
built for the task. Panning is currently not available, although
different parts of the image can be examined using the "windowing"
functionality. Update times are painfully slow (particularly at
magnifications < 1), as multiple large overlays are all resampled
and rendered independently; in the final software, fast "blitting" of
only the relevant "top" image will speed things up dramatically. 500 MB
RAM minimum is required for the full resolution demo. I might make a
smaller version (resampled by x 1/2) available.
[[ description of how coregistered images are loaded and converted to
coregistered images ("volumes") in Space ]]
All images as coregistered volumes (Space native format) in zipped
file: Marks_bookshelf_1_DIS_corrected_images.zip
(200 MB. Sorry this is so large; images don't compress well with
.gz/.zip, but they need coregistration info that can't be stored with
TIFF/JPEG formats. Ideally a GIS format would be used that would
integrate the two.) To view them in Space
software, unzip, drag all volumes onto a Space window, and use the
overlay viewer to select which image is "on top". [Warning: Display update is painfully
slow for purely technical, and correctable, reasons.]
[[ description of DIS navigation and navigation limitations in Space ]]
Demo
animations, from screen captures
[[ description of next steps for improving demo and streamlining the
whole process ]]
