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PS2a: Camera Calibration, Details.
(By Gabriel Brostow, TA for DVFX 1999-2002).
- Check camera inventory sheet when checking camera out, so you’re not held
responsible for things which were already missing. For this project, you
should not need to borrow equipment for more than a day, but you have it for
few days just in case. You should buy one or more of your own DV tapes. These
can be close to $5 if you buy in bulk (feel free to share shopping tips on the
SWIKI). When using a blank tape for the first time, record it from start to
finish with the lens covered, so it has a consistent time-code. This will
prevent problems when dumping to the drive.
- Remember to switch to manual focus! You can print a calibration file of
your own (make the squares are all the same and line them up) and mount it on
a perfectly planar surface. For a checker-pattern that is nominally (because
printers stretch printouts) 28mm^2, grab /net/cpl/NT/OpenCV_alpha3/calib/checkerGrid4x5_28mm.pdf.
You can also use the larger calibration pattern located in the test-space of
the DML. Remember to measure the
exact width and height of a square. For example, measure width of 4 boxes (B,
W, B, W) and divide total by 4 to get a more accurate X-dimension. Record in
cm. for later. Origin is in upper left of the pattern pictured here, and I
skipped the first and last row of boxes since the printer chopped them short.
Move the pattern around so it takes up most of the field of view, and try to
explore the space: show different positions and orientations, keeping the
origin-corner as the left-top most corner in the image. Without moving the
camera (or changing the zoom/focus!), move around in the scene or do something
simple – this is just to show that it’s a sequence and not a single frame.
Keep it short (30sec or less), and possibly make it relate to the 3D graphics
you’ll be adding later. Don’t worry if you don’t yet know where you’ll be
adding stuff to the scene (just glare in random directions for now). Snap
multiple “home” position images with the pattern appearing bigger or smaller
(see #8 below).
 
 
- Keep this under 30 sec. also, and don’t move the camera too drastically.
This will be used only if we can obtain some match-move software licenses, and
will only be important for the more ambitious groups.
-
Each camera has a DV cable, which doesn’t look like the RCA, or the
S-Video cables in the bag. It connects to the panel on the front of the DVFX
PC’s in the States PC cluster. DVRaptor software is installed on all those
machines, and you don’t have much need for the Navi software right now, unless
you already recorded so much data that you want to easily index to the tape.
Turn the camera to VCR mode before starting the software, and you’ll be able
to control the “deck” using the on-screen controls. Video plays back on the
LCD of the camera, and may not show up in the Raptor software’s preview window
– that’s ok. I usually use the batch capture utility (in one of the menu
options). It lets you mark the start (in) and stop (out) points of each part
of the tape that I want to dump to a separate file. Once marked and given a
file-destination, “ADD” each desired clip to the batch-capture list. You can
save and update this list if things break down for some reason. If you get an
error that the “in point” can’t be found, you may need to reboot the PC (so
save that list to a safe place). Dump to E since it’s fast and hopefully not
very fragmented, and either capture the data to smaller clips (calib clip,
action clip, moving camera clip, etc.), or segment a larger file in Premiere
so that you don’t have to burn drive-space by saving video which you won’t
use. That said, don’t trash data you “might” want later. CD-burners should be
working now (hint-hint).
- Premiere is ok for this, just export a frame instead of a movie. Don’t
dare change the image size or use lossy compression! You will want to copy &
rename the extracted images so that the last one still has a number less than
100. It hurts the calibration software to have to run through 2000 non-images
to get to frame_2001. Just count from 1, and everything will be fine.
- It seems to run faster when using a local copy:
\net\cpl\NT\OpenCV_alpha3\calib\TOOLBOX_calib\
- Follow the “First Calibration Example” on this page:
\net\cpl\NT\OpenCV_alpha3\CVL_html\appPage\calib_doc\index.html.
The calib_gui program’s interface is pictured below. Basically, follow
the onscreen instructions/questions and the hints that appear in the images to
perform all the tasks leading to “Exit” (starting in the top left and going
across). I’ll put up a
trace of the program when I used it. Reprojection error MUST be less than
a pixel, and ought to be around 0.1 pixels if you throw out the really bad
images and keep around 8 good ones. Reprojection: the corner-finder puts
little crosses when it finds something, and the corresponding point from the
3D model of your calib pattern is rendered using the calculated calibration as
a circle. These should line up.
- What can I say - it’s the box in the lower left. If you
add/suppress it into the list of active images, you can visualize (and
sanity-check) where it was relative to the other calibration images. If it’s
way off (because, say, it appeared too small in the image), you’ll want to use
a different image as the “home”.
- Remember the submission directory is /net/dvfx/DVFX/2004/PS2A/your_group_number/
as mentioned earlier, we w will use time-stamps of files to determine
when something was submitted, so if you choose to update the file, give it a
different filename.
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