National Instruments NI myRIO, Справочник по основам

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CHAPTER 4. CAMERA SETUP 31
dot locations and therefore reduces the errors associated with the simple point
distance calibration. NI Vision Assistant provides convenient tools to evaluate
the performance of the correction and to select an appropriate balance between
performance and computational effort.
I
Study the video Grid Calibration
4
to learn how to print a dot grid target
and then calibrate the camera according to the dot grid.
I
Print the dot grid on a laser printer. Measure the distances between the align-
ment marks to confirm that the dot grid has been printed accurately. Measure
in both the horizontal and vertical directions.
I Smooth the paper to be as flat as possible on the camera copy stand base.
10
Follow along with the steps in the video to set up a grid calibration for
your camera. Record the percent distortion and mean error for the five available
distortion models.
N
OTE
: The camera calibration step must be repeated any time that you
change the camera configuration. Also be aware that the calibration is only
valid for objects that are in the same plane as the dot grid, i.e., calibrating
on the dot grid and later refocusing on a much thicker object will yield
incorrect results.
11
Try some measurements of your ruler. Include ruler positions at the center
of the image and at the edges where the distortion corrections are more notice-
able. How do these measurements compare to the mean error value reported
by the grid calibration procedure?
I Intentionally introduce tangential distortion by tilting the camera relative to
the object plane; expect to see perspective distortion as imaginary lines intersecting
the dot grid now converge rather than remaining parallel.
12
Re-calibrate your camera and select the option to correct tangential distor-
tion. Screen capture both the uncorrected and corrected images for comparison.
13
Again, try some measurements of your ruler. Discuss the ability of grid
calibration to deal with perspective distortion.
4
http://youtu.be/eWqq65ZZ0NQ
(6:47)