Origin location in a corrected image (using calibratio​n data)

Question

Hi there,
I am using the vision toolkit to correct deformation in images.
I am not using the dot grid template to let the software generate the calibration
map, but instead use a set of points (which I can precisely define
in the deformed image) and provide their respective "real world"
coordinates. I can then apply this correction to other images.
Up to now everything is fine.
Now I want to overlay two sets of images, which are deformed
differently (for instance, they represent the same object, but have
been imaged in slightly different conditions). I
can correct one set using the approach above, and the other separately, using
its own separate correction, but I am stuck there.
Each calibration/correction is characterized by a scaling factor and an
origin coordinate. I need both to be able to overlay the two sets of images.
I have chosen non-linear calibrations that preserves area and I figured a
way to recover the scaling factor by computing the areas of all the square
pattern in my set of calibration points (as computed in the original image), taking
the average and then the square root of this average area: this gives the
spacing between equidistant points in the corrected image.
However, I can't figure out how to get the location of the origin in the
corrected image. One of my calibration points is associated with position
(0, 0) in the real world. However, this is neither its position (in pixel unit)
in the original image or the corrected image. I need the latter. How can I
obtain it simply (I can think of sophisticated ways to obtain it, but I suspect
there might be trivial one).
Thanks in advance for your answer.
Sincerely,
X.
Solved!
Go to Solution.

Answer 1

Dear X,
I'm not sure if this is the information that you are looking for, but take a look in the Vision Concepts Manual, page 3-10
start»programs»National Instruments»Vision»Documentation»NI Vision»Concepts_Manual.pdf
"If you specify a list of points instead of a grid for the calibration process,
the software defines a default coordinate system, as follows:
1. The origin is placed at the point in the list with the lowest x-coordinate
value and then the lowest y-coordinate value.
2. The angle is set to zero.
3. The axis direction is set to indirect."
If not, I don't know of a good way to recover the transformation between an original image and a correct image for a specific pixel. It would be one thing if it were a simple linear calibration, but we don't have access to the algorithm used to apply the nonlinear correction, which changes based on the locations of the distortion in the image.
What are you looking for in overlaying the images? Are you doing a comparison?
I don't know the constraints of your project, but one thing you could try is doing a pattern or geometric match for a unique feature in both of the corrected images that you want to overlay (one you know the location of, and that will serve to align the images properly), and then define the coordinate system based on that match.
Hope that helps!
~Nate