IMAQ Calibration Error Map Units

[chaning]

What are the units associated with the error map data available after using the IMAQ Calibration?  I can't seem to find it anywhere in the documentation.

Thanks!

[VGA_CD-ROM]

Hi chaning,
If I understand your question correctly, are you asking how you specify or determine the real-world units returned after a calibration? The IMAQ Learn Calibration Template VI in LabVIEW allows you to choose from the following units when you are specifying a calibration:

undefined, angstrom, micrometer, millimeter, centimeter, meter, kilometer, microinch, inch, feet, nautical miles, standard miles, steps

When a calibration step is applied, calibration information is retained with the image (and can be read out or saved to file as well). When you perform measurement steps in Vision Builder, Vision Assistant, or IMAQ VIs, they will usually return measurements both in pixel coordinates and real-world units which you specified during calibration. You can also use the IMAQ Convert Real World to Pixel VI and the IMAQ Convert Pixel to Real World VI to convert between pixel measurements and real-world units on an image which has calibration data associated with it.

[chaning]

Sorry, that isn't what I need to know.  Let me describe in detail what I'm referring to.

When using IMAQ Learn Calibration, an option 'Learn Error Map?' is available.  When this is set TRUE, it outputs a 2-D array the size of the image used for the calibration, with -1 for uncalibrated regions, otherwise the value appears to vary from 0 to 1, but I can't verify this.  According to the help documentation for NI Vision for Labview, "The image error map reflects error bounds on the calibration transformation".

Also, in the help under 'Perspective and Nonlinear Distortion Calibration', it again refers to the Error map and says the following:

"An error map helps you gauge the quality of your complete system.  The error map returns an estimated error range to expect when a pixel coordinate is transformed into a real-world coordinate...."

This statement is where my question comes from.  What are the units of the error map data?  Is it real world units, pixels, percent error, something else?

Thanks for the help!

[VGA_CD-ROM]

Ahh ok, I'm sorry that I misunderstood your question. The NI Vision Concepts Manual has an entire page, starting on page 3-12, regarding the error map and explains how it is calculated. It explains it better than I could type it out, but do let me know if you have any more questions on it.

The NI Vision Concepts Manual also contains a lot of other information on how various image processing algorithms are performed in our software, so it's a great reference all around.

[chaning]

Ahh,

I missed this when I read it before...

"The error value of the pixel coordinate (i,j) indicates the largest possible location error for the estimated REAL-WORLD coordinate (X,Y)..."

So the answer is, the error map is in REAL units.

Thanks!