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12/13/2005

Image Shift Measurement

Given a series of similar images that are slightly shifted relative to each other, this example shows how to crop out a region of interest from all of them, and measure the relative shifts using the Data Cube Tool, FFT Tool, Process Folder Tool, and Blob Tool.

The images are EBSD images, in a folder called tracking-sample, and were taken by Mark Vaudin of NIST.  The folder of images is available from the NIST Lispix website:  http://www.nist.gov/lispix/images/.

To view the images and their shifts, load them into the Data Cube Tool, and show them as a movie:

The cube, as loaded will look like this.  The slice slider is below the cube - move the slider to see different slices. 

In order to measure the relative shifts from one slice to the next, we will convolve all of the images by the first one.  The bright spot in the cross-correlation gives the shift of the one image relative to the other.  We wish to measure the positions of the bright spot in image 1 correlated with itself, then correlated with image 2, etc.  To convolve, first select a square area of interest, with edge length in pixels of a power of 2.


Alternatively, if you have not loaded into the data cube all of the images that you wish to crop, you can use the Process Folder Tool.  This ignores the above cube.  Briefly,

The resulting cube looks like this:  It will appear on top of the original cube, which you can now close by clicking on its "X" button.

This smaller cube consists of seven images, which must be saved so that they can be convolved with the first image, using the process folder Tool.

The next steps use the Blob Tool to measure the positions of the bright spot in the center of the convolved images.  A threshold must be chosen that selects the bright spot in all of the images.  A convenient way to do this is to read the images into the data cube tool, select a threshold with a Threshold Slider, propagate that threshold through the cube for examination, and then blob this folder of images.

When the thresholds all look OK, you can blob the images with the Blob Tool.  The temp2 cube can be closed, which will close both of the sliders associated with it. You may also close the data cube Tool.l

Now that the convolved files have been blobbed, all that remains is to write out the centroids to a file that can then be imported into Excel.