Greyscale threshold using ni black gives invalid results on large image

[glen.meiring]

I am analyzing images of hot steel slabs looking for cracks in the surface of the slab.  The images are 2048 pixels wide by 14000 to 40000 pixels high.  I am using NI Vision Assistant 8.5.

Sometimes the threshold does not work right when using the grayscale threshold, Local Threshold: Niblack.  For smaller images it works ok, but for larger images it will not work.  There are no errors, but the result is an image full of colored pixels rather than an image of particles.

I have attached some sample images.  These are small crops of the larger image.

The first one is from an image about 17000 pixels high which worked properly.  The second is from an image about 25000 pixels high, which did not work.

Is this strictly a size issue?  Is there something I can do to fix it?

Thanks!

[Vivek_N]

Hello glen.meiring,

This does seem a bit unusual. Was this completely done using Vision Assistant, or did you migrate the script to LabVIEW? If this was done in LabVIEW, could you post a screenshot of your VI? Also, could you attach your original image as well as the Vision Assistant script? This way, I can check if the issue can be reproduced on my side as well.

[glen.meiring]

I am having the problem within Vision Assistant.  I think at one point I migrated it to a LabVIEW vi and had similar results.

I am going to try to post a link to the files.  The images are rather large, about 5MB.  I have also attached the script.

The first file should work with the script, the second should not.

Thanks for looking at this!

Glen

http://us.share.geocities.com/theace999/Images/11192003.jpg

http://us.share.geocities.com/theace999/Images/11209307.jpg