I wanted to check regarding the units (I know it was mentioned somewhere on the blog that the standard units for motion is pixel/frame). This might be a silly question, but what are the units for the sheet thickness? I assume its mm, but I have no hands-on experience with macro-scale PIV, however, I seem to remember that in the PIV book by Adrian & Westreweel they mentioned order of mm for sheet thickness used in a "standard" PIV system).
While on the subject, could anyone tell me where does the "magic" integer 200 appear in lines 30 and 31 in the accuracy.m script? I am confused about what the z_move parameter actually means (some fraction of the sheet? millimeters? pixels?)
Thanks for your reply. Yes, this is about synthetic image generator.
I do find their description of the image generation on page 165. However, they don't seem to actually do any measurement for OOPM. And I am not entirely sure how you implement your OOPM (i.e. purpose of the magic number: 200).
Hi, I currently do not have computer access to check, but I think I did it like this:
The script in accuracy.m is copied from the main GUI.
The out of plane motion is given in percent to the script because of the GUI link.
100%/200% = 0.5
So with 100% out of plane motion, the particles in both images will come from two different random distributions (z0_pre and z1_pre) --> there is no link between the two images anymore.
I might be wrong, but I think because the distributions are random, there is a chance you will get an artificial match. In these images, you have z_move = 200% and the central bit does not change significantly.
Hi, that is true, but I can think of two reasons why this is not an issue:
1: PIV is matching particle patterns, and not individual particles
2: If you would have a real physical setup with 100 % out of plane movement, then the exactly same thing will happen: New particles enter the laser sheet and appear in positions that match previous particles that already left the sheet. The more particles you have, the more likely it is (imagine that every pixel is occupied by one particle).
I think (although I am not an expert in statistics) that this is normal behaviour of a statistical method like PIV.
How can you manage to get zero match between two random datasets? I currently can't think of a solution without putting artificial boundaries on the datasets (e.g. particles A must only be centered on even pixels and particles Bei must only be centered on odd pixels).
I think this is an interesting subject, and if you have further input, then I'd like to hear it.