WORKING WITH MICRO CT DATA

[akashvardhan]

Dear Daniel,
I have been using cloud compare to work with datasets obtained after doing a micro ct scan of my samples. I segment the areas which are of interest to me and then export them as stl's which I open in cloud compare. I am basically looking for the transformations that have occured in the two datasets that I am analyzing. For this I perform the Match Bounding Box centres operation, followed by scale matching and finally fine registration. The transformation matrix that I obtain at the end of these operations gives me the values of euler angles (phi,theta,psi) and the translations along X,Y,Z. I wanted to know what is the convention that is being followed by the program in computing euler angles. The help page contains a link which opens the wikipedia page for euler angles, am I to assume that it is the wikipedia convention that you guys follow? Also for the translations what are the units of linear translation, is it in mm or cm or mts??..I also wanted to know how does cloud compare orient these datasets in space when I call upon the individual stl files, I ask this question because the numbers I obtain after superimposing one dataset over the other seem to make no physical sense in the context of my data..does it preserve the coordinates of the exported stl and then orient it in space based on those values with reference to it's own coordinate system, or does it randomly place the objects, because a lot is depending on the values I obtain from these transformations in cloud compare..,
Regards,
Akash.

[daniel]

I'll try to answer your (many ;) questions in the right order:

- I believe the Euler angles we use are 'Tait-Bryan Z1Y2X3'. But I would strongly recommend to use the (vector, angle) form which is much easier and safer to use. By the way, the fine registration algorithm can find the scale as well (with the 'adjust scale' option).

- regarding units: CloudCompare doesn't uses any explicit units. It loads the data as is and displays the numbers as is (the user should know in which units they are expressed). If you need to change the units you can simply rescale the entities. And if you translate the entities, the quantities are of course implicitly expressed in those units.

- regarding coordinate systems: same thing, CC uses the coordinates as is by default (did you really believed to the 'random' placement?! ;). The only exception is when the coordinates are really big in which case CC suggests the user to shift the entities (otherwise a loss of precision might occur).

You should definitely check how your STL files are generated.

[akashvardhan]

Dear Daniel,
Thanks for your reply. Is there a way to mirror stl files in cloud compare about a given axis or a plane?

Best,
Akash.

[daniel]

The best way would be to use the 'Apply transformation' tool and set almost the identity but change 1 with -1 in the column corresponding to the mirroring dimension (1st = X, 2nd = Y, 3rd = Z).