Specifying "Reference" and "Compare" in Distance tool

[gvalch]

Hi Daniel,

I have started to use Cloud Compare for a project I'm working on (comparing a CAD model of an object with a scanned model). Congratulations - it's an awesome tool!

I wonder if you could help me out with a question I have. I'm comparing a CAD model (which is a solid model) and a scanned model of the object. I start by using the Cloud Compare fine registration (ICP), using the CAD model as the reference and align the surface model to it (they are already roughly aligned). I then use the Distance tool and again specify the CAD model as the reference and the now-fully-aligned surface model as the 'Compared'. My question is somewhat basis: how the sample points chosen for distance computation?

1) It seems that the 'source' points are chosen from "Compared" and CC then finds the closest 'target point in the Reference. Is this correct?

2) /If/ this is true, ideally I would prefer to have the CAD model act as the "Compared" and use the scan as the reference (since the CAD model is the truth). My problem is that the CAD model is a solid model...and if I use the CAD model as a reference, CC will choose points not on the surface of the model (and thus, understandably yields a bad match).

Assuming CC behaves as I described in 1), is there any way in CC to circumvent the problem described in 2). Or is the best solution to extract the surface from the CAD model. And if so, do you know of an easy way to do this.

Thanks for your any help you can provide!

-GV

[daniel]

Hi,

I think most of the answers are here: http://www.cloudcompare.org/doc/wiki/in ... D_entities.

In your specific case:
1) You're totally right
2) If you want to get an accurate result, you'll have to 'sample' points on your CAD model with the 'Edit > Mesh > Sample points" tool (you can sample a lot of points, don't hesitate ;). Then use this cloud as 'compared' entity. It will give you a good visual result, as well as accurate measures, histogram, etc (you can export the measures as an ASCII file to further process them in Excel or Matlab for instance). The only drawback is that you won't be able to project back theses measures on the CAD entity...

Daniel