Thank you for the quick reply.
Blobdetection will only draw an outer line around a cluster of pixels with the same colour, which is basically what i already have when i draw the outer perimeter.
BoofCV have an elipse function, but it seems that what it does is fit an elipse around a blob… and in this case the blob will be the whole test tube (or maybe more if they lay close to eachother, as some of them are in the attached picture).
What i need is to search out circles in the picture and then sort them by a min and maximum diameter.
Or other suggestions?