[bob.wolff68]

Hi,
I have some concern about the idea of assigning 'particles = *temp' and deleting temp rather than simply USING temp-> in the routine and then deleting temp at the end, but that isn't looking like the culprit. I see

			targetParticle = particles.at(0);

as a big issue -- when there are no particles at all in the image, particles.size() or temp->size() if not deleted are zero. This means you CANNOT assign targetParticle = particles.at(0); as there is NO ZEROTH ELEMENT at that time. If you, instead, carefully deal with placing that part inside an

if (particles.size()) {
  targetParticle = particles.at(0);
  flagNoParticles = false;
}
else
  flagNoParticles = true;

then that part won't crash. HOWEVER, it seems that elsewhere in the code you are counting on targetParticle to be valid. You might have to be a little creative (using a flag or re-thinking your logic) in treating this special case such that where you are counting on targetParticle, you are checking to see if you are in the situation where there are none instead and just returning in those cases. It's a little hard to describe in words...if this isn't clear, maybe we could talk it out on the phone.

bob