Defining objects in image chips for Deep Learning

[ElianeMcCarthy]

Hi all,

I am currently trying to train a deep learning framework for detecting bats from thermal imagery taken from a drone. You will see in the below image the bats are the detectable as the bright circles against a dark background:

So far, I have been defining training samples by drawing a circle from the center of each bat to just outside the edge of the animal. 

However, this is not giving me good results:

You will see in some instances, completely dark areas are classified as bats, or just the edge of a bat is detected. 

I suspect that the issue here is how I am defining the bats in the training samples. I am thinking about trying this process again with rectangular training samples. 

Is anyone able to offer advice on the best way to define a bat in my training images?

Thanks in advance for your help!

Elie