How to censor a shape?

Let's say you are dealing with images and you want to block out a particular shape. Now, you might say that you will use shape matching to identify the shape and then just block it out, right? But the problem here is that we don't have any template available. So, how do we go about doing this? Shape analysis comes in various forms, and we need to build our algorithm depending on the situation. Let's consider the following figure:

Let's say we want to identify all the boomerang shapes and then block them out without using any template images. As you can see, there are various other weird shapes in that image and the boomerang shapes are not really smooth. We need to identify the property that's going to differentiate the boomerang shape from the other shapes present. Let's consider the convex hull. If you take the ratio of the area of each shape to the area of the convex hull, we can see that this can be a distinguishing metric. This metric is called solidity factor...