Image Filtering - Rock Boundaries Detection

Hey,

I am currently working on a project requiring to detect the boundaries of a rock captured on an image sequence. The problem is that the color information is relatively poor because the object is located in the forest and there are others rocks in the background.

Here is an example of a picture I am trying to start with:

And now an example of the result I am looking for:

I tried all the basic edge detection methods: Sobel, Canny, Prewitt, Roberts, etc.

Nevertheless, none of them gave appropriate results. You can see below what I get. Note that, I played with the filter parameters (thresholds, etc.) but it did not solve the problem.

I would really appreciate if someone could suggest me a more appropriate filtering method or a direction I should take to get the results.

Thanks