Isolate a large black island from a similarly colored border

What kind of command will let me extract a large black island in the center of the image from its surroundings. Surrounding the large black island is a thick white band ( which I want to keep) and surrounding that is a thick black band (same color as the island that i want to extract). How do I get rid of the outer black band.

Answer 1

[Image Analyst]

Gosh, an image would have been so instructive. Since you forgot to post it, I can only imagine. Basically to turn the outermost black boundary white, I'd invert the image (so now it's white that touches the edge of the image, not black), then get rid of white blobs touching the edge of the image, then invert again to make your black band black again.

binaryImage = ~imclearborder(~binaryImage);

If that doesn't work, then attach your image.