Change the image background

Hi, I have some problem to my diesel spray and combustion image. I want to change the background of the image to black.because the quality of the image is bad, so I don't know how to mask the image. Thanks a lot.

[Rik]

What part of the image do you want to make black? It looks like it already is entirely black.

[Image Analyst]

Please create an image that you'd want as the output. And what do you want to do assuming you can make it a black background? Measure area fraction or blob size distribution or something?

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[Qiang Cheng]

Thank you very much for your reply. The output which i want have been marked as Yellow. Firstly, I think I have to find the boundary of the edge, and then cut the background.