Help with processing high-speed schlieren images

Hello everyone! I am trying to do the following on the image below:

1. Fill the black particles with the background. Basically, I don't want to see the particles there, but I also don't want to see a hole.
2. I am trying to extract the bow shocks behind and around the particles.
3. Obtain turbulent structure information from the image. There are some intermitent turbulent structures that cannot be seen in the image. Is there some sort of contrast that you recommend to make these turbulent regions more significant?

I have tried using regionprops to locate the location of the particles, yet I can't find a way to make the particle disappear nicely. Please let me know if you have any suggestions. I am also trying to track the bow shocks by a segmentation method, but I first need to remove the particles.

Experimental procedure:

• High-speed schlieren using MHz sampling. I can perform particle tracking velocimetry on the particles themselves, but I want some tracking or PIV for the bow shocks, as they don't travel with the particle entirely.
• Please note that these are a set of 256 images per experiment, so I can track the bow shock frame to frame.

Thank you!

[José Hermenegildo García Ortiz]

Dear Juan,

It's probably too late, but if someone else has this problem, did you try the imfill function? You can adjust the options, and it probably works in this case.

Regards!