From my experience, I think the dirt in your image hard to use imageJ to clean. but I think " MATLAB maybe solve this problem".

I use this method to separate the object.

https://www.mathworks.com/help/images/marker-controlled-watershed-segmentation.html

I think you can find the MATLAB programmer to help you for coding program to clean your image.

I confirm 100% if you use MATLAB you can solve this problem but you must have MATLAB programmer to help you.

Normally an inpainting technique could give a good result, but given the size of the dirt and that it's located in a corner of the image I don't think this technique is enough. In this case, I suggest you use photoshop to correct it manually.

Thanks Kritsada Saensomboon and Manuel G Forero

I will try use photoshop to get rid of them. I have one more question. Does anyone know how to stretch the image to fit the whole area using ImageJ? Thanks.

Use the Image->Scale command.

Manuel G Forero I tried the scale command, but it doesn't work as my expectation which stretching the target region selected using polygon to filling the whole rectangle area. It seems only for scaling in a rectangle way. Is my understanding right? Thanks

Fang, You are right. The tool I told you is used to scale the image. If you want to correct the perspective, a very easy way is to use Photoshop. Use Edit>Perspective Warp, as explained in https://www.creativebloq.com/adobe/3-ways-fix-perspective-errors-photoshop-71412122

Manuel G Forero This can be done in Matlab as well. But as most of my image process is done in imageJ, i though it would be more convenient to find a way to correct the perspective in it.

I find one ImageJ plugin-Transform->Interactive perspective. It works! Manuel G Forero