Hello Alexey!
I?ve read that discussion and I understand it. My problem is how to do it! I have 3 differnet cameras with 3 photos without the object in scene. I load photos of every position in a different chunk, along with the photo of background. But in the menu of import mask for active chunk, it ask you for a .png file not a .jpg. So before doing that I have to export the mask with "export for active chunk" just by clicking on the background photos, without doing nothing, is it correct or not? In any case, id does not work, I?m taking wrong one stpe and I dont know which one it is!