Blurred Image Stitching

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Hi everyone, I'm trying to create a panorama with a certain number of images by stitching only 2 images everytime. So, I get a portion of the final panorama with the first two images, then I update this portion combining it with the third image and so on. The problem is that the left part of the stitched image is going blurred at each step. Why? How can I avoid this?

I'm using the code stitching_detailed.cpp

Thanks a lot.