Learning Tomography: Non-centered projections

Why is my reconstruction so bad ?
This question often arises when the resulting image (or volume) is so disappointing... Specially when you are working with experimental projections, new artifacts and defects are appearing never met before in this series Learning Tomography where all the images are always perfect.
Thus, it is time to show some of these defects... to understand how to fix them.

Gray-level images and morphology

In MM [TOC], the gray-level images are considered as a stack of binary images, what they called level sets. Here is a small demonstration using ImageJ and an application to dilation.