Practice, practice and practice till you learn to see stereograms. You might strain your eyes a lot during this process, but I bet it is worth it. If you have chosen to work in this field you have to take this up seriously. There are different kinds of stereograms available, photographs, computer generated, random dot, etc, etc. Use whichever you are comfortable with. Use smaller images initially; you will not have to cross your eyes too much. Also I feel cross stereo is much simpler to learn than parallel stereo, because crossing your eyes is easier to moving them apart (at least for me). Here are some of the links from where you can find these different kinds of stereograms.
I have created a simple stereogram here in which the circle appears to be in front of the rectangle when viewed stereoscopically (crossed).
Once you have learnt to see stereograms, there is a small observation you will have to make. You can actually do it in the above diagram itself. The gap between the circle and the rectangle is not the same in the two images. This change in the relative distance between two objects in the images is what is called disparity. Solving for depth between the two images is actually solving for this disparity. So you cannot overlap these two images one above the other to fit both the objects perfectly. When your brain combines these two images the disparity that exists between them is converted to depth. If your observation is very keen you will also be able to observe that when your brain combines the rectangles the circles would have not overlapped perfectly and when your brain combines the circles the rectangles would have overlapped at an offset. You cannot combine two objects at different depths at the same time in your brain.
The gray color in the overlapped object is shown just to highlight the partial overlap that takes place for objects at different depths other than what is viewed. Our brain does not average the colors, so you won’t see this gray in the single image that your brain creates; instead it will either be the circle from the left image or the right one. This is called binocular rivalry and I want to have a separate post to explain this concept.
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