I don’t know how much efficient opengl is and how many things are possible using it. I am trying to learn things about it and was wondering if it is possible to get the pixels of a 3d model and calculate its area. I know 3d models are hollow from inside so is it possible to calculate their boundary? e.g i have a jar and i want to fix a ball inside of it, is it possible to calculate the circumference of jar or any irregular shape? at a certain height obviously e.g the height of the jar is 5 m and i want to calculate its circumference at 4 m. Can i do that is that really possible? I would be really thankful for your help 
OpenGL is a rendering library, not a modelling/geometry/whatever library. I can’t see how it would be of any use for this task.
well I have a similar task to do where I need to calculate the circumference of an irregular 3d model at certain points, can i do so using opengl?
You just re-asked the same question, so the answer will be the same: No. OpenGL is for rendering pictures. And while it’s possible to use graphics computational resources for arbitrary computations, OpenGL has no built-in functionality for doing what you want.