How do I find all objects in a certain area on the screen?

I want to make a system that allows me to select objects using a drag selection window, but I don't know how to do it. I managed to make the drag selection window but I just can't figure out how to find the objects! I was already thinking of spawning a part at the position of the corners of the window. However, this does not work because the selection window on the floor is a trapezoid and not a rectangle as you can see here

With this method I still miss the two corners. As shown here
Any help is welcome!

If the objects that you are trying to select are within a model then I would suggest the use of Camera:WorldToViewportPoint when (looping though)/(testing for) the objects and if they are on the screen.

I hope this answers you question, though I may have misunderstood.