Methods of zooming in/out with openGL (C++)

Question

I am wondering what kind of methods are commonly used when we do zoom in/out. In my current project, I have to display millions of 2D rectangles on the screen, and I am using a fixed viewport and changing glortho2D variables when I have to zoom in/out. I am wondering if this is a good way of doing it and what other solution can I use.

I also have another question which I think it is related to how should I do zoom in/out. As I said, I am currenly using a fixed viewport and changing glortho2D variables in my code, and I assumed that opengl will be able to figure out which rectangles are out of the screen and not render them. However, it seems like opengl is redrawing all the rectangles again and again. The rendering time of viewing millions of rectangles (zoom out) is equal to vewing hundreds of rectangles (zoom into a particular area), which is opposite of what I expected. I am wondering if it is related to the zooming methods I used or am I missing something important.

ie . I am using VBO while rendering the rectangles.

Answer 1

and I assumed that opengl will be able to figure out which rectangles are out of the screen

You assumed wrong

and not render them.

OpenGL is a rather dumb drawing API. There's no such thing like a scene in OpenGL. All it does is coloring pixels on the framebuffer one point, line or triangle at a time. When geometry lies outside the viewport it still has to be processed up to the point it gets clipped (and then discarded).