FPS how calculate this?

Question

In my OpenGL book, it says this:

"What often happens on such a system is that the frame is too complicated to draw in 1/60 second, so each frame is displayed more than once. If, for example, it takes 1/45 second to draw a frame, you get 30 fps, and the graphics are idle for 1/30 1/45 = 1/90 second per frame, or one-third of the time."

In the sentence that say "it takes 1/45 second to draw a frame, you get 30 fps", why do I get only 30 fps? Woudln't 45 fps be more correct?

Answer 1

The graphics card will normally only buffer one frame ahead.

If it takes 1/45 of a second to draw a frame, then at the 1/60 of a second mark, the previous frame will be redisplayed. At the 1/45 mark, the next frame is done - but the card doesn't have a free buffer to start rendering the next one, so has to sit idle until 1/30, where it can send out that frame and start working on the next one.

This is with VSync enabled - if you disable it, instead of getting the 30FPS framerate and an idle card 1/3rd of the time, the card will start redrawing immediately, and you'll get screen tearing instead.