We've been looking at the process of artificial image generation, as implemented via our TVs and Monitors.

...and, as it turns out, it's all...

JUST AN ILLUSION!!

This can manifest as "object flicker" if half-drawn images become visible on the display, or as "screen tearing"
if the scan ends up combining pixels from multiple
separate "frames" into a single hybrid picture.

If you do that, you'll never be "crossing the beam"
while it is scanning the image, so you will be
insulated from partial images.

The simpler, brute-force, approach to avoiding the display
of partial images is simply to avoid writing anything
to the frame buffer while it is being scanned. 

Historically, CRT-based systems had a short delay at the end (or start) of each "frame", during which the scanning beam from the electron gun would have to be temporarily
"switched off" and then moved back to the top-left
corner of the display for the next frame. 

This was called the "vertical blanking interval".

Instead of drawing the entire image to the frame buffer during the VBLANK we could, if we had enough memory, prepare the next image in a separate buffer ("at our leisure") and
then simply copy it into place during the VBLANK.

If you can perform the copy to the "front buffer"
during the 1ms period of the VBLANK,
then this approach becomes viable.

Double-buffering with V-Sync usually works very well,
but is not without its problems. 

The main drawback is that we sometimes have to wait for the graphics hardware to finish with the current "front buffer" before we can start using it again. 

In a well-balanced system running at the full hardware
frame-rate, this waiting period is minimal,
and causes few problems...

 -- The Godfather: Part II

Instead of waiting for the current front-buffer to become available at the VBLANK, we can just move our drawing into some 3rd buffer and continue working there without delay. Then, when the VBLANK arrives, the "buffer flip" occurs as before, but now it is essentially happening in the background and it doesn't slow us down!

A naive rendering system would be prone to unwanted "partial images" exhibiting both "flickering" and "tearing".

V-Sync prevents you from seeing "partial images" 
by only modifying the Frame Buffer in the period
when it isn't being scanned (i.e. during the VBLANK).

Double-buffering gives you the freedom to create your
next image "at any time" (i.e. not just in the VBLANK)
without the "work in progress" ever being seen.
This approach prevents "flickering".