Frame rate and Screen Tear

I realise that during reviews for games I mention the frame rate and any screen tear, but have yet to truly explain what that means. Time to change that.


To illustrate frame rate I present three examples the first runs at 100 frames per second, the second at 50fps and the third at 25fps. You should note that each takes the same time to complete one full cycle, but that the results look quite different.

[Update oh great Blogger doesn't allow GIF animations and for some reason the 100fps is running slow Okay hopefully the two 25fps and the 50fps illustrate enough]

When the frame rate stutters, the hardware or software engine can't produce the frames you want to see. So if you're looking at point A and turn to point E taking one second to do so you should see points B,C, and D in between. If the game can't cope and thus can't produce displays for B,C, or D then you can pass over those points and end up looking directly at E after one second or the game may wait until it's calculated the displays and thus you see point B after one second then point C etc. so a one second turn either seems to jump or take longer than it should.

Screen tear is another symptom of this. Rather than wait until finished, or jumping directly the game just tries to throw what it's got at the screen as and when it's requested.

To show no favouritism here are two examples from Assassin's Creed 2 on the XBox360 and PS3 taken from EuroGamer's excellent Digital Foundry

 

In all fairness to the game these stills were taken at 1/20th speed and therefore may not be noticeable at full rate, but they do illustrate the problem. In the first image his hands are moving upwards and the frame hasn't quite caught on leaving part of his wrists in the correct position and his hands in the wrong one. In the second example the camera is moving from right to left, so the lady although walking in the direction is viewed as left to right with her legs in the wrong position, more noticeable are straight lines and in this case the building on the left is also torn.

As you can see tearing can occur at any point of the screen and although not shown in some cases at more than one point..

Now imagine trying to play a game that stutters and/or tears - fun it's not.