What is the purpose of motion blur in video games?
In: Technology
You can see the effect of motion blur in real life by just quickly waving your hand. Your hand is moving faster than your brain can process, so your brain kinda just guesses what happened by blurring what it saw together. Subconsciously, we perceive something moving and blurry as fast, and something slowly moving as clear.
Since video games want to be realistic, adding motion blur helps maintain immersion.
First, our eyes don’t “update at 24-48 fps”, it is a continuous analog process where there are different response times to things like recognizing motion and processing what an image is of. 24 fps is just about the minimum required to convey smooth motion with motion blur.
Motion blur is the consequence of how cameras work. A light-sensitive sensor or photographic film is exposed to light during a period of time appropriately called an “exposure”. During that period of time things being imaged can move resulting in the light from their position being smeared across the frame. This results in motion blur, the blurry track of a moving object during an exposure. With multiple exposures this blurry track will merge with the end of the blur from the previous frame and start of the blur in the following frame, helping the viewer connect the images in sequence. Without motion blur a moving object seems to jerk between positions in an unnatural way (remember the eyes are anolog so motion is normally perfectly smooth).
Your brain will interpret 24 fps as continuous motion. But if those 24 frames are clear, in focus pictures they do not look realistic. Irl a car speeding by looks blurry to us, not a clear car moving bit by bit.
“Go motion” was invented for Star Wars so the space ships would look blurring fast when they moved. Older stop motion had an unnatural look because everything was always in clear focus when it moved.
Real life has motion blur so it mimics realism. It’s the reason you can tell the difference between 60 fps and 240 fps despite your brain only really updating around 24-48fps. Most people don’t like it because it occasionally blurs things they want to see, like enemies.
Motion blur is added to games to give you an incentive to check out the settings menu as you immediately search for a way to turn off the motion blur when you see that a game has it.
It’s one of a number of post processing effects that picked up popularity in a way that makes its absence feel like a lack of polish to many people. It comes out of the box (near zero cost for development) in popular game engines, and in the scheme of things is fairly trivial to implement if it isn’t already a feature. So why not add it and leave it in by default, so all the screenshots, videos, and previews for your game have the desired visual fidelity for marketing’s sake?
Also, the way many games implement motion blur doesn’t very accurately emulate how our eye or cinematic cameras “see” blur due to motion. For example, Unreal Engine keeps track of information about how objects are moving across the screen, and does a straight-line blur in front of and behind the object for each frame you see it moving fast enough on screen.
Contrary to what other posts are saying, this doesn’t necessarily feel more normal to your brain interpreting what it’s seeing. There’s a good reason VR applications don’t use motion blur. It’s not so costly that it drops FPS below the typical VR target framerate of 80 or 90 FPS. It’s just that the implementation used in common game engines just doesn’t do good enough of a job of emulating reality (ie what your brain expects to see), and makes you more prone to VR sickness during play.
If done properly it is barely visible and give fluidity to camera movement (instead of being a saccade if images).
Unfortunately it’s expensive to compute, and there is a shitty way of doing it that is cheaper: you blur consecutive images. This lead to a motion blur that is 2 to 3 times exaggerated.
In practice, run the game at 120 Hz and it will feel smoother than 60 Hz with motion blur. It’s old technology that can’t die soon enough.
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