What is "radial blur"?

Question

I've thought that radial blur is a form of motion blur that is only applied to mobile objects, not camera movements, but I may be misinformed.

Radial blur can sometimes, albeit rarely, be configured in a PC game's options menu.

motion blur in effect in Crysis

Pharap · Accepted Answer · 2018-08-15T07:14:41.277

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The existing answers are good, but I want to offer a more in-depth explanation of what radial blur is and where it's used.

Radial blur is a particular kind of blurring effect that originates at a single point within an image and blurs outwards (or inwards) from (or to) the point. The word 'radial' itself actually means "Arranged like rays that radiate from, or converge to a common centre", which perfectly describes how radial blur works.

In the context of video games, radial blur is used for a variety of purposes.

Firstly it is used a means of drawing the player's focus onto an object by converging the blurring around a (sometimes moving) object that the game wants the player to focus on.

This is common in racing games where the effect tricks the player into focusing on their car as well as faking motion blur, visually implying that said car is moving fast without actually doing proper motion blurring. It's cheap but effective and can be found on many older games designed for systems that aren't powerful enough for full motion blur.

In addition to being used around the car, it is sometimes used on the wheels to fake the wheels turning. (Note that this variant of the effect is sometimes referred to as 'circular blur', but there seems to be no official consensus.)

Alternatively it can be used in first person shooter games to get the player focused on their gun sight, leaving the rest of the world blurred out (as it would be if the player were really focusing on aiming a gun). This is especially common for sniper weapons that opt not to use a crosshair effect (or sometimes in conjunction with a crosshair).

Radial blur is often used in action games to stimulate action. For example it can be used to simulate explosions or gun recoil.

Lastly, it is sometimes used for UI effects in the kinds of minimalistic UIs that still show the paused game behind them.

For information about how radial blur works, the best resources I could find with a brief non-exhaustive search are this article using Java and this theoretical article.

Edit: Even better, I found an working example on ShaderToy.

Post Script: It's not particularly important but some people like to make a distinction between circular blur and true radial blur. Circular blur leaves a circle in the centre unblurred whilst radial blur blurs from a single point outwards, thus causing a complete blur.

edited Aug 15 '18 at 07:14

answered Apr 13 '17 at 12:02

Pharap

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The Fallout 3 screenshot looks a lot like depth of field. – user598527 Apr 13 '17 at 12:16
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@user598527 I'm assuming that's the one with the gun below the wheels. It's not depth of field, it's radial blur. Depth of field has even blurring like this image. If you look around the outsides of the image with the gun in it you will see line-like artifacts radiating out from the centre, which is the tell-tale sign of radial blur. – Pharap Apr 13 '17 at 12:28
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Your wheel example isn't radial blur - rather than blurring inward or outward from the center, it blurs around the center. Radial blur would make the wheel look like it's moving toward or away from you, rather than looking like it's spinning. The effect in the wheel example is described by your jhlabs link as spin blur, and by your chemaguerra link as circular blur. (None of your links describe circular blur as a radial blur with the center unblurred - you may have gotten that impression by misinterpreting the circularly blurred tiger photo.) – user2357112 Apr 13 '17 at 17:12
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@user2357112 That's a bit of a grey area. There seems to be little consensus on what constitutes 'radial blur' and what constitutes 'circular blur'. Without something more 'official' dictating the difference I'm going to leave it there for now. The image may not be 100% accurate but the principle is the same. Out of interest, this article classes both as radial blur and calls one 'Spin' and one 'Zoom'. Also radial gradients tend to form circles, so it's at least related. – Pharap Apr 13 '17 at 19:22
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Some programs, like IrfanView, definitely call the wheel example "radial blur", while the other ones "zoom blur". Though I am not sure which one is better. "Circular blur" sounds better for the wheels. – IS4 Apr 13 '17 at 21:21
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@Pharap: Well, whether or not the term "radial blur" is appropriate for a blur around a center rather than toward/away from it, you've defined radial blur as toward/away from the center in your answer and then given a picture that blurs around the center. It'd be useful to note that the wheel example is actually doing a different thing from the other pictures, and perhaps make note of the terminological ambiguity. – user2357112 Apr 14 '17 at 17:30
Also, do you have any references for the "leaves a circle in the centre unblurred" thing? – user2357112 Apr 14 '17 at 17:31
@user2357112 You're free to suggest an edit if you think there's a better way it can be written. – Pharap Apr 15 '17 at 17:58
@Pharap: Edit suggested. – user2357112 Apr 15 '17 at 19:24
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@user2357112 I was going to consider accepting and then adapt the wording slightly, but it seems two moderators called Vemonus and Dragonrage got there first and rejected it with the reason "This edit deviates from the original intent of the post. Even edits that must make drastic changes should strive to preserve the goals of the post's owner." Never had that happen before, must be a new 'feature'. – Pharap Apr 16 '17 at 01:36
@Pharap You can still incorporate the suggested edit into your post in an edit of your own. It was probably rejected because it was substantively different, and usually only the author of a post should be able to make those kinds of edits. – Brian McCutchon Apr 16 '17 at 04:36
@BrianMcCutchon I shall leave it for now, enough people have upvoted that I think it would be unfair to alter it futher (assuming I only made that one typo and there aren't any other spelling or grammar mistakes). – Pharap Apr 16 '17 at 05:03
@Pharap On the contrary, a popular answer ought to be up-to-date and accurate. – Brian McCutchon Apr 16 '17 at 05:07
@BrianMcCutchon Well, technically there haven't been any new resources come to light, just opinion. If someone finds something more official than a blog or a blog by someone more official stating otherwise, I'll definitely update and link to the resource. – Pharap Apr 16 '17 at 05:22
@user2357112 Good news, you won't be able to have your edit overturned without the author's consent ever again. They just added a feature to let a post author reject edit rejections. – Pharap Apr 23 '17 at 04:06

score 19 · Answer 2 · edited Apr 13 '17 at 00:17

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A radial blur is a great way to add motion to an image, and the entire effect can be completed in a matter of minutes.

The same concept applies to gaming. It is not motion blur. More info here: http://www.photoshopessentials.com/photo-effects/radial-blur-action-effect-photoshop/

Original Image:

Same Image with Radial Blur:

edited Apr 13 '17 at 00:17

Timmy Jim

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answered Apr 12 '17 at 22:55

Peterrrrrrr

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A matter of minutes? You must be using a terribly slow gpu. tongue in cheek – Pharap Apr 13 '17 at 02:37
@Timmy Jim Why did you change captions? Captions are usually written in italics, under the image, – user598527 Apr 13 '17 at 07:39
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@Pharap or he included starting up photoshop – ratchet freak Apr 13 '17 at 11:02
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@ratchetfreak I'd be worried if a game was trying to run photoshop to do its blurring instead of using shaders like normal games. also tongue in cheek – Pharap Apr 13 '17 at 11:18
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@user598527 looks more appropriate to me. Rollback if you wish. – Timmy Jim Apr 13 '17 at 11:21
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@TimmyJim: We'll see. I also try to avoid capitalization, there may be cultural differences. – user598527 Apr 13 '17 at 11:23
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@user598527 Placing captions under images used to be the undisputed norm for things like websites and magazines, but it's becoming increasingly common to put a title above an image because the title introduces the image and explains its purpose before it is shown. Ultimately it's a matter of preference, but in the case of stackexchange answers I think placing the title/explanation before the image tends to make more sense, especially with larger images. – Pharap Apr 13 '17 at 12:12
"This would be a nice answer on a photography site, but I would expect any answer here to at least reference its usage in games." – user598527 Apr 18 '17 at 18:56

score 14 · Answer 3 · edited Aug 13 '18 at 17:10

A blur is typically performed on an image using a convolution kernel. For example a simple 3x3 box blur uses a convolution kernel so the total of all the elements in 1.0:

+-----------------+
| 1/9 | 1/9 | 1/9 |
|-----+-----+-----|
| 1/9 | 1/9 | 1/9 |
|-----+-----+-----|
| 1/9 | 1/9 | 1/9 |
+-----------------+

Applying this filter to each pixel results in a blur over only three pixels horizontally and vertically:

You can use different shaped convolution kernels to achieve different effects. You can use a purely horizontal or vertical kernel to achieve blurring purely in the horizontal or vertical directions.

Or you can arrange your kernel to work on a diagonal:

+-----------------------------+
|     |     |     |     | 1/5 |
+-----+-----+-----+-----+-----+
|     |     |     | 1/5 |     |
+-----+-----+-----+-----+-----+
|     |     | 1/5 |     |     |
+-----+-----+-----+-----+-----+
|     | 1/5 |     |     |     |
+-----+-----+-----+-----+-----+
| 1/5 |     |     |     |     |
+-----------------------------+

and now you blur on the 45 degree:

Radial or circular blurs, you change the convolution kernel for each pixel, arranging the blur in the direction you want: radially or tangentially:

Bonus Reading: JH Labs - Blurring for Beginners (archive)

It's a good answer, but perhaps a bit too technical for a site dedicated to playing games. — Mage Xy, Apr 13 '17 at 17:00
This would be a nice answer on a photography site, but I would expect any answer here to at least reference its usage in games. — DCShannon, Apr 13 '17 at 23:30

score 5 · Answer 4 · answered Apr 13 '17 at 07:37

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From Unreal Engine 3 manual:

The radial blur effect allows you to mimic the look of a shockwave coming from an explosion. The effect blurs the screen in the direction eminating from the center of the radial blur actor that is placed in the scene.

answered Apr 13 '17 at 07:37

user598527

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What is "radial blur"?

4 Answers4