A Tale of Three Blurs

There are three Blur effects in After Effects that are commonly used, have been upgraded to 32 bpc floating-point color for version 7, and are generally misunderstood.

Gaussian Blur

Gaussian Blur has the simplest UI of the three — “Blurriness” and options for blurring in X, Y or both. Historically, Gaussian Blur was known as the premium blur for those who could afford its increased render times. While this was once true, it is now purely a myth. Gaussian Blur uses the exact same blur engine as

Fast Blur

…except that Fast Blur adds a very important “Repeat Edge Pixels” option. So for the record, there is no advantage in using Gassian Blur over Fast Blur. Gaussian Blur is obsolete.

Box Blur

Box blur is like a Volvo. It’s boxy, but good. Oddly, it’s best feature is that it can, if desired, be less boxy than the other two.

Box Blur is the simplest kind of blur, and at its defaults produces “squarish” results. However, it features an Iterations slider that allows you to perform this box blur operation any number of times you would like. At three iterations, Box Blur is identical in quality to Fast Blur. At four or five, it is better, producing a softer, rounder blur. When blurring small, bright floating-point things, you may be glad for this extra quality.

Or you may find that at one iteration, Box Blur’s simple blur is a handy utility. For example, used in Horizontal or Vertical mode, Box Blur is a better approximation of motion blur than is Directional Blur. Photographic motion blur should have that squared-off look, not a Gaussian-esque smoothness.

The only bummer is that Box Blur’s main slider works differently than Fast Blur’s, so if you need to “upgrade” to Box Blur you’ll have to eyeball your values to match.

Since Box Blur does everything the other blurs do and more, I often reach for it first, usually setting Iterations to three or four by default.

So just like Carson Kressley’s rule of suit jacket buttons (middle, top, bottom = always, sometimes, never): 

• Box Blur = Always
• Fast Blur = Sometimes
• Gaussian Blur = Never

Comments

[Dario]

Stu,

What do you think about the addition of Lens Blur into AE now?

It seems pretty powerful, but unfortunately it's not 32bit float.

What if it was? Would you prefer it over box blur, or are they just completely different?

[Stu]

Hi Dario,

Lens Blur will be super handy when it is upgraded to 32bpc and sprouts a few more options, such as control for the aspect ratio of the iris. It's a really important effect for photorealistic comping.

[Dario]

Hi Stu,

What to you recommend I do if I want to blur using a zDepth pass but in 32bpc?

I been using compound blur or frischluft's depth of field, but I don't think they work in 32bpc.

[Stu]

Dario,

I would recommend emailing Phillip at frischluft:

http://www.frischluft.com/support/contact.php

...and asking him to update his awesome Lenscare plug-ins to float.

Couldn't hurt to bug the AE guys about it as well, but maybe Phillip could beat them to the punch.

[Bruce Allen]

Stu,

Awesome article. I always thought that fast blurs looked pretty much like Gaussian blurs these days...

Just one problem - I did a test and it said that a radius 15 Gaussian Blur / radius 15 Fast Blur equals a radius _10_ Box Blur with 3 iterations. A radius 15 box blur is too blurry.

I am on AE 7.

My test project is on
http://www.boacinema.com/clients/for_stu/blurs_for_stu.aep

Bruce Allen
www.boacinema.com

[Bruce Allen]

Stu

I am a total idiot and didn't read your article properly. It clearly states that box blur doesn't use the same values.

All the best again.

Bruce
www.boacinema.com

[Sung]

I got to know this space early this morning. I just found out this awesome article. I've been working on this project where I used lots of blur effects and had been using Gaussian blur until I read your article. I tried Box Blurr instead, I could clearly see a better result. Thank you.

[Anonymous]

Took me a while to figure out I had to click on the line of ?????? to leave a comment...

I was just wondering, is there a mathematical correlation between box blur + iterations and fast blur?

I use fast blur heaps, and can guess pretty accurately what values I'll need. But after reading your article I thought I'd investigate the box blur more, but I find myself spending more time guessing what values I'll need, as the results are so different. Is there some kind of equation or rough guide?

[David]

I know this is an old article, but if you're still reading comments, I have a question regarding the various blurs in After Effect 7.

They all seem to share a feature that's driving me crazy, which is that they don't scale quite linearly.

I'm trying to do some multi-resolution work, and I've found that if I double the resolution of my comp, I can't simply double the amount of blur. I have to add a little extra to make it match the look of the lower-resolution image.

Oddly, the extra amount is not linearly dependent on the amount of blur. For most blur levels, if I increase resolution 16x, I have to multiply the blur level by 16 and then add 20.

This is driving me crazy because I have to do a lot of these and the resolution differences aren't consistent. I'd like to be able to write an expression that scales the blur, but there's no simple formula.

Any idea why this is or if there's any way around it?

Thanks,
Dave