Blurriness units to pixel radius conversion

[Walter Soyka]

[Pedro Amaral] “As we couldn’t find any CSS filters for the Fast Blur, for the time being we’ve been trying to match it visually – blurriness of 100 units (on fast blur) is about 35px (pixel radius) – but in the end the final result doesn’t seem to be quite the same. “

Fast Blur does measure in pixels. Observe:

Create a 1920×1080 comp.
Create a full-white, 100×100 solid, centered in that comp.
Set the project to 32bpc.
Apply Fast Blur. Set the Blurriness to 100.
Render out a PNG.
Open the PNG in Photoshop, then Image > Trim… Based on Transparent Pixels.
Image > Canvas Size will yield 300×300, confirming a 100px radius on each side of your original 100×100 image.

Your developers are probably not compensating for the difference between linear and gamma-encoded spaces as Ae does.

If you set Ae’s working space and choose “Linearize workspace” then I suspect you will see results in Ae closer to what your developers are accomplishing. They may need to make a gamma adjustment.

[Pedro Amaral] “Plus the developers still have to simulate the “Repeat Edge Pixel” from the Fast Blur by increasing the size of the content until it compensates the bulged margins from the blur.”

Yes, repeat edge pixels does exactly what it says.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive   |   RenderBreak [blog]   |   Profile [LinkedIn]

[Pedro Amaral]

Hi Walter,

Thank you for your reply, indeed it makes total sense.

My confusion came when trying to match the same blur result on both ends, the blurriness amount I had in AE was completely different from the pixel blur values in the devs code. (I have no coding knowledge whatsoever so maybe it is supposed to be this way)

Please open the following example on your Chrome browser:
https://jsfiddle.net/9rcf2gtk/3/

On the CSS area (top right) you see 35px which, I suppose, stands for the pixel radius. To match the exact same result there I’d need 100 units blurriness in AE.

Once again thanks for the help.