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10/24/2007 02:43:59 PM · #1 |
For those just joining us, I've gotten the answer several times over, so feel free to jump in, but I don't want to waste your time. :)
Here was my original post:
I'm looking for a technical discussion. I noticed that using a gaussian blur with a large radius and then fading it (using PS) creates a very different result from just using a lower radius.
For example:
blur 80 / fade 50%
is different from
blur 40
Does anyone know (or have a link) to what happens in a gaussian blur and what then happens when you fade it?
Message edited by author 2007-10-24 22:35:51.
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10/24/2007 03:08:49 PM · #2 |
The radius setting defines the effect, and the opacity determines how much the effect is applied. This is always true,k whatever tool you are using: the parameters define the effect, and the opacity determines how much of it is applied. In the case of Gaussian Blur, the effect at radius 40 is very different from the effect at radius 80, so there's a big difference in the end results.
Try using sharpening at a high radius and fading it vs sharpening at a smaller radius, and see the same process at work.
R.
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10/24/2007 03:29:06 PM · #3 |
There's a fairly simple explanation of the algorithm on wikipedia that's mostly correct
//en.wikipedia.org/wiki/Gaussian_blur
a 50% blur of a radius 80 Gaussian blur is no way equal to a radius 40 Gaussian blur.
The 50% fade halves the effect of the blur on the pixels that it affected.
The radius changes the pixels that get affected, not the amount that they are changed.
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10/24/2007 04:06:40 PM · #4 |
An even simpler explanation: Blurring means replacing each pixel with an average of the pixels around it. The radius parameter specifies how far out to go when computing the average. Small radius means you are only averaging the immediately surrounding pixels, large radius means you are drawing a big circle and averaging everything within it. Gaussian blur is just a specific type of blur, where the average is computed with Gaussian weighting.
Which is very different from the fade/opacity parameter, which, as Rob and Gordon explained, just refers to the strength of the effect. It works the same for every effect, including blur. |
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10/24/2007 04:10:16 PM · #5 |
Thanks, guys. Looks like Gaussian blur does exactly what I thought it did. I'd even visualized the 3-D graph of weights shown on that Wikipedia page.
But I'm still missing what fade does. Is it the equivalent of doing the blur on a duplicate layer over the original, then reducing the opacity of the blurred layer?
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10/24/2007 04:14:58 PM · #6 |
Originally posted by levyj413:
But I'm still missing what fade does. Is it the equivalent of doing the blur on a duplicate layer over the original, then reducing the opacity of the blurred layer? |
Yep! At least that's my understanding of it. |
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10/24/2007 04:15:33 PM · #7 |
[thumb]604303[/thumb]
I did a quick visual comparison for you.
The image on the left has a Gaussian blur radius of 2.5. The one on the right has a radius of 5.0 faded to 50% opacity. By the logic of you original post the two images should match.
However, you will see that while the radius of the right image is double, the fade allows details to be seen rather sharply, while adding a diffusion effect.
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10/24/2007 05:29:26 PM · #8 |
Leroy, I think that's a good visual for folks who haven't seen this effect before understand what we're discussing.
To be clear as to why I posted, though, I knew they were different. I was asking for the reason why they were different. And now I know! :)
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10/24/2007 09:46:13 PM · #9 |
Okay, I just did a test to confirm what was said earlier about the effect of "fade." Yep, these two are identical:
Do a blur and fade to x%
Duplicate the layer, do the same blur, and set opacity to x%
Even at the pixel level, they're identical.
And now I know why fading is at least a little controversial in basic editing: it simulates having two separate layers. Useful trick, though!
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10/24/2007 10:03:39 PM · #10 |
Here's a less aesthetic example, but you can examine closely it without looking over your shoulder first. ;)
While both "look blurry", the blur/fade approach leaves sharp details (like the line on the left) mostly intact, while providing a pleasing soft-focus effect. The blur-of-half-radius destroys these details.
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