Feather and Blur

[Doug Katz]

Would someone be kind enough to explain the differences between the Effect>Stylize>Feather command and the Effect>Blur>Gaussian Blur command?

In compliance with a certain geneologist's recommendation, I've been exploring AI 10.0.3's prefab stroke and fill styles. In these, Feather seems to produce a blurring effect. In fact, if I remove a Feather that, say, has been applied to a stroke (by deleting it in the Appearance palette), then replace it with a Gaussian Blur, I think I can produce a visually similar result by fiddling with the settings.

I'm familiar with feathers and blurs in Photoshop. As most of you know, filling a feathered selection with a color will yield a result that's very similar to filling an unfeathered selection with the color, then applying a Gaussian Blur to the deselected pixels. Is AI's Feather supposed to be mimicking the first of these two? And is this why it looks so similar to the Gaussian Blur, at least to my undiscerning eyes? (And if both of these are true, aren't the two Effects slightly redundant? Actually I'm sure they're NOT redundant... but what's the diff?)

[Teri Pettit]

The difference in visual appearance is that Feather affects only the opacity. The edges of the object get more transparent the closer they are to the edge.

Gaussian Blur, in contrast, takes neighboring pixels and averages both their colors and their opacity. Any transparency that arises towards the edges is due to averaging in the transparent background.

When the object to which the effect is applied is a solid color, they will tend to look similar, since averaging two pixels of the same color doesn't do much. :-) But if you fill an object with a pattern and Gaussian Blur it, you will notice quite a difference. You can also see the difference on objects that have both a stroke and a fill, since Gaussian Blur will blur the stroke into the fill, and Feather won't.

They are also implemented differently. Gaussian Blur rasterizes the object, as can be seen by doing an Expand Appearance afterwards. Feather creates a solid opacity mask the shape and size of the object, and then Gaussian Blurs that opacity mask. So if you do Expand Appearance on a feathered vector object, it remains a vector object, and it isn't until you look in the Transparency Palette and see that it has an Opacity Mask that it becomes apparent where the edge fuzziness is coming from.

Finally, since Gaussian Blur is a "pure" Photoshop filter, its units are expressed in pixels and thus do not scale when you scale the object, even if Scale Strokes and Effects is on. (A pixel is a pixel regardless of how big the object is.) But Feather is a native Illustrator effect with the blur radius expressed in ruler units, so if you have Scale Stroke and Effects on, the feathering radius scales.

[Doug Katz]

Excellent, excellent, excellent. These are not only differences but useful differences. For example scalability's no trivial matter. Nor is vector preservation. I'm off to apply some feathers and blurs to pattern fills.

[Adam Schneider]

I don't get the explanation of why Gaussian Blur can't be scaled. Okay, so a pixel is a pixel, but i don't see why Illustrator can't change an 8px blur to a 2px blur when the object is scaled to 25%!

[KATHY G]

"Finally, since Gaussian Blur is a "pure" Photoshop filter, its units are expressed in pixels and thus do not scale when you scale the object, even if Scale Strokes and Effects is on." .....Then if you have a number of files that use the same copy w/ drop shadows, what's the best way to make drop shadow? Do I have to create a new one for every file? Can't cut and paste and then scale down? And if I want to scale them all down to put on one page for a proof???

[Teri Pettit]

Kathy,

If you're specifically referring to Drop Shadows, you could just use the Drop Shadow effect instead of the Gaussian Blur effect. Since Drop Shadow is a native Illustrator effect instead of a Photoshop effect, the blur radius in Drop Shadow is expressed as a distance, not a number of pixels.

Even with the Gaussian Blur effect, the objects scale fine, their blur radius just stays the same number of pixels. (Which is the same thing that happens in Photoshop if you scale a selection on a layer with a Gaussian Blur layer effect.)

Adam,

Regarding my earlier admittedly simplistic explanation of why the blur radius on Gaussian Blur can't be scaled. I have a tendency to get more detailed than people want, so I kind of policed myself and glossed over it. But here is a more complete explanation.

The effects below the "Warp" group are real Photoshop filters. Illustrator has a Photoshop adapter that provides a veneer that makes it look like Photoshop to a filter. (This is no Adobe secret, it is the same thing other programs that run Photoshop filters must do.)

So Illustrator can't "look inside" the filters, it just pretends to be Photoshop as far as the filter is concerned. (Specifically, Photoshop 5.) The only kind of things that Illustrator can ask the filter to do, or the filter can ask Illustrator to do, are the things that the filter is already prepared to communicate with Photoshop about! Since the communication protocol between Photoshop and filters does not include any way for Photoshop or the Photoshop-pretender to say to a filter "Please scale your distance-relevant parameters" or "Which of your parameters refer to distances?", this is not one of the things Illustrator or any other application can tell a Photoshop filter to do.

It does include a protocol for saying "list all your parameters and their current settings", and another one for saying "please accept these new values for your parameters". These were originally intended to let Photoshop or the host app implement the "Repeat Last Filter" command. But Illustrator can repurpose them to record the filters as live effects. There isn't that much difference to the filter between being recordable once for "Repeat" vs being recordable per-style for a live effect. These two messages can even be used to allow such a non-Photoshoppish feature as the Blend tool to blend parameters between two objects with different settings of the same Photoshop effect, since it can pair up the two sets of parameters and go down the list interpolating them all.

But the parameters expressed in these protocols are just naked numbers without any units or identification as to which of them are distances and which are other kinds of attributes like angles or percentages. So it wouldn't work for scaling. You wouldn't want scaling an object by 200% to make a 30 degree angle change to 60 degrees or a 10% opacity change to 20%. It will only work for features where all the parameters can be treated uniformly and it doesn't matter what meaning they have to the filter.

Now since Adobe controls the Photoshop SDK, theoretically we could add a way for a filter to answer such questions about units or scale-sensitivity of its parameters. But since Photoshop itself doesn't have a use for it, probably few filter vendors would be motivated to supply the new information. Using Photoshop filters inside Illustrator is still such a small fraction of the way they are used that Illustrator just doesn't have that degree of influence on the way Photoshop filters are written.

Which is probably way more detail than you wanted to know. But this time I tried to anticipate all your possible questions.

[James E. Talmage]

> I have a tendency to get more detailed than people want

No you don't. Please don't think that, and please keep it up.

Most of us have to make our "best guesses" about the rationale behind a
feature or about what it is actually doing "behind the curtain."

I really wish all software documentation (at least the section
overviews) were written similarly to your informal explanations in this
forum. Most manuals pretty much simply tell us to use the smoke and
mirrors and 'Pay no attention to that man behind the curtain!'

I, for one, very much appreciate it, Toto--er,--Teri. ;-)

JET

[Doug Katz]

I, for two, VERY much appreciate it. It's the detail that makes this complicated and important stuff so suddently clear to me. Thanks to your detail, I'm guessing it will only take me 10 years rather than twenty to fully understand and wisely use this wonderful program you've written.

[Mark Durgee]

Teri,

I also appreciate the all the great info you give here. If you ever write
the Adobe Illustrator the Missing Manual, I'd certainly buy it!