by infinite_resolution


May 7, 2009

Blobacious: adjective: Extremely bold or daring; original; without restriction

Contributed by Brenda Sutherland, Illustrator Team Rowing Captain
I know we’ve all had this experience. You take on a project that looks so simple you’re confident you’ll knock it out in no time. Then it turns out to be far more complicated then you ever expected, and you realize that to do it well is going to take a lot more time and effort. That’s exactly what happened when we implemented the Blob Brush in CS4. We thought it would be a pretty simple thing to do, as we had already implemented the Eraser tool back in CS3, and the idea behind the Blob Brush was to use Pathfinder to create shapes like the Eraser does, but instead of erasing them, fill them with color. What could be simpler than that?

I suppose if we had left it at that, it just might have been that simple, but there was another piece that seemed essential to making this tool complete, and that was merging. If you’ve been following my series on Pathfinder and the features in Illustrator that use the Pathfinder Engine, then you’ll recognize the connection here. The Blob Brush works in a very similar way to the Eraser tool in that is starts off by creating a Calligraphic Brush object, expands it into simple paths, then runs Pathfinder to create either a simple or compound path, eliminating all the overlapping brush strokes as well as the original path.

Because it starts out as a Calligraphic Brush object, it has all of the same functionality available that Calligraphic brushes have. If you are using a pressure sensitive tablet, you can even vary its settings just as you can with a brush. And like the Brush and Eraser tools, you can increase and decrease the size with the square bracket keys.

So one question you might have is, if the Blob Brush is so similar to the Calligraphic Brush, why did we go through all this trouble to create a new tool in the first place? The answer is that as cool as brushes in Illustrator are, there are times when all you want to do is create a simple path, a blob so to speak, that doesn’t have editable brush attributes after it’s been created, but can easily be erased, have gradients or live effects applied, and if it’s not the right shape, can be added on to without having to expand and then run pathfinder on.


If you’d like to see a demo of the Blob Brush in action, I highly recommend Mike McHugh’s, of Creative Sweet TV, short demo on the Blob Brush. Mike also shows how you can use the new graphic style library “Additive for Blob Brush” to create airbrush like effects with your Blob Brush created objects.So it’s pretty clear that the merging behavior is an integral part of how the Blob Brush performs it’s magic. Merging helps you build up your blob shapes into a single object, rather than a group of objects with the same color fill piled on top of each other. But to be a true Blob Brush Ninja, you need to understand the rules (or should I say discipline) of merging.The first and foremost rule is the Blob Brush will merge will other objects (created by the Blob Brush or not) if they have the same fill color and no stroke.Stacking order is important. If there is an object between the new object and the object you want to merge with, then the merge will not occur. This is because when objects merge, the new object is created at the top of the stacking order. If merging an object from below into this new object would change the appearance (because the underlying object is partly covered by the other object, and merging would mean putting it above) then the objects will not merge. Maintaining the appearance is more important in this case than merging.blob_2.jpgA similar rule is that the Blob Brush will not merge with existing objects that are in two different groups, unless it merges with (and hence flattens into a single path) every single path in the entire group. This is again to avoid disrupting the group or layer structure that you have set up.There is also a way to manually control merging. In the Blob Brush options dialog (double click on the tool in the tool panel) you’ll see an option for “Selection Limits Merge”. When this option is on, and an object is selected, merging will only occur with the selected object. Of course, all the other rules of merging still apply. If the option is on, but there is no selection, then merging will occur as it would if the option was off. In other words, this option only makes a difference if there is a selection.Cheryl Graham does a great job describing merging behavior, including some of its limitations, in this tutorial for Layers Magazine. She also explains the Selection Limits Merge behavior.Another thing you might notice when using the Blob Brush is that you might start out with a colored fill, then after using the Brush, you have a fill of none, and a colored stroke. Why does it do that? You have to remember that although the Blob Brush creates filled objects, it’s actually a brush (remember how it starts off creating brush strokes and then runs pathfinder to merge them into the simple object?) Like all brushes, it needs to paint the brush stroke with the stroke color. Since this is a bit confusing, we didn’t want you to have to always make sure that the stroke color was set and not the fill, especially if you have started off by selecting or eyedropping off a previously created Blob Brush object, which would be filled, but not stroked. So when using the Blob Brush and a fill color is selected but not a stroke, Illustrator will automatically switch the color to the stroke. If there is both a fill color and a stroke color, then the Brush will use the stroke color and drop the fill.I hope this was useful, and not more information about the Blob Brush than you will ever need. We really worked hard at making this tool do the right thing for you, so you wouldn’t have to think about things like object stacking order, or whether to pick a stroke or fill color. It may have turned out to be a bigger task than we’d anticipated, but I think it was worth the effort.


  • By shoaf - 10:47 AM on May 8, 2009  

    Just want to take the opportunity to thank everyone involved in adding the Blob Brush to AI CS4. For me, this is probably the single most useful addition to Illustrator in the 16+ years I’ve been using this app.Wow, what a nice compliment! On behalf of the Illustrator team, and the Blob Brush feature team, you’re most welcome!

  • By Leha Carpenter - 8:25 AM on May 10, 2009  

    I love the Blob Brush tool. Thank you for all the work you did to make it happen. I have to admit, at first I didn’t really get it, but this is turning out to be my favorite tool for so many things. Especially when paired with the Wacom tablet, the Blob Brush just seems to invoke creative sparks. For rebuilding logos from tiny JPEGs or scans of the client’s business card, the Blob Brush is the fast answer. For tracing just about anything where you need to get nice, clean, simple paths, the Blob Brush beats Live Trace (and is faster than the Pen tool). For creating layered skin tone effects, Live Paint groups, or anything involving complex shapes with fills, it’s the Blob Brush. I can’t figure out how I lived without it before…

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