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TECHNIQUES OCTOBER 16 , 2001

Stroking a Path with Gradients and Motion Blurs
Using the Stroke Path command in conjunction with tool options in Adobe Photoshop

by David Nagel
Executive Producer
[email protected]

A long time ago, I mentioned that I was looking for a way to simulate a motion blur in Photoshop. I don't mean just a simple motion blur like the one you find in the Filter menu. I mean a more versatile blur, one that would allow me to blur in multiple directions. In other words, a motion blur along a path. Well, I just might have found the answer. You decide.

This is not a particularly difficult tutorial. I actually stumbled upon this technique when answering a user question in the Adobe Photoshop forum here at Digital Media Net. The question was, "How do I stroke a path with a gradient?" I thought about it for a second, came up with the answer and then thought, "Wait a minute! This could be applied to blurring as well! Hot diggity!" (No, I don't actually think phrases like "hot diggity!" I'm just toning it down a bit for the fragile sensibilities of my bosses.)

Stroking a path with a gradient
For the gradient portion of this tutorial, it works like this.

1. Draw the path to be stroked. In the example above, I just drew a rough stroke with the Freeform Pen tool.

2. Select one of the painting tools in the Tool palette. Pick your brush, and then adjust the settings in the tool bar up at the top.

3. In the little brush icon in the toolbar, you'll see three options: Size, Pressure and Color. To create a color gradation, simply turn on "Fade" in the color option. This will transition the stroke from the foreground color to the background color over the course of the steps you set. (See note below.) To create a gradation that goes from a color to transparent, select Fade in the Pressure option. Again, this will transition the stroke from the foreground color to transparent over the number of steps you set. (You can also combine the two, along with size.)

4. Now, in the Path palette, click on the flyaway menu and select "Stroke Path." The default option will be your currently selected tool.

Voila! You now have a gradation along a path. Can you see where this is heading? That's right: Motion blur. But first a note.

NOTE: The most difficult part of this process is determining the proper number of "steps" for your gradation along any given path. In Photoshop, a "step" is simply an application of the selected brush. Brushes "step" according to the Spacing percentage set in the Brush Options dialog, which you can access by clicking on the brush icon in the tool bar.

So let's say you're using a 10 pixel brush spaced at 100 percent intervals. In 10 "steps," your brush will cover 100 pixels, right? Well, unless my first-grade arithmetic fails me (which isn't out of the question), that's right. So now all you need to know is the length of your path. If it's a straight path, no problem. You can just use the Measure tool. Otherwise, I haven't found a way to measure a curved path yet. So it's a matter of trial and error.

Stroking a path with a motion blur
Now, for some of you, I might be using the phrase ":motion blur" a bit too loosely. We'll see what you think after you read this. Using the same principles outlined above, you can stroke a path with either the Blur tool or the Smudge tool. But there's a problem with the Blur tool: It's isn't directional. So, if you stroke a blur along a path, you're just going to wind up with a curvy smudge—no motion to it at all. And forget stroking a path with the Motion Blur filter. It only blurs on one direction, regardless of the shape of your path.

Your other option is to use the Smudge tool, which, aside from smudging, also creates a directional blur effect. It's a bit of a heavy-handed effect, but it gets the job done. You just need to make a couple of adjustments. First, look at the example below. It's a little mouse, and I want to create the comic effect of a long motion trail behind it. So what I do is to draw the path leading from its back side.

Then I'm going to stroke the path with the Smudge tool. The problem? Well, it doesn't really look like a motion blur without a little tweaking. So, in this case, I masked off my mouse and moved it to the top layer. Then I created a transparent layer and a white background layer underneath. To get the proper effect, I'm actually going to apply the motion blur effect to the center layer, leaving the top layer (my original mouse) in tact. By checking the "Use All Layers" option in the tool bar, the stroke will actually draw the images from all visible layers but only apply the results to the currently selected layer. (Pretty slick, eh?) Here's the result.

As you can see, it's still not exactly perfect. I want to reduce the opacity of my top layer just a bit so that part ofd the smudge, or motion blur, shows through. And here's the result.

It might not be exactly what you're looking for in a motion blur, but, then again, this is just one way of doing it. By tweaking around with apply modes and playing with some of the options available, you should be able to achieve the look you're going for. Know an easier way to do this? Let me know by e-mailing me at [email protected].

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Dave Nagel is the producer of Creative Mac and Digital Media Designer; host of several World Wide User Groups, including Synthetik Studio Artist, Adobe Photoshop, Adobe InDesign, Adobe LiveMotion, Creative Mac and Digital Media Designer; and executive producer of the Digital Media Net family of publications.