The following tutorial is courtesy of "Illustrator CS Most Wanted Techniques and Effects" by Matt Kloskowski

In Illustrator you can create two objects and blend them together and Illustrator will do all of the work of interpolating how the "in-between" objects fit together. However, it arranges them in a straight line. This straight line is called a spine. The default spine has one point for each object in the blend. When you only use two objects, there are only two points. If you connect those two points together you would get a straight line. You can see this by opening the Blend layer that's created. Alternatively, viewing the illustration in Outline mode will give you a good representation as well. You'll notice the two objects you created but you'll also see a third path as well. This path is the spine. The spine behaves much like a regular path. You can add, delete, and move the anchor points that comprise the spine. You can also modify the handles just as you can a regular path. In this exercise you'll learn how to create a custom path to apply to a blend by replacing the original spine.

Step 1

This image contains a space scene with an alien spaceship flying away from the earth. However, it seems to be missing something. Perhaps a motion trail behind the spacecraft would add a little movement to the illustration. You're going to draw two shapes and blend them together. Then you'll drop the opacity of one shape so it appears as if the blend is fading away. But you're going to take it a Step further. You're going to draw a new curved path and replace your blend's straight path spine with the curved path.

Step 2

Take note of the layers in this file. The top layer contains the spacecraft SPACECRAFT, the middle layer MOTIONTRAIL will contain the motion trail that you're about to draw, and the bottom layer BACKGROUND contains the space scene background.

Step 3

Lock the top SPACECRAFT and bottom BACKGROUND layer and select the middle layer MOTIONTRAIL.

Step 4

Select the Ellipse tool. Draw a large oval behind the spacecraft. This oval should be entirely hidden by the spacecraft. However, a reference image is included here to give you an idea of the size of this oval.

Step 5

Set the fill color of this oval to a light yellow R:255 G:255 B:102. Set the stroke to None.

Step 6

Next, draw a small oval on top of the earth. Use the same fill and stroke settings as the previous oval.

Step 7

Select the two ovals and choose Object >Blend>Blend Options. Enter settings similar to this reference image.

Step 8

Choose Object>Blend >Make (CTRL/CMD+ALT/ OPTION+B). You should now have a smooth blend between the two oval shapes.

Step 9

Now, much like the spotlight effect, select the small oval and change the opacity settings to 0% in the Transparency palette.

Notice how the blend follows a straight path much like the spotlight effect. If you expand the arrow next to the blend layer in the Layers palette you'll notice a straight path in a layer on top of the two oval objects that you've blended together. This is the spine. You're now going to draw a curved path to replace that spine.

Step 10

Select the Pen tool. Create a new layer. Draw a curved path similar to the reference image here. Press ENTER/RETURN to close the path.

Step 11

Select the curved path that you just drew. Hold down the SHIFT key and select the blend that you created in Step 8.

Step 12

Choose Object >Blend > Replace Spine. Your blend should now follow along the curved path that you created in Step 10.

Step 13

Remember, this new spine is merely a path and subject to all the rules and regulations of a path. You can add, delete, and modify anchor points just as you would a regular path. You can select the entire path and move it wherever you like and the blend will follow. In this instance, I wanted to modify the path so the curve was reversed.

The best way to do this is to lock the blended objects so you don't accidentally select one of them instead.

Step 14

Select the Path sublayer in the Blend group (the spine).

Step 15

Select the Convert Anchor Point tool from the Pen flyout menu.

Step 16

Click the middle point in the curved path and begin dragging. Drag this point up and to the right.

Step 17.

Release your mouse button and your blend will instantly redraw itself.

Courtesy of Layers magazine.