How do I interpolate curves in Illustrator?

Question

Imagine a vectorialized typographic "C" which stroke width varies along the curve, as it often happens in typography.

I'd like to draw a curve between those two boundary curves in such a way that each point of the curve is at the same distance ratio from these two curves, for example it is always at the midpoint (distance from outer = distance from inner) or at 33% (distance from outer = twice the distance from inner).

Tracing several different curves with different distance ratios will produce a series of "concentric" curves that gradually change in shape from the outher curve from the inner curve, a little like the lanes of a street seen in a distorted perspective.

(While I used the fixed ratios procedure to let you all understand the desired result, other methods that produce a similar result are fine.)

How can this be done in Illustrator?

Answer 1

If I've understood your question properly, it sounds like you are describing a Step Blend.

Try this . . .

1. Type a letter C, and do Object > Expand to give you outlines, set no fill and set a thin stroke so you can see it.

2. Using the Direct Selection tool A (black arrow), select and delete, the end segments.

For example

3. Do Object > Compound path > Release, then Object > Ungroup

4. Select both curved pieces

5. Do Object > Blend > Make

6. Do Object > Blend > Blend Options - and choose "Specified Steps" and set the number of steps required

7. Note: a Step Blend is a path effect, so if you want to bake in the effect to give you actual curves you can edit, then do Object > Expand

Step Blends also work with any curves, not just those created from letters, and also with shapes