Perspective/non-linear blends with a common centre
In illustrator, if you want to take a blend between two objects with a specified number of steps and give it a perspective-like effect - changing the curve of the blend from linear to non-linear so the blend has different density at different points - it's pretty straightforward. You add a spine to the blend and edit the spine until you are happy with it.
(to add an editable spine, an easy way is to plonk a line or shape on top of the blend, select it and the blend, then Object > Blend > Replace Spine)
However, this method does not always work well when the shapes you a blending have a common centre. For example, in A below, manipulating the spine requires pushing the blend out in a direction, like in B. Using this technique, the blend has to be lopsided, and can't be neatly centred and symmetrical like C (which I made for this examples via the tedious process of expanding the blend then resizing each line by hand).
How can you create an effect like C from a blend like A?
I'll accept methods that are not based on blend, so long as, like editing a blend's spine, they can be editted, tweaked and adjusted conveniently after they are applied.
Edit: After asking, it occurred to me that the blend tool spine alone will never be the right tool for this task since it controls only the positions of the intermediary shapes, while what I'm trying to do requires changing their sizes. So treat the blend spine thing as an illustrative example of the idea only - an answer is not likely to involve that technique. I imagine there is probably a good answer that involves envelope distorts, but I'm not having much luck making the details work.
I think I've cracked it. This method is highly flexible, straightforward/reliable, and modifiable. The only weakness is it does distort the outer and inner shapes of some shapes (which can be a bonus depending on the effect desired). The other method using polar grids and the object replace script is probably better if you need to avoid shape distortion.
- Create the blend
- Select blend, make a 3x3 envelope mesh (Object > Envelope Distort > Make with mesh)
- Select the corners of the middle square of the mesh (easy way: draw around them with the Lasso select tool (Q), make sure Edit Envelope is selected in the top left)
- Use Object > Scale with 'Preview' ticked and adjust until it looks right. Both the blend, and it's envelope distortion, remain completely editable.
You'll see that the concentration of lines is highest towards the middle of the envelope, whereas in my example I wanted the concentration to be highest at the edges. You can control this by putting an invisible shape (grey in my example to be clearer) behind the blend to push the middle of the envelope to the position in the blend where you want the concentration to be highest - changing the size of this box gives you a lot of control on where to focus the distortion.
You can also get more control by adding more grid lines to the envelope mesh. Here's some examples that just use Scale at different points in the grid. Naturally you can tweak any point in the grid to any amount to get really precise effects.