Adobe introduced 3D shapes to Illustrator several versions ago, and allows direct generation of vector lines within the program. While its function is much more limited than that of coupling a full-fledged modelling suite to a vector renderer (like Blender with Pantograph, Autodesk Maya/Swift3D, or Sketchup Pro), it works very well in a pinch and for simple projects. I’ve used it to generate 3D bar-charts (see figure) and geometry for inking over; in this tutorial I’ll show a simple example in making a few Tensegrity instructional illustrations for my friend Alejandro.
An examples of the finished illustration. We’ll briefly cover the processes in (i) cutting the shapes, (ii) extrusion parameters, (iii) 3D-2D conversion, and (iv) touch-ups.
Cutting the Shapes
There are several operations that generate 3D shapes, including extrusion, revolution, and rotation. They are useful for 3D-objects of different symmetry, and their effects on a 60% transparent shape are shown below.
All of these rely on first generating a path to be operated upon. The tensegrity sticks need to have volume, and have no rotational symmetry; they are a clear candidate for extrusion. To generate the outline I simply removed two (small) rectangles from a large rectangle using the ShapeBuilder tool. To soften up the shapes a little, the corners were rounded with the Effect -> Stylize -> Rounded Corners effect. This last round-corner operation has the side-effect of generating more anchor points, and dirty up the 3D effect somewhat later on.
At the end of this operation I inverted the fill and stroke (shortcut: shift-X), to generate a shape with purple fill and null stroke. The reason for this is to simplify the resulting expanded appearance (see next section).
The extrusion parameters control both the visual appearance of the 3D object, as well as the properties of the expanded 2D appearance. For example, with simple shapes like this, the appearance of Diffuse and Plastic shading may appear identical, but the expanded objects are constructed differently, and this affects what subsequent operations can be made. The following figure systematically shows what the different options do. The first three are of identical extrusion parameters, but different stroke/fill options; the remainder shows the effects of varying extrusion parameters. The lower row shows the outlines after the appearances have been expanded.
Because I’m interested in generating the cleanest outline, I went with a fully opaque fill (and no stroke) on the H-shaped paths, and extruded with no bevels. The various copies were then placed on one another in proper Z-order.
At this point I duplicated everything on a new layer and hid the layer containing the 3D objects (select all, copy, new layer, paste in place; toggle visibility of lower 3D layer). This gives us something to fall back on. The appearance was then expanded (Object -> Expand Appearance) to give easily worked with paths. The strokes were set to black 0.75, and we have ourselves a mostly OK illustration! Note that the fill cannot be set to null; otherwise the previously masked back-faces will be exposed. A white fill should be used instead (on the expanded appearances) if plain outlines are to be obtained.
Note that stray lines will be present for more complicated shapes. For those cases, manual intervention will be necessary. For the tensegrity illustrations, the rounded corners present some of these elements, and they’re cleaned up where needed.
The forward-backward ordering of entire objects works well in many cases, but not for interlocked objects. For example, the purple stick on the right should be between the blue sticks, not under; but this is not compatible with its need to be partially behind the back green stick.
We simply need to touch up the rest, in this case, by drawing over the offending blue-stick, using the Eyedropper tool (shortcut: I) to match the style and color.
Other graphic elements are drawn in a new layer over the sticks to ensure their visibility. At this point, all of the elements can be quickly manipulated and stylized to will, something that would have been time-consuming to do were it constructed as raster graphics.
The procedure outlined here, once worked out, is convenient, flexible, and quick to scale up, at the expense of perfect perspective and realism. As with most things graphics, there are numerous ways of going about this — feel free to let me know of alternatives in the comments!