Tutorial: Working with 45-Degree Curves in Adobe Illustrator
I got an request from an anon last week which asked:
"Hey! Could you do a video tutorial on how to bypass Illustrator’s annoying round corners effect in case of 45 degrees? It would be a lifesaver!"
Now, while you’re not going to get a video tut out of me — I don’t have the resources, time or know-how to produce one of those — I can and will share my battle-tested personal approach to this problem.
As our anonymous friend says, the “Round Corners” effect in Adobe Illustrator is essentially broken when it comes to making transit maps. Because of the way it measures the “radius” of curves, it produces unexpected (and useless) results on curves that aren’t a bog-standard 90 degrees. I wrote in depth about this problem on my design blog a while back: I suggest you head over and read the article if you want to fully understand the issue at hand here.
The other problem with the effect is that it applies the same radius to every curve along the entire length of the path, which isn’t always what you want: a route line might be the outside line (a larger radius) at one corner, but the inside curve at another (a smaller radius).
So I’ve long since given up trying to fight the inadequacies of the “Round Corners” filter, and instead generate all my curves manually from a “master set” of curves that I create at the beginning of a project.
STEP 1 above shows a pretty standard initial set-up: four concentric circles that are aligned to the grid that’s being used by the map. Here, we’ve got a 10-point grid, and the four circles have radiuses of 10, 20, 30 and 40 points. The line thickness is 8pt, so there’s a nice 2pt gap between each route line. When you’re setting up your master curves, be sure to create enough circles to account for the maximum number of adjacent route lines you’ll have on your map. Often, it’s only two or three, but for my map of TGV Routes in France, I had to set up a staggering eighteen!
STEP 2: From here, it’s a simple matter to use Illustrator’s Scissor tool (shortcut: “C” key) on the existing anchor points in each circle to split them up into four 90-degree segments. I’ve moved one set of 90-degree curves away from the others so you can see what you should have. Keep this set for when you need 90-degree curves, then duplicate it so we can use what we’ve already created to create a new 45-degree set.
STEP 3: Draw a line using the Line Segment Tool (shortcut: “” key) that passes through the centre point of all the circles and crosses all the paths at a 45-degree angle. (Hint: start at the centre point and hold down the Option/Alt key — to extend the line equally in each direction from that point — and the Shift key — to constrain the line to 45-degree angles).
Use the Rotate tool to make a duplicate of this line that’s rotated 90 degrees. Select both lines and press Cmd/Ctrl-5 to turn them into guides. Make your guides visible (Cmd/Ctrl-;) and turn on Illustrator’s Smart Guides (Cmd/Ctrl-U). Then use the Scissor tool to cut each of your route lines where it crosses these new guide lines. The Smart Guides will help a lot by giving you feedback when you’re positioned correctly over the guides: a little “intersect” tool-tip (just visible in the STEP 3 screenshot at top right) will appear near the cursor. Click to cut when you see this and you’ll be golden. Repeat for each line you need to cut. You don’t need to have any paths selected to cut, just position your cursor, click and Illustrator is clever enough to work out what you need. You only need to cut these 45-degree points because we already cut the 90-degree points in STEP 2.
STEP 4 show the result. Again, I’ve moved one set of curves away from the others to show you what you should have: eight complete sets of 45-degree curves, ready for use on your map!
STEP 5 shows a common scenario where three separate routes go around a 45-degree curve together. Draw them so the separate segments for each line touch, but there’s absolutely no need to join the lines at this stage.
A note: If Illustrator took a leaf out of CAD software and included a “Fillet” effect, we could forego this entire workaround. You’d simply select each segment of a route line, invoke the filter, set a curve radius (an actual, proper radius!), and the software would then create the curve accurately and join the lines seamlessly for you, each and every time. After 14 versions of Illustrator, I’m really not holding out hope for this functionality any time soon, however…
Another note: decide which of your curve radiuses represents a “standard” curve and always use that curve when a single route goes around a corner. Then, decide what happens when you have two curves: do you use the next size up or down for that curve? And so on for each combination of curves. Always apply your curves according to the rules you set here — this is what gives your map consistency and visual flow. Don’t cheat and use a smaller or non-standard curve to make things fit!
Here, I’ve decided to use the three largest curves for my three route lines (I often feel that the smallest radius can be a bit tight and look ungainly, so I only use it when I really have to), and I’ve pasted in the appropriate curves from my master set (I’ve changed their colour to magenta to make them easier to see and place correctly).
STEP 6: It’s always easier to align the curves to the horizontal or vertical line segment first. Again, this is easier if you have Illustrator’s Smart Guides on. Drag the curves over by one of the left points, holding down the Cmd/Ctrl key as you do. This ensures that Illustrator provides you with the correct visual feedback that things have aligned properly. Be warned: sometimes, Illustrator reports that things are aligned when they’re not. Be sure to zoom in enough and check things out if you suspect things aren’t quite right.
STEP 7: Now align the right points of the curves with the 45-degree segment, by holding down the Cmd/Ctrl key and the Shift key (to constrain the movement) as you drag across. Again, Smart Guides will give you feedback when things are aligned.
STEP 8: Now that the curves are in the right place, you simply have to align the end points of all the line segments with the end points of the curves, then join everything together. Fortunately, Illustrator’s Join command s a lot less finicky than it used to be. You can now just use the “black arrow” Selection tool to select each segment and hit Cmd/Ctrl-J to join them all together in a logical progression. You used to have to select individual end points, two at a time, and tell Illustrator to join just those segments!
An approach I like to use is to leave all this curve work until the routes are substantially laid out. Then I place all my curves, delete the original straight line segments, and simply join all the remaining curves together. Because they’re positioned accurately, you know you’re going to get nice straight lines between each and every one of them! And it saves having to tediously move end points around until things line up before you join things together.
That’s it! If anything’s unclear, drop me a line or leave a comment and I’ll try to clear things up for you.
STEP 9 shows the finished result: beautifully nested 45-degree curves with no unnecessary additional points. Perfectly executed and reproducible across the entire map!