Tool: The Two Erasers
There are two erasers in the toolbar, and for a while I thought that was a mistake I would eventually get around to fixing.
It isn’t. They do different things, and the reason they have to is one of the oldest confusions in vector drawing — a confusion the pixel world simply doesn’t have.
In a paint program, an eraser is unambiguous. There is one kind of thing on the canvas, pixels, and the eraser removes them. Drag it anywhere and the pixels under it go away. Nobody has ever had to ask what an eraser does.
In a vector program, there are two completely different kinds of thing on the canvas, and they only look alike. There is a filled region — a rectangle, a disc, the closed silhouette of a letter, an area with an inside. And there is a stroke — a line, an arc, a pen path, a thing with length but no inside, that is visible because it has been painted along.
An eraser dragged across a filled disc should take a bite out of it, leaving a disc with a scoop missing. An eraser dragged across a line should not take a bite out of the line. It should shorten it. The line has no interior for a bite to be taken out of; what you obviously want is for the part you rubbed to stop existing and the rest to remain.
Those are different operations. Not different in degree — different in kind. One subtracts area, and the other trims length. And because a vector drawing contains both kinds of object at once, a single eraser cannot be right; it would have to guess which one you meant, and it would guess wrong about half the time, and you would slowly come to distrust it.
So: the Eraser cuts shapes. Drag it across filled regions and it removes the area you dragged over, reshaping the outline to go around the bite. The disc comes out with a scoop.
And the Path Eraser trims paths. Drag it along a path and the part you dragged over is deleted, and the path ends where you stopped. The line gets shorter.
Once you can name the two kinds of object, the two tools stop being confusing and start being obvious. Which is, I think, the actual lesson here, and it’s not really about erasers.
Almost every persistent confusion in a piece of software is a place where two different things have been given the same name, or the same appearance, or the same button. The user’s difficulty is a symptom; the underlying disease is that a distinction the program depends on was never made visible. People are not confused because they are careless. They are confused because they have correctly noticed that the system is telling them two things at once. [1]
The honest fix is not to merge the tools into one clever eraser that guesses. It’s to make the distinction legible — to name both things, put both in the toolbar, and let the person who understands the difference act on it. That costs a sentence of explanation, once, and then it never costs anything again. The clever merged version costs a little bit of trust every day, forever.
There is a third tool nearby that is not an eraser at all and gets used as one, which is Shape Builder with alt held down. That removes a face — a region bounded by the shapes you already drew. If the area you want gone happens to be enclosed by existing geometry, alt-drag is better than the eraser, because the resulting edge is exactly the edge of the shape that was already there, rather than the wobble of your hand. Use the eraser when you want a hand-made edge, and Shape Builder when you want a geometric one. [2]
[1] The tell is when experienced users develop a superstition — “don’t use the eraser on lines, it does something weird” — instead of a rule. A superstition means the distinction is real and undocumented, and everyone has independently discovered its shadow.
[2] I use alt-drag ten times for every one time I use the eraser, which is probably true of anybody who draws with geometry rather than by hand. The eraser earns its place for the cases where you want the mark to look made rather than computed.