Glossary

The 18 words this manual marks with a wavy underline. Each one also appears on hover where it is used, so this page is for looking one up, or for finding the chapter that needed it.

Antialiased: Softened at the edges. A diagonal drawn on a grid of square pixels is faded across the boundary ones so it reads as a line rather than as a staircase.
Arity: How many curve segments a slot holds, fixed for the whole rig. A rig of straight lines is one segment per slot. A rig with a circle in it needs four, because that is what a circle takes.
Centroid: The average position of a set of points, which is to say the shape's balance point.
Congruent: The same shape and the same size, allowing for having been turned or moved. Two icons are congruent when one could be laid exactly over the other without stretching it.
Correspondence: The answer to which stroke becomes which stroke. Every hard decision in morphing is this question, and everything after it is arithmetic.
Cubic: A curve defined by four points: where it starts, where it ends, and two control points that pull it into shape. Every shape in this manual is stored as a chain of them, straight lines included.
Driver: The small piece of JavaScript that plays a compiled morph, for when you need to reverse it, interrupt it, or scrub it by hand. Everything it does can be skipped if the animation only ever runs start to finish.
Interpolate: To produce the in-between values. Given a start and an end, interpolating at the halfway point gives you the value halfway between them.
Parameterisation: How a curve's points are spaced out along it as you sweep from one end to the other. Two curves can draw the same line and still be travelled at different speeds.
Parking: What a slot does when the icon it belongs to has no stroke for it. Rather than vanish, it hides on another slot and is faded out, so it has somewhere to be born from next time.
Polar: Describing a point by its angle and its distance from a centre, rather than by how far across and down it is. Rotations are simple in those terms and awkward in the other, which is the whole of Part 6.
Procrustes: A standard way of finding the single rotation, scale and shift that best lays one set of points over another. Named, grimly, after the Greek innkeeper who made guests fit the bed.
Residual: What is left over after the closest possible fit. Zero means the two shapes matched exactly, and a bigger number means the fit had to give something up to get there.
Rig: A set of icons built to agree with each other, so any one of them can become any other with no matching step at runtime. The agreement is the whole trick, and writing it down is what this manual is about.
Slot: One of the fixed positions every icon in a rig is divided into, drawn as a single path. Slot 0 of one icon always becomes slot 0 of another, which is how the rig knows what turns into what.
stroke-dasharray: A CSS property that breaks a stroke into dashes and gaps. Set the gap long enough and the line disappears, so animating the offset makes a shape look like it is drawing itself. What moves is how much of the line you can see, never the line's shape.
Subpath: One continuous stroke inside a path, from a pen-down to the next pen-up. A path with three separate lines in it has three subpaths, and each one is a candidate for becoming something.
Tier: How much machinery a morph needs to play, from compiled CSS with no JavaScript at all up to the driver. The tiers are a delivery choice and they change nothing about the correspondence underneath.