Select parts of a mesh

Sometimes you want to change part of a shape and leave the rest alone. Panels on some faces of a door, moss only on the top, tiles missing from a floor, windows only on the wall that faces the street. The pattern is always the same: work out a number for every face, decide which faces that number picks, then act only on those.

When you need this the change you want applies to part of a shape, not all of it.

Say what counts as one face

Under the surface every shape is triangles, so "one face" needs deciding. Set Faces does that: in Quad mode a pair of triangles that form a square counts as one face, which is almost always what you mean by a tile, a panel or a brick.

Skip it and a change lands on half a tile, because a triangle is what the shape actually had. Put it early, right after the shape is built.

The other thing to know is that changing a shape's topology throws the grouping away. If you thicken, cut or subdivide after grouping, group again afterwards, or the selection will be describing a shape that no longer exists.

Get a number for every face

This is the step that confuses people, so it is worth being plain about it.

Before you can pick faces you need something that varies across the shape. Not a selection yet, and not a yes or no. Just a number, and every face gets its own.

Noise is the usual one. Picture a soft grey cloud laid over the shape: some places light, some dark, drifting smoothly between. Each face reads whatever is underneath it, so neighbouring faces get similar numbers and distant ones do not. Its Scale is how big the blotches are, in real units, so on a wall four units wide a scale of one gives you roughly four blotches across. Its Seed picks a different cloud.

Height and Normal work the same way but mean something. Height gives each face how far up it is, which is how you get snow only near the top. Normal gives each face which way it points, which is how you get "only the side facing front".

The fastest way to understand any of these is to look at one. Feed it into a Color Ramp and the shape paints itself with the numbers, and suddenly the thing you were about to threshold is visible.

Turn the number into a choice

Compare draws a line through those numbers. Set a threshold, and every face lands on one side or the other: picked, or not picked.

Which side gets picked is the operation. Less Than picks the faces below the line, so raising the threshold picks MORE of them. Greater Than does the opposite, and that is worth checking whenever a control feels like it is working backwards, because both are correct and only one matches the label you gave the knob.

That single number is usually the most useful control on the whole graph. It is the difference between a floor with two tiles missing and a floor with two tiles left.

A selection does not have to be a yes or no

This is the part worth slowing down for, because it is where selections stop being a switch.

Some nodes read the selection's actual value rather than just whether it cleared a line. Feed a raw number straight into Inset and each face insets by its own amount: no threshold, no yes or no, a different result per face. Feed the same number into Color Ramp and you can see it, which is the easiest way to understand what a selection is really made of.

Nodes that scale by the value: Set Position, Inset, Extrude Tiles, Bevel, Color Ramp, Refine, Loft, Metaball Mesh. Nodes that only ask yes or no: Delete Geometry, Grow Selection, Extrude Along Curve.

If you want uneven results, you need one from the first list. A yes-or-no node gives every selected face the identical treatment, however varied the number feeding it was.

Growing and shrinking a selection

Grow Selection takes a selection and spreads it to neighbouring faces, which is how you turn scattered picks into patches. It is also the way to leave a margin: grow a selection, then use it to keep an area clear.