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: with A Face Is set to Quad, 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.

When there are no faces worth selecting#

Quad mode can only find a face where the shape put one. A sphere, a cone tip, anything hulled or fractured, and a shape that arrived as triangles have no squares to find, so Quad mode hands back triangles and everything downstream works on wedges.

Quad Subdivide fixes that by making faces rather than looking for them. It turns every face into one square per corner: a triangle becomes three, a square becomes four, a hexagon six, so whatever goes in comes out as squares you can select, inset and extrude. It only adds edges. Nothing moves, so the shape you see is the shape you had, with more to grab hold of.

One pass is almost always enough. Each further pass multiplies the face count by four, and Iterations stops at three for that reason.

It is not a smoother, which is the easy mistake to make given the name. Subdivision rounds a shape off; this one leaves it exactly as it was.

Joining faces together#

Set Faces decides what one face is by a rule. Merge Faces decides it by pairing: each face takes one neighbour, and the two become one face from then on. Like Set Faces it changes no geometry at all, only what the nodes after it treat as a face.

The unusual part is how you control it. Both of its inputs are read at the EDGE between two faces rather than at the faces themselves, because what is being chosen is which joins happen, not which faces are involved. Feed White Noise into Priority and the joins are picked at random, with the seed as the only thing that changes them. Feed a Compare into Selection and you set how many joins are allowed at all, which is really a dial between a regular surface and a ragged one.

Faces that find no partner are simply left alone, which is normal near an edge and not a problem.

Putting something in every face#

Once a shape has faces worth selecting, Fit To Faces puts a copy of something into each one. Not dropped on top of it, but reshaped to it: a tile lands in a four-sided face by having its corners moved onto that face's corners, so on a grid where no two cells are the same shape every tile still fits its own.

Wire the shape into Cells and the thing you want repeated into Tile. If the tile is several pieces joined together, all of them go into every cell. The Selection input decides which cells get anything, and Index decides which piece they get, which is how a cell next to a built one can take a different tile.

Two things to know. The tile fills its cell edge to edge, because the tile's own size is what defines the cell - Margin is what pulls it back in if you want streets between buildings or grout between paving. And Keep Shape mode stops the reshaping for anything that would look wrong distorted, like a barrel.

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.

Position and Normal work the same way but mean something. Position with Axis set to Y 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, Merge Faces.

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.

Asking about a face's corners#

Everything above asks a question about a face as a whole, because that is how fields work: each face gets one number, read at its middle. Corner Pattern asks about the four corners instead.

How Many is the plain reading, and it is useful on its own. If the selection is "this is land", then a face with all four corners selected is inland, a face with none is open water, and a face with one, two or three IS the coastline. That is a shoreline in one node, and it works on a face with any number of corners.

The other three readings need squares, and they are how a shape can choose a different tile depending on what is around it. Four corners that are each on or off make sixteen arrangements, but most are the same arrangement turned round, so they come down to six: nothing, one corner, two side by side, two opposite, three corners, everything. Which Tile tells you which of the six, and Turns tells you how far to turn it. Feed those into Fit To Faces and a set of six tiles arranges itself.

One thing to watch. The selection has to be something that varies across the shape, like a Noise or a Position, or all four corners give the same answer and every face reads as nothing or everything.

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.