Triangulation: Untriangulate and Triangulatev3

Screens draw triangles, so every flat face of a part is cut into triangles, often more than it needs. A polygon, also called an n-gon, is a flat face with any number of corners: several triangles in one plane, joined into one face.

The Triangulation panel has two actions, and a quick way to make a part lighter, Quick decimate, further down. Untriangulate joins flat triangles into polygons and takes away the vertices in the middle of each flat face. Triangulate is the way back: the flat faces stop being polygons and show their triangles again.

When to use it

• Untriangulate: on a part whose flat faces carry vertices they do not need, for example around holes you closed with Remove holes. The part loses those vertices and the triangles that hung on them.
• Triangulate: to go back to plain triangles, or to even out long thin triangles on flat faces.
• Untriangulate with Pair into quads on: on a mesh that was made of quads and arrived as triangles, before you take it into a modelling program. Neighbouring triangles become four-sided polygons again, on curved surfaces too.

Steps

1. Select the parts to work on. With nothing selected, the panel works on every part.
2. Click Remesh on the bottom toolbar. Its icon is a grid. The panel opens at the bottom left of the viewport. You can also find it with the search (CtrlK, “Remesh”).
3. Choose Untriangulate or Triangulate at the top of the panel. The settings of the one you chose show under it.
4. Press the button under the settings, or Enter. It carries the name of the action you chose.
5. A message says what was done, for example how many triangles a part had before and after.

From the keyboard

U

Untriangulate the selection

ShiftU

Triangulate the selection

The two keys work on the selection only, so a key never changes the whole scene by accident. With nothing selected, they only say “Select a part first”.

The settings of Untriangulate

Pair into quads

Instead of joining flat faces, pairs neighbouring triangles into quads, the way Blender’s Triangles to Quads does. The best pairs go first and each triangle goes into one quad at most. Two triangles are never paired across the boundary of a CAD face or across a UV seam. With it on, the panel shows only its two angles below. It is off at first.

Fold up to

With Pair into quads: two triangles are paired only if the angle between them is no more than this. It starts at 40°.

Corners within

With Pair into quads: every corner of the quad two triangles make must be within this of a right angle. It starts at 40°.

Flat faces within

How far neighbouring triangles may turn from each other and still count as one flat face, in degrees. It starts at 1.

Even triangles

Turns the edges inside each rebuilt face until its triangles are as even as its vertices allow, instead of long thin fans. No vertex is added or taken away.

Flatten shading on flat faces

A vertex whose triangles all lie in one plane is shaded flat. The rim of a hole you closed needs this. It still carries the tilted shading it had against the wall of the hole, and a face like that is not joined.

Shading tolerance

A flat face is left alone if the shading at its vertices differs from the face by more than this, because joining it would change smooth shading. It starts at 5°.

Surface may move

A vertex that is a little off the plane of its face, as on a gently curved panel, may still go if the surface moves no more than this, and never more than 1% of the part’s size. It starts at 0: only vertices in the plane go.

Points on straight lines

Also takes away the points that only sit on a straight line of a face’s outline, such as a long edge cut into pieces. They go from every face that touches them at once, so nothing cracks and the shape does not change.

Dissolve nearly flat within

For faces that are only nearly flat, such as a panel with a little noise or a gentle bend. A point that is left goes if the triangles round it turn from each other by less than this angle and the surface moves by less than Surface may move. 0 turns it off.

Untriangulate repeats until nothing more can be joined. Joining faces makes bigger faces, and a vertex that was on the edge between two of them is then in the middle of one.

The settings of Triangulate

Even out the triangles

Turns the edges inside each flat face until the triangles are as even as its vertices allow, so there are fewer long thin ones. The number of triangles and the vertices do not change.

Show their triangle lines

Draws the triangle lines of these parts in the wireframe, whatever the Lines setting is.

Flat faces within

How far neighbouring triangles may turn from each other and still count as one flat face when they are made more even.

What does not change

The shape of the part. Untriangulate takes away only vertices that lie in the middle of a flat face, so the face keeps its outline. Only when you raise Surface may move above 0 can the surface shift, and by no more than the distance you typed. Triangulate adds no vertex and takes none away.

Pair into quads moves no point and takes no triangle away: it only says which two triangles make one quad. Export to OBJ or FBX with Keep quads on to write the quads into the file. A quad on a curved surface is a little bent, so another program may cut it along its other diagonal when it draws it.

How it shows in the view

In the wireframe view (Alt2), a part that has been through Untriangulate draws its polygons, and a part that has been through Triangulate draws its triangles. That holds whatever Lines is set to under Settings, Viewport.

Quick decimate

Quick decimate makes the selected parts lighter by choosing a level, with a live preview, instead of typing numbers. It is a separate tool from Untriangulate and Triangulate: it removes triangles, so it is lossy, like Decimate.

1. Select the parts to make lighter.
2. Press B, or find Quick decimate with the search. A column of levels opens under the pointer. Each level is how many of the triangles stay, written as a number without a % sign.
3. Move up and down the column. The selected parts take the level the pointer is on straight away, so you judge the result on the real model. Moving back up gives the detail back.
4. Let go of the mouse button on a level, press Enter, or let go of B if you held it, to keep that level.

Keeping a level is one undo step. Esc or a press beside the column puts the parts back as they were. It needs a selection: with nothing selected it says “Select a part first”. It does not run while Edit mesh is on.

A small or thin part can lose its shape if too much is taken away. If a level would leave a part with less than about 60 % of its surface, or would shrink it by more than about 10 % of its diagonal, that result is refused and the part takes the next gentler level instead. So a small bolt does not shrink to a scrap while the big parts beside it go down to the level you chose.

Undo

A run is one undo step. Press CtrlZ and the mesh is as it was.