What Are Facets in 3D Printing?

Facets are the flat triangles every STL file is built from. How many there are decides how smooth your model looks, and how big the file gets. Too few and your sphere looks like a disco ball; too many and the file is the size of a small novel.

The same circle drawn with 8, 20 and 90 flat sides. The 8-sided one is clearly an octagon, the 90-sided one looks round. — A mesh can only use flat sides. With enough of them, your eye cannot tell a polygon from a circle.

A facet is one flat triangle: three corners and a direction it faces. An STL surface is thousands, or millions, of them side by side. Why triangles? Three points always lie on one flat plane, so a triangle can never twist. Any other shape can. That makes triangles the simplest building block for any surface. See what an STL file is for the full picture.

Embossify turns brightness into height. It lays an even grid of squares over your picture, gives each square a height from the brightness of its pixels, and splits every square into two triangles. A flat back and sides close the shape into a solid.

The same picture drawn on a coarse grid, a medium grid and a fine grid. The finer the grid, the closer it follows the picture. — The three grids are the idea behind Standard, High detail and Ultra detail. Real grids are far finer than this drawing.

The numbers

Because every square becomes two triangles, and a binary STL spends 50 bytes on each triangle, you can estimate a file size yourself: about 100 bytes per square.

Type	Squares across	Example: a 3:2 photo
Standard (free)	200	about 27,000 squares, 53,000 triangles, roughly 3 MB
High detail	1,500	about 1.5 million squares, 3 million triangles, roughly 150 MB
Ultra detail	2,000	about 2.7 million squares, 5.3 million triangles, roughly 270 MB

Embossify uses an even grid, so file size depends on the type you pick, not on how busy your picture is. The Gallery shows the same close-up at every type, so you can judge the difference with your own eyes.

How many is enough?

A finer grid only helps when it is finer than the thing that will use it. A 120 mm wide model at High detail has squares about 0.08 mm across. Most filament printers lay down lines 0.4 mm wide, so they cannot show that, though a CNC with a fine bit or a resin printer might. A blurry or small picture gains nothing from Ultra: you only get a bigger file.

Print or carve small and simple? Standard is often plenty.
Large, detailed, or going to a CNC? Choose High or Ultra.
Slicer slow or running out of memory? Use a lower type.

Tips

Start with a sharp, high-contrast picture. Facets can only record detail your picture has.
Use Smoothing for speckles. Grain and JPG noise turn into tiny bumps. The Smoothing slider softens them.
Look at the triangles. The Embossify Lens has a wireframe style that shows the mesh, and the converter preview has a Mesh button.

Quick answers

Are facets the same as polygons?

Facets are polygons, but in an STL they are always triangles.

Why does my smooth ball look faceted?

Because it is made of flat triangles. A finer mesh hides it, but you may also see the printer's own layer lines, which are a separate effect.

Keep exploring. Compare the types in the Standard, High, Ultra and Pixel Grid comparison, or read what an STL file is.