A triangle mesh format built for printing, not for design, and why that distinction matters when you slice it.
Convert an image to STLA what is an STL file question usually gets a one-line answer: it is a file that describes the surface of a 3D object as a cloud of triangles. Nothing else. STL stands for stereolithography, the name of the 1980s printing process it was invented for. Each triangle records three corners in space plus a vector pointing outward from the surface, and the whole model is simply those triangles butted edge to edge.
No colours, no textures, no material settings, no assembly tree. If you export a red part and a blue part from the same design software, the two STL files are identical apart from their geometry. Some CAD tools write a header comment into the file, but slicers ignore it. What arrives at the printer is a skin, and the printer works out what to fill it with.
The format also carries no unit. Coordinates are bare numbers, and a value of 25 could mean 25 millimetres or 25 inches. Your slicer decides the scale, and most default to millimetres. That is why a model sometimes lands on the bed the size of a grain of rice, or so large it falls off the build plate entirely.
The path from a solid model to a printed part runs through four stages.
You build or download a shape in CAD or a sculpting tool. It is a mathematically exact solid at this point, with smooth curves and precise surfaces. The STL does not exist yet.
On export, the software approximates every curved face with flat triangles. Fewer triangles means a coarser curve. More triangles means a bigger file and a slower slice. This step is lossy and it is not reversible.
The slicer intersects the mesh with a stack of horizontal planes. Each intersection becomes a closed polygon, and those polygons become the toolpaths for each layer. Holes or flipped normals in the mesh show up here as gaps or solid-filled cavities.
The slicer writes G-code, and the printer executes it layer by layer. G-code is machine specific; the STL is not. The same STL will slice differently for a 0.4 mm nozzle than for a 0.6 mm one.
Everything in the left two columns is real; everything on the right is gone by the time you have an STL.
| Property | Stored in an STL | What you lose |
|---|---|---|
| Surface geometry | Yes, as triangles | Smooth analytic curves and exact radii |
| Triangle orientation | Yes, one normal per facet | Any record of the original solid volume |
| Scale | Bare numbers, no unit | Whether the part is 25 mm or 25 inches |
| Colour and texture | No | All appearance data from the source model |
| Design history | No | Feature tree, parameters, constraints, dimensions |
Three groups lean on this format for three different reasons.
It is the common handoff format. A model downloads as an STL, drops into a slicer, and prints. There is no alternative that every slicer reads as reliably, which is why the format has outlived several attempts to replace it.
They rarely author in STL. They keep a native CAD file for editing and export an STL only as a frozen snapshot for manufacturing or for a colleague to inspect. Once exported, the model can no longer be edited parametrically.
A height map, a logo or a silhouette has no solid model behind it. A converter builds a mesh directly from pixel brightness, so the STL is the first real 3D representation of that image, not an export of an existing design.
If you have read this far you already know the trade-off: you get triangles, and nothing but triangles. Upload a JPG or PNG and the converter builds that triangle mesh from the image, with the bright and dark areas setting the height.
Not in the sense that matters. A CAD file holds a parametric solid with editable features, dimensions and history. An STL is a tessellated export of that solid, a frozen set of triangles with no way back to the original. You can open an STL in CAD software, but you cannot edit it as a design.
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