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STL (from English stereolithography) is a file format that is widely used to store three-dimensional object models for use in additive technologies. Information about the object is stored as a list of triangular faces that describe its surface, and their normals. An STL file can be either text (ASCII) or binary. It got its name from the abbreviation of the term “Stereolithography”, since it was originally used in this technology of three-dimensional printing.
Since an ASCII STL file can be very large, there is a binary version of this format. The file begins with an 80-character header (which is usually ignored, but should not begin with ‘solid’, since an ASCII STL file begins with this sequence). After the header is a 4-byte unsigned integer indicating the number of triangular faces in the file. After that, there is data describing each triangle, in turn.
Each triangle is described by twelve 32 bit floating point numbers: 3 numbers for the normal and 3 numbers for each of the three vertices for the X / Y / Z coordinates. After that there are 2 bytes of unsigned ‘short’, which is called ‘attribute byte count’. In a regular file, it should be zero, since most programs do not understand other values.
Floating point numbers are represented as IEEE floating point numbers and are considered inverse byte order, although this is not indicated in the documentation.