STL (file format)        http://en.wikipedia.org/wiki/STL_file

STL (STereoLithography) is a file format native to the stereolithography CAD software created by 3D Systems. STL is also known as Standard Tessellation Language^[1] This file format is supported by many other software packages; it is widely used for rapid prototyping and computer-aided manufacturing. STL files describe only the surface geometry of a three dimensional object without any representation of color, texture or other common CAD model attributes. The STL format specifies bothASCII and binary representations. Binary files are more common, since they are more compact.^[2]

An STL file describes a raw unstructured triangulated surface by the unit normal and vertices (ordered by theright-hand rule) of the triangles using a three-dimensional Cartesian coordinate system.

An ASCII STL file begins with the line:

solid name

where name is an optional string (though if name is omitted there must still be a space after solid). The file continues with any number of triangles, each represented as follows:

facet normal ni nj nkouter loopvertex v1x v1y v1zvertex v2x v2y v2zvertex v3x v3y v3zendloopendfacet

where each n or v is a floating point number in sign-mantissa 'e'-sign-exponent format, e.g., "-2.648000e-002". The file concludes with:

endsolid name

The structure of the format suggests that other possibilities exist (e.g., facets with more than one 'loop', or loops with more than three vertices) but in practice, all facets are simple triangles.

White space (spaces, tabs, newlines) may be used anywhere in the file except within numbers or words. The spaces between 'facet' and 'normal' and between 'outer' and 'loop' are required.^[2]

Because ASCII STL files can become very large, a binary version of STL exists. A binary STL file has an 80 character header (which is generally ignored – but which should never begin with 'solid' because that will lead most software to assume that this is an ASCII STL file). Following the header is a 4 byte unsigned integer indicating the number of triangular facets in the file. Following that is data describing each triangle in turn. The file simply ends after the last triangle.

Each triangle is described by twelve 32-bit-floating point numbers: three for the normal and then three for the X/Y/Z coordinate of each vertex – just as with the ASCII version of STL. After the twelve floats there is a two byte unsigned 'short' integer that is the 'attribute byte count' – in the standard format, this should be zero because most software does not understand anything else.^[2]

Floating point numbers are represented as IEEE floating point numbers and the endianness is assumed to belittle endian although this is not stated in documentation.

UINT8[80] – HeaderUINT32 – Number of trianglesforeach triangleREAL32[3] – Normal vectorREAL32[3] – Vertex 1REAL32[3] – Vertex 2REAL32[3] – Vertex 3UINT16 – Attribute byte countend

There are at least two variations on the binary STL format for adding color information:

• The VisCAM and SolidView software packages use the two 'attribute byte count' bytes at the end of every triangle to store a 15 bitRGB color:
  • bit 0 to 4 are the intensity level for blue (0 to 31)
  • bits 5 to 9 are the intensity level for green (0 to 31)
  • bits 10 to 14 are the intensity level for red (0 to 31)
    • bit 15 is 1 if the color is valid
    • bit 15 is 0 if the color is not valid (as with normal STL files)

• The Materialise Magics software does things a little differently. It uses the 80 byte header at the top of the file to represent the overall color of the entire part. If color is used, then somewhere in the header should be theASCII string "COLOR=" followed by four bytes representing red, green, blue andalpha channel (transparency) in the range 0–255. This is the color of the entire object unless overridden at each facet. Magics also recognizes a material description; a more detailed surface characteristic. Just after "COLOR=RGBA" specification should be another ASCII string ",MATERIAL=" followed by three colors (3 × 4 bytes): first is a color ofdiffuse reflection, second is a color ofspecular highlight, and third is an ambient light. Material settings are preferred over color. The per-facet color is represented in the two 'attribute byte count' bytes as follows:
  • bit 0 to 4 are the intensity level for red (0 to 31)
  • bits 5 to 9 are the intensity level for green (0 to 31)
  • bits 10 to 14 are the intensity level for blue (0 to 31)
    • bit 15 is 0 if this facet has its own unique color
    • bit 15 is 1 if the per-object color is to be used

The red/green/blue ordering within those two bytes is reversed in these two approaches – so while these formats could easily have been compatible the reversal of the order of the colors means that they are not – and worse still, a generic STL file reader cannot automatically distinguish between them. There is also no way to have facets be selectively transparent because there is no per-facet alpha value – although in the context of current rapid prototyping machinery, this is not important.