2.2 STEREOUTHOGRAPHY DETAILS

4.2.2

StereoUthography

Details:

The"

.STL"

File Format

Introduced by 3D Systems Inc. in 1987, the ".STL" file format has become the de

facto standard, even though other "direct slice" methods have been tried. The" .STL"

method tessellates the CAD model with triangles just like the hexagons and pen-

tagons on the surface of a soccer ball.

The ".STL" file is (a) a header, (b) the number of triangles, and (c) a list of the

triangle description by vertices and the normal vector to the triangle. Table 4.2 shows

the layout. The size of the ".STL" file is (50 x number of triangles)

+

84. Thus a

1O,OOO-triangle

object needs 500,084 bytes.

TABLE4.2

The

N

.5Tl

N

File Format

Entity

Described

by

The header

80 bytes

The number of triangles

Unsigned long integer

(4

bytes)

For each tessellation triangle (50 bytes

of

information)

See below

Floating point integer (4 bytes)

Normal vector

I

Normalvector J

Normal vector K

First vertex

X

Ploanng polnt integer te bytes)

First vertex Y

Flontingpo;ot

integer {4 bytes)

Fint vertex

Z

Second vertex X

Second

vertex

Y

Second vertex Z

Third vertex X

FLoating point integer (4 bytes)

Third

vertex

Y

Third vertex Z

Attribute

Unsigned inleger (2 bytes)