Introduction
StereoLithography(StL) is something that is widely used in CAD/CAM, RapidPrototyping etc. The concept is that any surface or solid is exported to StL format by data exchange packages after they are modeled in commercial CAD packages. This data consists of triangulated facets that approximate the surface of the solid. No topological or geometric information is exported. The surface is subdivided into small triangles. The approximation uses chordal deflection for curved surfaces to smoothen the surface. More the smoothening required, more number of smaller triangles are generated by subdivision resulting in larger size of data file. Now, along with triangles, their facet normals are also generated. The data is written both in ASCII and binary formats. The data in ASCII is written like this:
facet normal 0.000000e+000 -1.000000e+000 0.000000e+000
outer loop
vertex -1.499999e+002 -1.000000e+002 0.000000e+000
vertex -1.500000e+002 -1.000000e+002 0.000000e+000
vertex -1.500000e+002 -1.000000e+002 -5.000000e+001
endloop
endfacet
The facet normal tells the three components of the facet normal followed by three vertices of one triangle, enclosed by the statements.
facet normal 0.000000e+000 -1.000000e+000 0.000000e+000
and
endfacet
Thus all the triangles are written one after the other. This data can be then used as input for generating Rapid Prototype models as well as for NC toolpath generation. The Normal data helps to compute tool offsets etc.
Here, for displaying the data in the OpenGL viewer, I have written the code for reading the data and displaying each triangle using glBegin(GL_TRIANGLES) and the normals for lighting. The viewer and geometry coding is same as my earlier project CadSurf. You can read as many StL (ASCII only) files into the viewer zoom, pan, rotate the views, select the objects, change attributes like color and material. The viewer provides object oriented context menus. I.e., when you click the right mouse button in the view with no active selection, you get a context menu for setting the viewer attributes, whereas with StL object selected, you get menu for the object attributes. Some sample StL data is also provided in the Data folder of the project.
Please let me know your comments. Thank you.
