There are a number of way models can be represented. The two more important are in polygonal form and in parametric surface form. Our main interest is in the latter and in particular in the trimmed NURBS surfaces. The real time rendering of NURBS surfaces is a very important task in an interactive modeling system. A real time rendering algorithm use a piecewise planar approximation or triangulation of the surfaces in order to produce reasonable images in a reasonable time, overall if we are not supported by a graphics hardware.

During our research we have realize a graphics library, named

To render a trimmed NURBS surface, the library provides two strategies: a

Once a suitable, planar approximation for the surfaces is obtained, and a PSP structure is defined, the library provides the following five different rendering methods:

- wire frame representation of the Segments in the PSP Structure;

wire frame trimmed NURBS wire frame mesh

- as the previous but following the strategy of drawing each segment
with a different grey level in accordance with the distance from the
observer (depth cueing);

depth cueing trimmed NURBS depth cueing mesh

- as the first but following the hidden-line strategy. The implemented
proposal consists of a quasi exact hidden-line algorithm highly suitable
for a real time visualization;

hidden-line trimmed NURBS hidden-line mesh

- shading algorithm (Z-buffer) and more precisely Flat, Gouraud and
Phong strategies.

Flat shading trimmed NURBS Flat shading mesh

Gouraud shading trimmed NURBS Gouraud shading mesh

Phong shading trimmed NURBS Phong shading mesh

**G.Casciola, G.DeMarco, trim library**: Programming Guide - Version 1.0, (1999).**G.Casciola, S.Morigi, Graphics in parallel computation for rendering 3D modelled scenes***Parallel Computing*, 21 (1995).**G. De Marco,**, degree thesis in Computer Science, Univ. of Bologna, (2000) trim.ps.gz (183Kb)*xcbool*un sistema per la composizione di solidi sculturati**T.Moeller, E.Haines, Real-Time Rendering**, A.K.Peters, (1999).

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treefiles.tar.gz containing some .tree files