Hello, I am new to this forum and hoping someone here might give a pointer on a problem I am having:

My app is a 2D Vector drawing program based on OpenGL. The main thing it does is draw filled polygons.

A polygon starts as a set of one or more contours, which are triangulated in gluTesselate. The triangles are then drawn in the normal way using glBegin GL_TRIANGLES, etc.

The problem is that a small number of users are reporting that the app is rendering corrupted. From their screenshots it appears as if most vertices are drawn correctly, but occasionally a vertex is drawn way off its true position.

The app works fine on 99% of machines, both Mac and PC, and I myself cannot reproduce the issue.

Screenshots of the problem uploaded to:

http://www.aspexsoftware2.com/image1.jpg
http://www.aspexsoftware2.com/image2.jpg

Any clues on what could be causing this much appreciated.

Tom