VUTRAX SOFTWARE ADVICE NOTICE 207 - 17 Jan 2001
Image Wrapping on
Highly Zoomed Drawings
Extreme ZOOM on large drawings may cause wrap-around effects on some system/graphics controller combinations. The problem is theoretically only possible in Windows 95/98/ME only, but poor implementations in NT and 2000 may also cause problems.
In particular, clipping (stopping drawing at the window, toolbar etc. edges) is normally left by the operating system/graphics controller by setting up 'clip regions'. Other aspects of clipping (e.g. overlapping windows) have to be handled by the operating system anyway, so it is more efficient to let it perform all of these functions.
Unfortunately, some aspects of Windows 95/98/ME, and their drivers, and even some graphics controllers, only support clipping for a limited number of coordinate bits. When the full image would be very big, coordinates may overflow these bounds, and clipping goes wrong in various ways.
Before you attempt workarounds, establish an easy to reproduce sequence from the initially opened state - in some cases the problem only occurs at some positions on the screen or particular ZOOM factors.
Note: A similar, but not necessarily directly linked, problem may effect Groundplane generation. This is discussed in SAN 201.