This text was supplied by Michel Verheughe

"Unofficial" texts are member contributions that have not yet been discussed or approved by the ESP community.

Venetian blinds.

X-Plane uses OpenGL technology to display the scenery. OpenGL is a rendering engine that allows real-time texturing, light setting, shading and smoothing. It works by "depth sorting", i.e. things are displayed from the furthest away, to the closest object. OpenGL is used to display the ground polygons, obstacles and custom objects. After that, the eventual runways and light points are drawn.

Last, the cockpit panel is drawn. This is why, in mountainous country, a hill may hide custom objects but not lights and runways.

Venetian blinds effect (interlaced stripes) occurs when two close surfaces are drawn by OpenGL. It can hardly result from two ground polygons, so it is usually caused by a custom object polygon near a ground polygon.
The reason it happens is that the "depth sorting" has to happen real fast (real-time rendering) and to do so, distances are rounded up. Say that this rounding is 0.1 % of the distance. I means then that two polygons being at 1000 m and 1001 m from the POV (Point Of View) will be
considered juxtaposed and will display with an unpleasant Venetian blind effect, as both surfaces claim the same area.

Venetian blinds are often encountered when a custom object has a polygon very near the underlying ground polygon. Avoid this by keeping polygons a couple of meters from each other. Note that there is no Venetian blinds on the runways because they are drawn after the depth sorting.