Tuesday, February 13, 2007

Warning: Orthophoto Polygons Are Not a Magic Bullet

With the new version of DSF2Text and X-Plane 8.60 it is possible to overlay orthophotos on top of DSFs. I'll get into this in more detail in a future blog post...the purpose of this post is to be a wet blanket.

Using polygons to overlay orthophotos on the base mesh might seem perfect. It's easy to do and you can clip polygons to any shape. Perfect, right?


There are a few major problems with overlay polygons that make them well suited to some cases but not a general purpose cure-all for orthophoto-based scenery. Here's the fine print:
  1. Orthophoto polygons draw over the base mesh. In particular the base mesh is still drawn underneath the orthophoto. This means that wherever you use them, X-Plane is drawing twice! This is bad for performance...it will cut the speed of mesh drawing in half. Ouch! (This is not an issue when working on very small areas like airports, but it is an issue when working on very large areas.)
  2. Polygons are not built up until you fly near them. Polygons are part of the 3-d "clutter" part of the scenery not the base mesh, so we defer building them until we get near them to save memory. (This is the same as airports - look at an airport in textured map mode when far away from it -- no taxiways!) But this means that if the orthophoto is huge (and thus visible far away) its absense will be quite obvious.
  3. Polygons burn CPU time when they are built up. For very small polygons this isn't that noticable, but the larger the polygon, the more work X-Plane does "draping" it over the base terrain. So on single-CPU systems with large polygon orthophotos, the framerate may be adversely impacted.
Basically overlay polygons are great for texturing small areas like airports or special landmarks within a city, but are not appropriate for making wide-scale orthophoto base meshes. For that you really need to replace the base mesh DSF!


Anonymous said...

So how could we put Orthophoto into X-Plane ?

Benjamin Supnik said...

I will describe this in my blog in a few days...the polygon system is complicated!