Wednesday, November 01, 2006

DSF Object Order is Not Draw Order

The order objects are listed in a DSF file (either the OBJ definitions or the actual OBJ instances) is not the same as the order X-Plane will draw them! X-Plane will change the order to maximize frame-rate, and the way it organizes them has changed in 850 (and will change again in futur versions).

Until X-Plane 850 there was no way to control the order of objects (although some packages do rely on X-Plane 840's behavior and now look funny in 850). But with 850 there is a clean way to control draw order that will work well into the future.

Starting with X-Plane 850, there is a new command let's you override what part of the rendering process your object is drawn in, which is useful in a number of cases. Here's how it works:

X-Plane draws the scenery in a series of steps or "layer groups". Because of the way OpenGL transparency and polygon-offset affect Z buffering, X-Plane must draw the terrain first, all decals over the terrain second, 3-d buildings third, and anything translucent (clouds, smoke puffs, etc.) last. Layer groups organize all of the scenery into these catagories.

Objects are normally drawn in the "objects" layer group, which is intended for 3-d buildings, static airplanes, jetways, and anything else you might model with an OBJ. However ATTR_layer_group will let you specify that some objects be drawn earlier or later than others.

The command's syntax is:

ATTR_layer_group objects 1

Where the name ("objects", but others are allowed - see the OBJ spec) specifies a group, and the number makes the object be drawn after (positive numbers) or before (negative numbers). Objects with the same group and number are drawn together (in an unspecified order), but X-Plane goes by the numbers.

So for example if you want to ensure that a transparent hanger is drawn after a static plane, use ATTR_layer_group objects 1 on the hanger to draw it after all other objects. To draw some apron lines before the terminal, use ATTR_layer_group objects -1 to draw before buildings.

A few other tips:
  • Try to use as few offsets as possible - e.g. try to put all "draw last" objects into the group objects/1 - rather than using a different number for each.
  • Avoid using layer groups when they are not needed - they inhibit X-Plane from optimizing framerate.
  • If you do not use a layer group, but do use polygon offset, X-Plane sets the layer group to objects with a negative number. This is inhibited when you specify the layer group yourself, so be sure to use a negative offset layer group for polygon offset!
  • Try to organize your textures so that one texture is not used in multiple layer groups. For example, if you have translucent buildings that must be drawn later, group them in their own texture.
  • Related - try to keep polygon-offset geometry (pavement markings) in their own texture and objects. Share one texture for all markings, but don't put the markings and object together.

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