He emailed me a while ago with some questions about layer groups, and I saved them to rewrite into a blog entry. A little bit of background:
X-Plane 850 and 860 introduce the concept of "layer groups", which provide a way to control the draw order of scenery to some extent. Objects naturally fall into a layer group based on their type (e.g. objects go into the "object" layer group by default, and runways always go into the runway layer group). However, some scenery elements let you customize their layer-group placement in two ways:
- By changing which layer group the element goes to entirely, or
- By providing a "bias number", which indicates that within the catagory of scenery elements, this one must be drawn early or late.
- Make sure that polygons and objects are drawn under runways or over taxiways when needed.
- Make sure that runway markings with polygon offset are drawn before 3-d objects.
With that in mind, some Q and A. (I will elaborate on my answers from what I originally sent Jonathan.)
J: I assume that objects and polygons within a single layer can be drawn in any order - ie there's no defined drawing order between different scenery types.
B: You assume correctly! Within a layer group, X-plane is free to reorder to improve fps. So you cannot rely on the draw order of any two scenery elements without assuring that they are in different layer groups, either by using different group names, or different relative offsets.
J: But is there are defined drawing order between objects/polygons and apt.dat-generated scenery?
B: Yes because airport scenery goes into specific layer groups! (In fact, all scenery has a "default" layer it goes into, and they usually vary by type of scenery element.)
J: eg if I have an object or polygon with ATTR_layer_group runways 0 will it be guaranteed to be drawn after the runway?
B: You're close. The runways go into the runways group so
ATTR_layer_group runways -1
will always be before runways and
ATTR_layer_group runways 1
will always be after.
ATTR_layer_group runways 0
is the default layer group for runways, so your object or polygon would be in the layer group with all of the other runways, and X-Plane would be free to change the order amongst your object and runways in any way that would optimize framerate.
J: If not, is there any other way to insert objects and/or polygons between the taxiways and the runways?
B: The numeric offset is provided for this. The "spacing" of the layer group numbers is such that you can have up to 5 groups before and after each "named" group. So anything from runways -5 to runways +5 is fair game. (In other words, you can separately control up to 5 different "layers" of elements with a well-defined order for any layer group name.)