! The Four Basic Drawing Environments
X-Plane has four fundamental drawing environments:
- The 3-d world
- The 2-d cockpit
- The 3-d cockpit
- The 2-d user interface
A warning about these environments: there isn't a clear correlation between the current view and what drawing phases are called. There is also not a clear correlation between the current view and whether the 3-d cockpit will be drawn. Rather these things are a function of the specific view and specific plane. Your best bet is to draw in response to certain drawing phases.
! General Rules
Some basic rules to follow for safe drawing in X-Plane. Follow these rules to increase the likelihood of your plugin working in future sim versions.
- Make no assumptions about OpenGL state when you enter your plugin.
- Unless you change OpenGL state with an XPLM routine, leave OpenGL state the way you found it.
- OpenGL state may change when calling XPLM routines, including reading data-refs!
- Try to do drawing in large blocks to avoid having to deal with the above state problems frequently.
- Lighting and fog are only trustworthy in 3-d world viewing.
- Do not assume a correlation between drawing callback and sim frame-rate.
- Use the elapsed time to determine animation.
- If you have expensive calculations, try to move them to a processing callback, or do them "on demand" (e.g. once the first time they are required per sim frame). Otherwise multiple calls to your drawing callback may thrash your code.
- Try to structure drawing to not depend on the sim's current view; some views may be composites of several view-like states.
- Read datarefs that affect drawing from within the drawing callback. Do not cache dataref values. Some view-related datarefs are only valid from within a view callback.
- Do not use the "first" and "last" callbacks (either for 3-d or the panel). See specifics on drawing phases later in this technote.
! Drawing to the 3-d world
Use XPLMWorldToLocal to find local coordinates.
__TODO__: identify the right 3-d drawing callbacks. The xplm_Phase_Objects callback is probably best for scenery clutter, and the planes callback for aircraft enhancements.
Do not assume that drawing to the 3-d world happens only once. For example, the Elumens dome hardware will cause X-Plane to render the scene multiple times from different angles to form the 180-degree field of view for the dome.
OpenGL lighting and fog are appropriately set in the 3-d world; pass true for lit and fog to XPLMSetGraphicsState to blend with scenery.
! Drawing in the 2-d cockpit
Note that the xplm_Phase_Gauges phase is called one or more times to prep the 3-d panel texture, or one or more times to draw the 2-d panel.
Use the view rect datarefs from the gauges callback to determine the right location to draw. Do not cache these - they can change per callback!
The xplm_Phase_Gauges drawing phase is a relatively safe one to use - it is only called when 2-d instruments are needed for some purpose, and the view rects will indicate which ones. (Gauges can be needed for the 2-d panel, interior 3-d panel, or even 3-d panel visible from outside through the cockpit windows. You can use the view_is_external dataref to simplify drawing if you need to reduce the framerate cost of drawing your panel.)
! Drawing in the 3-d cockpit
Right now there is no good way to draw in the 3-d cockpit. We recommend either:
- Draw to the 2-d panel's opaque area (from xplm_Phase_Gauges) and use the panel texture. (You cannot draw into transparent areas.)
- Use an animated 3-d cockpit object and drive animation datarefs from your plugin.
! Drawing to the 2-d user interface
Use an XPLMWindow where possible. Use the before/after xplm_Phase_Window as needed.
! Specifics on drawing phases:
xplm_Phase_FirstScene Do not use xplm_Phase_Terrain Use to disable 3-d drawing mostly for framerate xplm_Phase_Airports Cosider using "objects" phase instead xplm_Phase_Vectors DEPRECATED - this phase is not called in X-Plane 8. xplm_Phase_Objects Use this phase for drawing 3-d clutter xplm_Phase_Airplanes Use this phase to hide all airplanes or draw onto the airplanes xplm_Phase_LastScene Do not use xplm_Phase_FirstCockpit Do not use xplm_Phase_Panel Use with caution - consider gauges phase instead xplm_Phase_Gauges Use to modify 2-d or 3-d panel xplm_Phase_Window Use for UI when XPLMWindow isn't adequate xplm_Phase_LastCockpit Do not use