DrawingRules

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This tech-note is a work in progress.

! 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.

OpenGL state:

  • 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.

Drawing callbacks:

  • 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

Use the xplm_Phase_Gauges phase to add elements to the 2-d panel. Try to avoid using the xplm_Phase_Panel phase (which draws the background) - replacing the background is probably not a good idea.

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