X-Plane 930 beta 4 will be out real soon, and it has a somewhat significant new instrument feature: all instruments (not just generic instruments) can have additive lighting.

But wait – what is additive lighting?  Additive lighting is the basic equation X-Plane uses for “lit” stuff.  Basically it says that the _LIT texture is added to the daytime texture to simulate the effect of an object that reflects sun light and emits its own light.  Here’s a more detailed description.
It used to be that the _LIT textures for instrument overlays were used instead of the daytime ones at night.  Generic instruments introduced a new lighting mode, “back-lit”, where the LIT texture is added to the daytime texture.  This lets you make an instrument that has a light behind it to illuminate the markings at night.
In particular, when an instrument is back-lit, the amount of LIT texture added in is a function of the instrument light levels (the pilot can turn this up and down) while the amount of day time texture is a function of the sun and spot light shining on the area.
Originally I did not want to extend the legacy instruments to support back-lit lighting.  What finally made me change my mind was the amount of detail in some of the standard mechanical instruments.  As an exercise, I converted the six pack and nav instruments of Max’s default Cessna to generic instruments.  
The conversion required a lot of new (and very weird) datarefs, some esoteric extensions to the generic needle instrument, and it lacked some of the finesse of Austin’s built-in instruments. All of these problems stem from one limitation: the animation action of generic instruments cannot be nested.
(Of course there is a practical consideration too – for an author with legacy instruments, rebuilding with generics takes time. Typically it took about 4-6 generics to model each built-in instrument in the six pack. The conversion only took me about eight hours, but I have access to the source code of the built-in instruments, a luxury authors would not be able to leverage.)
So in the long term, I am at least investigating the notion of nested animations and movements for generic instruments; I think that this would be the final flexibility needed to model just about anything with generic instruments.
But for the short term, you can back-light your built-in instruments; just set the lighting mode to “additive” and create the LIT textures.

About Ben Supnik

Ben is a software engineer who works on X-Plane; he spends most of his days drinking coffee and swearing at the computer -- sometimes at the same time.

4 comments on “Back-Lighting For All Instruments

  1. Yes – they do work in the 3-d pit.

    The only feature that isn’t available in 3-d (in concert with other new features) is the 2-d spot lights. (Use the 3-d spot lights instead.)

  2. Acording to some of your posts, we can’t expect to see this for sceneries right soon, really? If yes, do you have any idea of a framerate loss according to what happens in the cockpits? I talk about adding this to a very small area around the plane, maybe 1 mille around

  3. Alejandro: additive lighting (which is what back-lit instruments are) has been available for scenery since X-Plane 6 – it’s the _LIT texture. Or are you referring to the 3-d spot lights? Those aren’t available for scenery.

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