I am stuck on lightmap - emissive textures - they don'[t seem to work for me in some cases. How do you tie it to just having the panel lights being on. I can light up the panel - turning on panel lights, but I want the text decals to appear on with the panel lights switch.
It might be easier to explain this with a couple of images. Here is the diffuse texture (albedo in P3D) for the gauge faces (and other parts) for my Ryan Navion for FSX:
Pay particular attention to the white text on the radio faces and audio panel and to the gauge faces themselves.
Here is the light map texture (emissive in P3D) and you can start to see where the magic takes place:
All the gauge faces are covered by the graduated reddish discs and in the sim with the panel lights on it looks like the gauges are lit from the outside edge with the light fading in intensity towards the middle of the gauge face. Anything black on the sheet gets no light addition in the sim.
Now look at the radio and audio panel areas. The panel faces themselves are black (with the exception of some red light 'spill' from the flood lights on certain parts) and therefore have no emmissive light. The text is all pure white (255, 255, 255) and therefore will have full emmissive light and will appear as backlit text in the VC at night and with the panel lights on.
Here is another example from the same aircraft. This texture is for all of the switches in the VC:
The buttons are black with white lettering during the day.
Here is the light map (emmissive) texture:
At night the switches have a backlit effect with green illumination and the text area is 'tweaked' so that it appears to glow and spills green light from the backlighting onto the faces of the switches. The effect is subtle but looks really good in the sim in a dark cabin and with only the panel lights on.
The bank of numbers at the bottom of the sheet are on the heads of the push-button breakers which are not lit at night which is why that entire area of the light map is pure black (0, 0, 0) and it gets no additional lighting at night.
The alpha channels for the gauges are pure white (255, 255, 255) over the main parts while the switches have a white value of 245, 245, 245 to give them a small amount of shine during the day.
Hopefully the images above will give you a better understanding of how light map/emmissive textures work and how to make them.