SIZE ON SCREEN
Instead of using distance from camera to define the displayed LODs, Microsoft Flight Simulator uses screen space as aircraft can vary significantly in scale and shape, and this in turn would affect the point you are measuring from. To give an example, let's look at this comparison between a Boeing 747 and a Cessna 172:
By using screen space for the LOD calculations, we can work out the number of tris needed for a LOD based on the plane size. Using the two aircraft shown above again as an example: if you viewed the 747 on screen and saw it as the same size as the Cessna, the 747 would have to be significantly farther away from the camera than the smaller aircraft, and as such be at a lower LOD level containing approximately the same number of tris as the Cessna. This helps keep the load on the render engine consistent and maintain performance.
It should be noted that in order to calculate the LOD used, the engine will use the bounding sphere of the aircraft model. You can visualise this using the Display Bounding Spheres option from the Debug LODs window. As a rough visual guide this is the LOD range in relation to the size of the sphere as seen in the build on a small plane:
NOTE: For a full and detailed explanation of the LOD limits in general, please see the following section - LOD Minsize Rules
- LOD x0 – drawn when the bounding sphere occupies 110% or greater screen space in relation to the vertical screen size:
- LOD x1 – This is drawn between 110% and 75%:
- LOD x2 – This is drawn between 75% and 50%:
- LOD x3 – This is drawn between 50% and 30%:
- LOD x4 – This is drawn between from 30% to infinity (this will only occupy a few pixels on screen and will only require one material ID and a handful of tris):
Note that a much larger aircraft - like a 747 - would have a different set of values to calculate the LOD ranges, but by the time we reach LODs3, 4, and 5, the actual tri count will be very similar for all aircraft.