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Why and how are LOD/Triangle Counts used?

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I've been reading into the P3D SDK and came across a section called Triangle Counts. What I can gather from what exactly they are is the resolution of the object, maybe if you had a circle and reduced the LOD/Triangle Count to its most simple form, it would be a triangle. I don't know really, it was just a guess. But are there different LODs for different objects? It says "Generally speaking, LOD 35 and up will have animations while LOD 15 and below will not." I don't know how level of detail relates to animation, is it referring to switches, control surfaces, etc? And I heard somewhere about different layers of LOD as well and am a bit confused by that. I just need a good explanation on what LODs are in respect to 3D modelling a complex aircraft and how I utilize them within the software.

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You would have an easier time with this if you downloaded the freeware Model Converter X and opened up a stock P3d aircraft MDL file.

https://www.scenerydesign.org/modelconverterx/

And without getting into great detail here, an MDL file can have multiple LODs. The highest LOD in the MDL file has the most polygons and that model is what is displayed when the user is very close to the object. As the user's viewpoint moves further away from the object, the model's lower numbered LODs (with far less polygons) become visible. Some MDL files might only have one LOD (100). Other MDL files may have four or five, including the lowest LOD (usually 10) that  is empty. An empty LOD causes the model to completely disappear at distance. With MCX, one can look at the complexity of each of the model LODs. The highest order LOD looks realistic and the lower ones become more and more simplified, in a fashion like the clones in the Michael Keaton movie Multiplicity.

Unrelated to the complexity of the model and its LODs, the textures with which the model is "painted" may also be mipmapped, which means that lower resolution textures from a specific texture sheet file are used at distance. All this was done to increase performance.

So when an object is farther away from the user's viewpoint, usually the geometry of the object is simpler and the texture being used has lower resolution.

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Thanks for letting me know about this program, it is really helpful for reference on the planes I'm practicing to model on.

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