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Sudden fps change at a certain altitude because of autogen trees


I brought up the following problem in the Forest tree placement tool subforum, but that place is kinda dead. So I try to get some helpful answers here.
After making the bgl's for autogen vegetation for my photoscenery with the above mentioned tool I observed that while on the ground the fps is close to 0 (it's like a slideshow), but as soon as I slew up to about 800-1000 ft, fps instantly comes back to a nice 25+ value.
I suspected that the 300k+ trees will be a bit too much (I even cut down their quantity to this amount because of OOM), but this phenomenon is very strange.
Why is that that the sim can't handle these trees at a low altitude, where it actually has to draw less objects, while a little higher, when there are much more trees in the view, it has absolutely no problem with it? Because the trees are still there, so the fps increase is not the consequence of the disappearance of the autogen objects. Also, I use Orbx Global vector and LC sceneries, which are next to my photoscenery with thousands of trees, and this problem is does not happen there, only when I hover above my custom scenery with the trees.
I even tried all the autogen and scenery density sliders to set to minimum, also the autogen tree density settings in the cfg file (AUTOGEN_TREE_LOD, AUTOGEN_TREE_MAX_DRAW_DISTANCE, AUTOGEN_TREE_MIN_DISTANCE_TO_LOD), but no use at all. When I delete the tree bgl's from the scenery folder, there is no problem, so there is something about my trees.

Is there a cutoff altitude where the sim handles the autogen objects differently, causing this change in performance?
Anyone also experienced this?

Are you sure you are using autogen? If the trees are placed with BGL files, instead of AGN files they are not autogen object. Then they are placed as library objects. Compared to autogen such library objects have much worse performance and when placed in huge quantities can kill your performance.

Not sure why it would be elevation dependent though, that's a bit weird...
Hm, now that you mention it, you are right. It seems that this tree placer tool places individual objects not autogen. My mistake.

But the weirdness still remains, why there is no fps drop above a certain altitude when the thousand of trees are still seen, while below that level my sim almost freezes as if it suddenly realized that there are some trees around :)
I attach two screenshots to illustrate the amount of trees and the fps I got at two different altitudes. Screenshot (33).pngScreenshot (34).png
Not sure if it helps, but why not.
Could it be your tree models have levels of detail? So that much more triangles have to be drawn when close to the ground?

The trees look more complex (and better) than the default trees.
No, this tool takes the trees from the default object library. So it's basically just a tree placer.
But I also suspect that maybe there should be different detail levels, so when I get closer to ground the sim can't handle the sudden polygon increase it has to draw. But I thought that the default objects are much simple, either they are visible or not. There is no lower or higher detailed version of the same object in the scenery at the same time.

Oh wait, now while writing this I remember that I also have FTX Trees HD installed. I'm not sure how it enhances the trees but it probably changes the default tree textures to higher resolution. Can it be the origin of the problem?
I'll try to deactivate this HD addon and see whan happens then. Maybe the HD textures are loaded only when the trees are closer...
Unfortunately reverting back to the default tree textures did not solve the problem.
The more I experiment with this the more I'm convinced that this cutoff level is at exactly 1000 ft. At 1050 fps is 25+, at 950 around 1. While nothing changes, the same trees are in the view, just 100 ft lower (closer)...

Well, I guess I have to look further for a solution.
Could it be your tree models have levels of detail? So that much more triangles have to be drawn when close to the ground?

The trees look more complex (and better) than the default trees.
While I still couldn't figure out what's happening here with my custom forest, one question I could not find the answer.
If the problem is really caused by the tree models with more detail levels, is there a possibility to make a low poly, one level-of-detail version of a default scenery object? Or do the default trees have LOD at all? If not, then I guess I should find a solution elsewhere...
So what I'm trying to acheive here is to place the trees as scenery objects which behave like fps-friendly autogen :) I rather have low-detail trees but many of them, than have 3 pieces of HD trees where a dense forest should be.

I know, one solution is not to use this tool, but instead place normal autogen trees with the annotator, but I still try to fix this first.
I'm experiencing the same thing and many others too. Some are suspecting the new dynamic lights. I have been having it happen in high Autogen building locations in Los Angeles. After about 500-100 AGL something switches off and I jump up to 35-45 fps.

I've investigated and my GPU is near idle, with 2 of 4GB video RAM used, my CPU running at 15-17% and RAM use only 3.8 GB of 16GB RAM used. So it doesn't seem that its a graphic or CPU bottleneck but rather a bug in the code somewhere, I know crosswinds kick in about 500-1000' AGL based on my contract work that involved FSX base code. In my testing as I accelerate down the runway (once the sim links my throttle to the sim throttle), frames move up to 4fps, and then as soon as I'm 500' AGL boom frame rates jump up to 35-45 (locked at 45 because I sometimes fly in VR).

I have a suspicion that the move to 64 bit has highlighted a possible old bug in the code. Starting about FS2000, then FS2002, FS2004, then FSX whenever anyone would approach a high detail airport like LAX for example frame rates would drop significantly. We all thought it was CPU or GPU and were usually overcome in time as videocards got more powerful. However now that most of the pipeline bottlenecks are gone it appears it may be something nefarious going on in the core sim code (like how they discovered what was causing blurries over 250 knts and fixed it for P3D).

Phillip Talyor (ACES) former graphics guru has given some info on tools to try and track down what's causing the problem. So once I'm finished with my rendering by this weekend I'll be able to start digging into what may be causing the sim to choke between 0-500/1000' AGL.
While it's good to hear that this problem does not only affect me, it sounds bad that it might be a problem only LM could fix (maybe in the next release of p3d).
Since then I drastically reduced the number of objects in my scenery because of this and I upgraded my card to a monster 1080ti that obviously handles p3d much faster. But I still see this sudden fps change with my photoscenery. Now it's around 12-18 fps on ground then 30+ above 800-1000 feet.
Let me know if you find a solution for this.
Yeah I'm needing to find a solution, I will re-install 4.2 and test with that and see if I get different results, I found out how I can activate the correct license without needing the 4.2 installer for the different license. If I have the same problem there, it may be some legacy FS code.

I'm about to move to a 2080Ti that's just been announced. Hopefully enough of us bug LM about the apparent bug...

Also the blurring issue with photoscenery at speed seems to be back
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