X-Plane 11.10 beta 5 is here, and a before and after says it all.
Tyler heard your pain (as did the rest of us, as we received approximately 8,414,493 requests to kill the FPS nag) and we have changed the low frame-rate warning from the perpetual flashing "you're too slow" shown on the left to the pop-up warning on the right. And if you like flying a slide show, you can click "don't show this again" and ... we won't!
The new warning is a bit more conservative - you should only see it if you've got persistently low framerate - a transient dip due to texture paging should not make it pop up. If you see it appearing spuriously, please file a bug. In these pics I've set my poor 3-year-old iMac to maxed out AG, maxed out shadows and maxed out shaders all at once with the 747 to dip below 20 fps.
The rest of this post discusses the "why" behind this dialog box.
X-Plane Requires 20 fps (or: How Did We Get Here)
X-Plane has some fundamental rules about how it simulates flight - these are our design parameters.
- X-Plane simulates one physics frame and renders one graphics frame in lock-step - physics and graphics do not run independently.
- X-Plane's physics model will not "step" the simulation forward more than 50 ms in each step, to avoid computational flutter and other flight dynamics problems.*
- X-Plane allows you to set your rendering settings as high or low as you want, even if you don't have hardware that can render a graphics frame at those settings in 50 ms.
If you consider all of these things, you can see the problem. You can crank up your settings on an old machine and the rendering engine needs more than 50 ms but the physics engine won't simulate more than 50 ms in a single step.
X-plane does the only remaining option: if your computer cannot sustain 20 fps rendering, your flight will be simulated slower than real time. For example, at 10 fps, a flight from Boston to JFK will take 90 minutes, not 45.
It turns out that lots of X-Plane users don't know about this behavior, and as a result, Austin receives a steady stream of emails complaining that the sim is unrealistic because en-route times are too long - virtually all of them are caused by low fps.
So for 11.10, Austin did what Austin does: he put a big loud message on screen to tell users that they were below real-time simulation.
We Should Probably Say Something
The result of this message was not users going "oh, I've learned something, I'll turn down my rendering settings." It was a huge number of bug reports that X-Plane 11.10 had catastrophically worse FPS than X-Plane 11.05. In many of these cases, users thought they were getting 20 fps because the flight model reads 20 fps in the time window data output (a very confusing display) and there is no bitch-o-gram.
I considered the option of just nuking the message a second time, but there may be problems with running at low fps that users don't realize. For example, if you're on VATSIM doing 170 knots on final like the controller asked at 10 fps, you're doing...85 knots and your 737 is looking a lot like a C172.
So we went with Tyler's popup + never-show-again design; it brings the issue of low fps to the surface, but lets users dismiss it forever if they can live with non-real-time simulation.
Why Don't You Just Split the Physics and Graphics
When discussing this with users I did get asked a lot, "why don't you run the physics and graphics independently so we can run in real-time at < 20 fps". There are a few reasons why we did not (and are not) considering this approach.
- Separating the execution of the physics and graphics code would require significant engineering work - right now they run in an alternating pair, and therefore they don't need to be coordinated at all. Once they run independently, a bunch of new code is needed to mediate their interaction. That engineering time would take away from just making X-Plane faster.
- The cost in CPU time of coordinating the independent FM and graphics would not be free, so this would make the actual frame-rate for the sim worse in pretty much all cases.
- Our goal is to make X-Plane run at 60 fps, and we consider 20 fps (the minimum) to already be really quite awful. So we really want to focus on making X-Plane faster, not on making it work better at a frame-rate where the user experience is very sub-standard.
So we're going to keep focusing on performance optimization that will benefit everyone and not take a side-tangent into making 15 fps flight run in real-time.
* If the time between frames is too large, self-damping forces on the aircraft and feedback-based control systems will not react properly because they will not respond quickly enough, due to there being no simulated frame shortly after a large force is introduced. The result can be incorrect handling at the outside of the flight envelope, for example.