This post has been on my todo list for a while – long enough that X-Plane 11.40 came out before I had time to write up a post saying “X-Plane 11.40” is coming. But just to put 11.40 into context, here’s what our patch roadmap looks like for X-Plane 11 this year.
All Physics All the Time
X-Plane 11.40 is a physics release. Almost all of the changes in X-Plane 11.40 come from Austin’s work on the physics engine over the last six months. This is a new approach for us. In the past, when we’ve updated the physics or systems, it would be in a giant “omnibus” release, where everybody’s latest code went out at once (e.g. X-Plane 11.10).
The problem with the omnibus releases is that they would take forever to get debugged. With so many people changing so many things, we never knew what had gone wrong when a bug report came in. And with all of the code changed, we had to investigate every single bug report carefully (no matter how unlikely or vague the report) because anything could have been broken.
So far, at risk of jinxing the beta, it appears that the physics-only approach is working a lot better. It has been quicker to find bugs when they are reported, and the overall level of crazy is a lot lower than in past releases.
There aren’t many open bugs left in the 11.40 beta, but one particular bug has caused the beta count to run up: we were seeing crashes due to NaNs in the flight model.
NaN stands for Not A Number, and it’s what you get when you have divide-by-zeros run amok in the physics. To catch them, we’ve turned on a lot of auditing code and we’ve been collecting automatic crash reports. At risk of jinxing it, I think Austin has fixed one of the two root causes in beta 8. We are going to keep chasing them until the other one is fixed, then turn down the checks once we’re done. So we may make it to beta nine or ten and we may have another week with two betas; the high tempo is just to get more checks in fast.
There have been a number of questions in the comments on the state of the experimental flight model, so I want to clarify how it works and what is happening in 11.40.
Normally, new X-Plane features get beta tested during the beta of an X-Plane patch. This means we have somewhere between two and eight weeks to debug the feature and get it ready to ship. Once it ships, if we change the feature, we have to consider how this would affect authors using the feature and whether it would screw up their add-ons.
That’s not a lot of time to debug! In particular, it’s really not enough time for the flight model, where people need months just to develop the aircraft and measure the performance to get us feedback.
The experimental flight model is basically a giant year-long open beta of a future revision of the flight model that hasn’t shipped yet. By checking the “experimental FM” box, you’re getting to beta test the flight model of the future, now. By keeping the experimental flight model as an experiment for so long, this frees Austin up to simply fix bugs and improve it, as opposed to worrying how the X-Plane 11.40 experimental FM changes will affect X-Plane 11.30 users.
To be clear: there is no attempt at backward compatibility between the experimental flight model from one sim version to another! The goal is to have the experimental flight model not interfere with the “normal” flight model at all.
Physics changes in 11.40 fall into two broad categories:
- Simulation changes that can change how an aircraft flies, improving the accuracy of X-Plane’s predictions about the airframe. An example of this is the delay in wash propagation from the prop to the tail of the aircraft. This makes the aircraft more stable, but may change how it flies; an author might have added artificial stability or reduced control surface efficacy to work around the lack of this feature in the past, and these work-arounds would be inappropriate with the new, more accurate physics.
- Simulations that change how the aircraft flies in unusual circumstances that you can’t tune your aircraft to. Examples of this include stalls and wake turbulence. There isn’t going to be a book value for the effect, so all we can do is try to produce the most sane results given an aircraft that simulates properly in regular flight.
Features in the first category require the experimental flight model to be enabled, while the second category of features is always on.
At some point in the future, the experimental flight model will become the flight model for X-Plane, but we are not there yet, and we are not planning to do this as part of making X-Plane 11.40 final.
Third Party Aircraft
If you develop a third party aircraft, you need to test it now against both the experimental and non-experimental flight mode. If the non-experimental flight model doesn’t fly the same as 11.36, please file a bug, and please provide flight testing details. For the experimental flight model, you may see book numbers change a little bit; the real question is whether the overall physics response is better or worse.
Vulkan and Metal
X-Plane 11.50 will be the next major patch once X-Plane 11.40 is out of beta, and it will feature Vulkan and Metal support.
The marketing guys showed the Vulkan build of X-Plane live at Cosford last week; that build did not have any support for texture paging. Since then, Sidney has a basic texture paging implementation running, so hopefully we’re in good shape to get this into developer’s hands after 11.40.
Our expectation for add-on compatibility is:
- Add-ons doing supported things, like 2-d panel drawing and UI should just work in Vulkan and Metal – we’ll take bug reports to fix compatibility issues.
- Add-ons doing unsupported things won’t work in Vulkan and Metal at all. Your 3-d drawing callback won’t be called, or your attempt to grab internal GL resources will just fail (because none of our resources are GL).
- X-Plane running under OpenGL should “just work” for pretty much every add-on, including ones doing sketchy things, and should be faster than 11.40 but not as fast as Vulkan or Metal.
I expect the Vulkan beta to be a relatively long one. We want it to start this year, but it probably won’t end this year, and my guess is that initially Vulkan will be fantastic for some users and will crash for others. During the beta we’ll gain useful information about how well Vulkan works “in the field” for different cards and drivers.
One reason I am looking forward to the Vulkan beta: we now have tremendous visibility into what the rendering engine and driver are doing. With OpenGL, the driver was often a black box. We still get reports of “the 3-d mouse in VR make my machine really slow” and frankly, we may never know why this happens to just some users and not others with the same hardware, drivers, and version of X-Plane.
With Vulkan and Metal it is going to be different. A lot more of the graphics work happens inside X-Plane, and the work that happens inside the driver is much more predictable, bounded, and can be viewed via modern profiling tools.
So while we will have a lot of debugging to do based on user feedback, it should be straightforward to get the information we need to really make the Vulkan renderer scream.
It’s a little too soon to discuss what comes after Vulkan, but I can say this: for almost two years now, Sidney and I have been rewriting the rendering engine with a rather strange goal: performance and predictability, but with the same visual output. So anything that looked ugly on the screen is supposed to keep looking ugly. Vulkan was a change of the how but not the what of our rendering engine.
Once Vulkan is out the door, that all changes. We have a number of fundamental changes we want to make to how we deal with light, with the atmosphere, with color, and with organizing our frame. Once we have Vulkan, we get to use it as our foundation for what comes next.