For the last few weeks we’ve had the Vulkan/Metal version of X-Plane in a private test program with third party developers, letting them kick the tires to discover the limits and bugs in our plugin compatibility code.

While the initial bridge to connect OpenGL-based plugins to X-Plane worked pretty well, compatibility wasn’t perfect and we had a few bugs. We also realized that we needed to change the design of that bridge to get much better compatibility. That rework of the bridge is now done and things are looking quite a bit better on all fronts.

We found one unexpected result from this redesign: when we finished rewriting the bridge, we ended up with code that could, in the case of Vulkan only, be used for 3-d drawing. This means we can now support OpenGL-based weather/cloud plugins from inside a Vulkan render, which we did not think was possible before.

The Skymaxx and xEnviro developers are in our private test program and have been great about jumping on new builds while remaining radio silent while we test this new tech. I am pleased to now be able to say: it works!

Those pics are a special build of SkyMaxx running in 11.50 in Vulkan mode. No more having to pick – you can have your third party clouds and Vulkan at the same time.

The Details: Vulkan, not Metal

3-d drawing compatibility will be available for Vulkan, but not Metal. The technique we are using is not available on OS X, so running a cloud plugin on Mac will require the OpenGL driver, not Metal. This isn’t something we can fix with some future bit of cleverness – it’s a limitation of OS X’s APIs. (We would revisit this if Apple introduced a new API but I do not expect this to happen.)

To be clear: on Windows what we are supporting is 3-d OpenGL drawing by plugins inside the Vulkan renderer, not native Vulkan rendering by plugins inside the Vulkan renderer. Native Vulkan rendering by plugins is an incredibly complex SDK problem, and not something we are looking at for 11.50.

2-d Drawing Just Works; 3-d Drawing Requires a New Plugin

For plugins that draw in 2-d we’ve gone for a “just works” level of compatibility; while we have seen some plugins that need to update due to illegal techniques and bugs, our goal is that a well-authored add-on just works even when Vulkan and Metal are in use.

By comparison, OpenGL drawing inside Vulkan is going to require plugin authors to sign up for a new 3-d drawing callback. We’re making this opt-in because, during the beta, we’ve found a lot of add-ons using 3-d drawing callbacks inappropriately, and 3-d drawing callbacks are no longer free. By requiring plugins to opt-in when they _really_ want 3-d drawing in Vulkan, we should get better performance.

The new 3-d drawing callbacks are really only meant for custom drawing like clouds. Other techniques like adding objects to the sim and drawing markings are better handled with other APIs. Our plan is to have detailed guidance on what technique is best for developers by the time we are ready for public beta.