Microsoft wants developers to build apps for the Universal Windows Platform (UWP), the set of APIs and frameworks that allows developers to build applications that are substantially compatible with desktop PCs, tablets, smartphones, Xbox One, HoloLens, and soon, a range of affordable virtual reality headsets.

The company's annual Build developer conference is taking place in May, which means that it's going to come after the expected release date of next major Windows update, the Creators Update. But there's new developer content in the Creators Update, so instead of having a big conference to talk about that, Microsoft has had a "Windows Developer Day" streaming session to talk about some of the new things coming in the Creators Update for developers. At Build, the company will be talking about what's coming after the Creators Update, looking forward at the next year or so of Windows development.

For the Creators Update today, Microsoft has announced that the current preview SDK is feature complete, meaning that developers can use it to get complete access to every feature that'll be coming in the update when it ships.

The substantial majority of these new features are exposed as UWP APIs, with the highest profile addition being the integration and inclusion of the holographic APIs used for virtual reality and augmented reality headsets. These build on the work already done for HoloLens, extending it to include support for headsets that completely occlude your vision. Microsoft's belief is that the cheaper price of Windows Holographic headsets—they should start at $299—will open up virtual reality to a much wider audience than can currently use the Oculus Rift and HTC Vive, and is hoping that developers will embrace the holographic APIs in their UWP applications.

While virtual reality games will tend to use 3D engines such as Unity, Microsoft envisions that developers will create regular applications that work in virtual reality too. At the most basic level this might be simply pinning a 2D application into a 3D workspace, but the UWP APIs should offer the ability to incrementally add 3D support. It's unlikely that most traditional applications will want to construct entire 3D worlds, but selective use of depth when presenting data, for example, can add value to mostly-2D software. We've seen similar things with HoloLens software, mixing conventional interfaces and 2D software with 3D for data visualization.

Apart from HoloLens, at a rather steep $3,000, there's currently no Windows Holographic hardware on the market. Developer kits should become available at or around GDC at the end of the month, and judging by the low cost of the retail headsets, we'd expect these dev kits to be much less expensive than HoloLens is.

The Creators Update is merely Microsoft's first step toward producing a full mixed reality platform, of interest primarily to developers and enthusiasts. The company will make a wider mainstream push with the next Windows update (codenamed "Redstone 3"). In the Creators Update, the hardware requirements for virtual reality will be comparable to those of the Rift and the Vive, with a discrete GPU mandatory. The Redstone 3 release will cut those hardware requirements to include support for integrated GPUs, greatly expanding the number of systems that are VR capable.

As well as 3D output, the Creators Update also builds on Windows' input mechanisms, with new integration points for Cortana, to offer natural language interaction with UWP applications, and support for devices like the Surface Dial.

The Creators Update should make UWP a better gaming platform, too. As well as Game Mode, which can offer a performance boost of a few extra frames per second, with the Creators Update DirectX 12 games will have greater access to Nvidia and AMD's proprietary extensions for features such as multi-GPU support and HDR rendering.

Gamers should be able to get started in their games sooner. In a feature shared with some third-party stores, UWP games sold through the Windows Store can be played as they download. Microsoft calls this "Intelligent Install:" app and game packages can specify which order their individual component parts are downloaded and installed, allowing a game developer to ensure that the introductory parts of their game are available first, allowing the rest to be streamed in while the game is running. Blizzard, EA, and Ubisoft all support this for games installed through their launchers, and use it as a matter of course. Steam technically has the ability—Mortal Kombat X used a streaming install—but developers appear to be avoiding it, at least for the time being.

Windows 10 and Xbox One will also both offer integrated support for Dolby Atmos surround sound.

To further help UWP developers, third party component vendor Telerik has released a number of UWP controls aimed at line-of-business app developers as open source. These include such bread and butter mainstays as data grids and calendars.

Most applications aren't UWPs, however. Microsoft's Desktop Bridge (formerly known as Centennial) went live with last year's Anniversary Update, and it lets developers package many of their existing Win32 applications and sell them through the Windows Store. Legacy Win32 applications packaged in this way can't access the full range of UWP features, and they're not run in the same restrictive sandbox, but Centennial apps do still get to use the Store's upgrading/servicing model, for example, so patches can be automatically installed, and they offer the same clean installation and uninstallation that Store apps do. Centennial apps can't access the full range of the Win32 API, but additional parts are being supported in the Creators Update, to improve compatibility.

Redmond also released a new SDK for Android today, for what it calls Project Rome. Project Rome is a set of services designed for cross-device experiences, such as transferring playback of an audio app from your phone to your Xbox when you get get home, or a TV remote control that runs as a phone app. With the new Android SDK, these experiences can now roam between Windows and Android phones.

UWP is a new platform, and unlike Win32, it's one that's seeing considerable new feature development. Microsoft is so keen to get developers to embrace UWP that it's deciding to be a little more open and accessible in how it develops the platform. The UWP bug tracker and feature backlog—the place that Microsoft records bugs and feature requests, and tracks work done on new features—is being made public. This will give developers much greater insight into fixes that are being made, and improvements that are coming down the pipeline. This is similar to what Microsoft has done with the Edge browser; there's a public issues list, and the development of new features can be tracked. Prioritization of new features can even be influenced, by casting votes.

UWP apps are only going to become more important to Microsoft. Xbox is likely to use more and more UWP technology and software, and Windows Holographic and the forthcoming Windows 10 Cloud are almost wholly dependent on developer support for the new technology. Microsoft is clearly working hard to boost developer engagement with and interest in UWP as a platform. Being more open with them, and involving them more heavily with future developments, is likely to help, but there's still something of a chicken and egg problem. The size of the Windows Holographic market is currently "several thousand HoloLenses," which is to say, irrelevant for most developers. Win32 is harder to use and in many ways inferior to UWP, but it boasts of greater reach, since Win32 apps run on Windows 7 too. The company is arguably doing the right things to create that developer momentum, but it's still facing an uphill struggle.