In the previous instalment we looked at the background for Qt 5.10’s upcoming Vulkan support. Let’s now start digging out into the details.
Obtaining a Vulkan-Enabled Qt Build
When building Qt from sources on Windows, Linux or Android, Vulkan support is enabled automatically whenever a Vulkan header is detected in the build environment. Windows is handled specially in the sense that the environment variable VULKAN_SDK – set by the LunarG Vulkan SDK – is picked up automatically.
Check the output of configure (also available afterwards in config.summary):
Qt Gui: ... Vulkan ................................. yes ...
If it says no, go to qtbase/config.tests/qpa/vulkan, run make, and see why it did not compile.
As mentioned in part 1, neither the QtGui library nor the platform plugins link directly to libvulkan or similar. Same applies to Qt applications by default. This comes very handy here: a Vulkan-enabled Qt build is perfectly fine for deployment also to systems without any Vulkan libraries. No headache with missing DLLs and such. Naturally, once it turns out Vulkan is not available at runtime, QVulkanInstance::create() will fail and will return false always. It must also be noted that the applications themselves can choose to link to a Vulkan (loader) library, if they have a reason to do so: all it takes is adding LIBS+=-lvulkan or similar to the .pro file.
Getting a Vulkan Instance
In Vulkan all per-application state is stored in a VkInstance object, see the the specification for a detailed overview. In Qt, Vulkan instances are represented by QVulkanInstance. This is backed by a QPlatformVulkanInstance following the usual QPA patterns. The platform plugins, at least the ones that are interested in providing Vulkan support, are expected to provide an implementation for it under the hood. As described earlier, this currently covers windows, xcb and android.
Following the familiar pattern from QWindow and the QOpenGL* classes, QVulkanInstance performs no initialization until create() is called. The loading of the Vulkan library (or the loader library which in turn routes to a vendor implementation) happens only at this point. (with a few exceptions, see below)
The resulting VkInstance can be retrieved via vkInstance().
While the Qt APIs allow including unsupported layers and extensions too – since it filters them out automatically – it may …read more