diff options
Diffstat (limited to 'src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc')
| -rw-r--r-- | src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc | 341 |
1 files changed, 322 insertions, 19 deletions
diff --git a/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc b/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc index d7d2fea281..9ce26e1bb8 100644 --- a/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc +++ b/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc @@ -31,33 +31,88 @@ \section1 Scene Graph Adaptations in Qt Quick -Originally Qt Quick only had one available renderer for parsing the -scene graph and rendering the results to a render target. This renderer -is now the default OpenGL Renderer which supports rendering either using -the OpenGL ES 2.0 or OpenGL 2.0 APIs. The Qt Quick APIs are designed -with the assumption that these two APIs are always available. It is -however possible now to use other graphics API's to render Qt Quick +Originally Qt Quick only had one available renderer for parsing the scene graph +and rendering the results to a render target. This renderer is now the default +OpenGL Renderer which supports rendering either using the OpenGL ES 2.0 or +OpenGL 2.0 (with framebuffer object extensions) APIs. The Qt Quick APIs have +originally been designed with the assumption that OpenGL is always available. +However, it is now possible to use other graphics API's to render Qt Quick scenes using the scene graph APIs. +\section1 Switching between the adaptation used by the application + +The default of the OpenGL, or - in Qt builds with disabled OpenGL support - the +software adaptation, can be overridden either by using an environment variable +or a C++ API. The former consists of setting the \c{QT_QUICK_BACKEND} or the +legacy \c{QMLSCENE_DEVICE} environment variable before launching applications. +The latter is done by calling QQuickWindow::setSceneGraphBackend() early in the +application's main() function. + +The supported backends are the following + +\list + +\li OpenGL - Requested by the string \c{""} or the enum value QSGRendererInterface::OpenGL. + +\li Software - Requested by the string \c{"software"} or the enum value QSGRendererInterface::Software. + +\li Direct3D 12 - Requested by the string \c{"d3d12"} or the enum value QSGRendererInterface::Direct3D12. + +\endlist + +When in doubt which backend is in use, enable basic scenegraph information +logging via the \c{QSG_INFO} environment variable or the +\c{qt.scenegraph.general} logging category. This will result in printing some +information during application startup onto the debug output. + +\note Adaptations other than OpenGL will typically come with a set of +limitations since they are unlikely to provide a feature set 100% compatible +with OpenGL. However, they may provide their own specific advantages in certain +areas. Refer to the sections below for more information on the various +adaptations. + \section1 OpenGL ES 2.0 and OpenGL 2.0 Adaptation The default adaptation capable of providing the full Qt Quick 2 feature set is the OpenGL adaptation. All of the details of the OpenGL -adpatation can are available here +adpatation can are available here: \l{qtquick-visualcanvas-scenegraph-renderer.html}{OpenGL Adaptation} \section1 Software Adaptation -The Software adaptation is an alternative renderer for \l {Qt Quick} 2 that uses the Raster -paint engine to render the contents of the scene graph instead of OpenGL. -As a result of not using OpenGL to render the scene graph, some features -and optimizations are no longer available. Most Qt Quick 2 applications -will run without modification though any attempts to use unsupported -features will be ignored. By using the Software adpatation it is possible to run Qt -Quick 2 applications on hardware and platforms that do not have OpenGL -support. +The Software adaptation is an alternative renderer for \l {Qt Quick} 2 that +uses the raster paint engine to render the contents of the scene graph. The +details for this adaptation are available here: +\l{qtquick-visualcanvas-adaptations-software.html}{Software Adaptation} + +\section1 Direct3D 12 (experimental) + +The Direct3D 12 adaptation is an alternative renderer for \l {Qt Quick} 2 when +running on Windows 10, both for Win32 and UWP applications. The details for +this adaptation are available here: +\l{qtquick-visualcanvas-adaptations-d3d12.html}{Direct3D 12 Adaptation} + +*/ + + +/*! +\title Qt Quick Software Adaptation +\page qtquick-visualcanvas-adaptations-software.html + +The Software adaptation is an alternative renderer for \l {Qt Quick} 2 that +uses the Raster paint engine to render the contents of the scene graph instead +of OpenGL. As a result of not using OpenGL to render the scene graph, some +features and optimizations are no longer available. Most Qt Quick 2 +applications will run without modification though any attempts to use +unsupported features will be ignored. By using the Software adaptation it is +possible to run Qt Quick 2 applications on hardware and platforms that do not +have OpenGL support. The Software adaptation was previously known as the Qt Quick 2D Renderer. +However, unlike the 2D Renderer, the new, integrated version supports partial +updates. This means that the full update of the window or screen contents is +now avoided, and only the changed areas get flushed. This can significantly +improve performance for many applications. \section2 Shader Effects ShaderEffect components in QtQuick 2 can not be rendered by the Software adptation. @@ -72,15 +127,263 @@ It is not possible to render particle effects with the Software adaptation. When possible, remove particles completely from the scene. Otherwise they will still require some processing, even though they are not visible. -\section2 Sprites -The Sprite item depends on OpenGL functions and will not be visible. - \section2 Rendering Text The text rendering with the Software adaptation is based on software rasterization and does not respond as well to transformations such as scaling as when using OpenGL. The quality is similar to choosing \l [QML] {Text::renderType} {Text.NativeRendering} with \l [QML] {Text} items. -\section1 Direct3D 12 (experimental) +*/ + + +/*! +\title Qt Quick Direct3D 12 Adaptation +\page qtquick-visualcanvas-adaptations-d3d12.html + +The Direct3D 12 adaptation for Windows 10 (both Win32 (\c windows platform +plugin) and UWP (\c winrt platform plugin)) is shipped as a dynamically loaded +plugin. It will not be functional on earlier Windows versions. The building of +the plugin is enabled automatically whenever the necessary D3D and DXGI +develpoment files are present. In practice this currently means Visual Studio +2015 and newer. + +The adaptation is available both in normal, OpenGL-enabled Qt builds and also +when Qt was configured with \c{-no-opengl}. However, it is never the default, +meaning the user or the application has to explicitly request it by setting the +\c{QT_QUICK_BACKEND} environment variable to \c{d3d12} or by calling +QQuickWindow::setSceneGraphBackend(). + +\section2 Motivation + +This experimental adaptation is the first Qt Quick backend focusing on a +modern, lower-level graphics API in combination with a windowing system +interface different from the traditional approaches used in combination with +OpenGL. + +It also allows better integration with Windows, Direct3D being the primary +vendor-supported solution. This means that there are fewer problems anticipated +with drivers, operations like window resizes, and special events like graphics +device loss caused by device resets or graphics driver updates. + +Performance-wise the general expectation is a somewhat lower CPU usage compared +to OpenGL due to lower driver overhead, and a higher GPU utilization with less +wasted idle time. The backend does not heavily utilize threads yet, which means +there are opportunities for further improvements in the future, for example to +further optimize image loading. + +The D3D12 backend also introduces support for pre-compiled shaders. All the +backend's own shaders (used by the built-in materials on which the Rectangle, +Image, Text, etc. QML types are built) are compiled to D3D shader bytecode when +compiling Qt. Applications using ShaderEffect items can chose to ship bytecode +either in regular files or via the Qt resource system, or use HLSL source +strings. Unlike OpenGL, the compilation for the latter is properly threaded, +meaning shader compilation will not block the application and its user +interface. + +\section2 Graphics Adapters + +The plugin does not necessarily require hardware acceleration. Using WARP, the +Direct3D software rasterizer, is also an option. By default the first adapter +providing hardware acceleration is chosen. To override this, in order to use +another graphics adapter or to force the usage of the software rasterizer, set +the environment variable \c{QT_D3D_ADAPTER_INDEX} to the index of the adapter. +The discovered adapters are printed at startup when \c{QSG_INFO} or the logging +category \c{qt.scenegraph.general} is enabled. + +\section2 Troubleshooting + +When encountering issues, always set the \c{QSG_INFO} and \c{QT_D3D_DEBUG} +environment variables to 1 in order to get debug and warning messages printed +on the debug output. The latter enables the Direct3D debug layer. Note that the +debug layer should not be enabled in production use since it can significantly +impact performance (CPU load) due to increased API overhead. + +\section2 Render Loops + +By default the D3D12 adaptation uses a single-threaded render loop similar to +OpenGL's \c windows render loop. There is also a threaded variant available, that +can be requested by setting the \c{QSG_RENDER_LOOP} environment variable to \c +threaded. However, due to conceptual limitations in DXGI, the windowing system +interface, the threaded loop is prone to deadlocks when multiple QQuickWindow +or QQuickView instances are shown. Therefore the default is the single-threaded +loop for the time being. This means that with the D3D12 backend applications +are expected to move their work from the main (GUI) thread out to worker +threads, instead of expecting Qt to keep the GUI thread responsive and suitable +for heavy, blocking operations. + +See the \l{qtquick-visualcanvas-scenegraph.html}{Scene Graph page} for more +information on render loops and +\l{https://msdn.microsoft.com/en-us/library/windows/desktop/ee417025(v=vs.85).aspx#multithreading_and_dxgi}{the +MSDN page for DXGI} regarding the issues with multithreading. + +\section2 Renderer + +The scenegraph renderer in the D3D12 adaptation does not currently perform any +batching. This is less of an issue, unlike OpenGL, because state changes are +not presenting any problems in the first place. The simpler renderer logic can +also lead to lower CPU overhead in some cases. The trade-offs between the +various approaches are currently under research. + +\section2 Shader Effects + +The ShaderEffect QML type is fully functional with the D3D12 adaptation as well. +However, the interpretation of the fragmentShader and vertexShader properties is +different than with OpenGL. + +With D3D12, these strings can either be an URL for a local file or a file in +the resource system, or a HLSL source string. The former indicates that the +file in question contains pre-compiled D3D shader bytecode generated by the +\c fxc tool, or, alternatively, HLSL source code. The type of the file is detected +automatically. This means that the D3D12 backend supports all options from +GraphicsInfo.shaderCompilationType and GraphicsInfo.shaderSourceType. + +Unlike OpenGL, there is a QFileSelector with the extra selector \c hlsl used +whenever opening a file. This allows easy creation of ShaderEffect items that +are functional across both backends, for example by placing the GLSL source +code into \c{shaders/effect.frag}, the HLSL source code or - preferably - +pre-compiled bytecode into \c{shaders/+hlsl/effect.frag}, while simply writing +\c{fragmentShader: "qrc:shaders/effect.frag"} in QML. + +See the ShaderEffect documentation for more details. + +\section2 Multisample Render Targets + +The Direct3D 12 adaptation ignores the QSurfaceFormat set on the QQuickWindow +or QQuickView (or set via QSurfaceFormat::setDefaultFormat()), with two +exceptions: QSurfaceFormat::samples() and QSurfaceFormat::alphaBufferSize() are +still taken into account. When the samples value is greater than 1, multisample +offscreen render targets will be created with the specified sample count and a +quality of the maximum supported quality level. The backend automatically +performs resolving into the non-multisample swapchain buffers after each frame. + +\section2 Semi-transparent windows + +When the alpha channel is enabled either via +QQuickWindow::setDefaultAlphaBuffer() or by setting alphaBufferSize to a +non-zero value in the window's QSurfaceFormat or in the global format managed +by QSurfaceFormat::setDefaultFormat(), the D3D12 backend will create a +swapchain for composition and go through DirectComposition since the flip model +swapchain (which is mandatory) would not support transparency otherwise. + +It is therefore important not to unneccessarily request an alpha channel. When +the alphaBufferSize is 0 or the default -1, all these extra steps can be +avoided and the traditional window-based swapchain is sufficient. + +This is not relevant on WinRT because there the backend always uses a +composition swapchain which is associated with the ISwapChainPanel that backs +QWindow on that platform. + +\section2 Mipmaps + +Mipmap generation is supported and handled transparently to the applications +via a built-in compute shader, but is experimental and only supports +power-of-two images at the moment. Textures of other size will work too, but +this involves a QImage-based scaling on the CPU first. Therefore avoid enabling +mipmapping for NPOT images whenever possible. + +\section2 Image formats + +When creating textures via the C++ scenegraph APIs like +QQuickWindow::createTextureFromImage(), 32-bit formats will not involve any +conversion, they will map directly to the corresponding \c{R8G8B8A8_UNORM} or +\c{B8G8R8A8_UNORM} format. Everything else will trigger a QImage-based format +conversion on the CPU first. + +\section2 Unsupported Features + +Particles and some other OpenGL-dependent utilities, like +QQuickFramebufferObject, are not currently supported. + +Like with the \l{qtquick-visualcanvas-adaptations-software.html}{Software +adaptation}, text is always rendered using the native method. Distance +field-based text rendering is not currently implemented. + +The shader sources in the \l {Qt Graphical Effects} module have not been ported +to any format other than the OpenGL 2.0 compatible one, meaning the QML types +provided by that module are not currently functional with the D3D12 backend. + +Texture atlases are not currently in use. + +The renderer may lack support for certain minor features, for example drawing +points and lines with a width other than 1. + +Custom Qt Quick items using custom scenegraph nodes can be problematic. +Materials are inherently tied to the graphics API. Therefore only items using +the utility rectangle and image nodes are functional across all adaptations. + +QQuickWidget and its underlying OpenGL-based compositing architecture is not +supported. If mixing with QWidget-based user interfaces is desired, use +QWidget::createWindowContainer() to embed the native window of the QQuickWindow +or QQuickView. + +Finally, rendering via QSGEngine and QSGAbstractRenderer is not feasible with +the D3D12 adaptation at the moment. + +\section2 Related APIs + +To integrate custom Direct3D 12 rendering, use QSGRenderNode in combination +with QSGRendererInterface. This approach does not rely on OpenGL contexts or +API specifics like framebuffers, and allows exposing the graphics device and +command buffer from the adaptation. It is not necessarily suitable for easy +integration of all types of content, in particular true 3D, so it will likely +get complemented by an alternative to QQuickFramebufferObject in future +releases. + +To perform runtime decisions based on the adaptation in use, use +QSGRendererInterface from C++ and GraphicsInfo from QML. They can also be used +to check the level of shader support (shading language, compilation approach). + +When creating custom items, use the new QSGRectangleNode and QSGImageNode +classes. These replace the now deprecated QSGSimpleRectNode and +QSGSimpleTextureNode. Unlike their predecessors, the new classes are +interfaces, and implementations are created via the factory functions +QQuickWindow::createRectangleNode() and QQuickWindow::createImageNode(). + +\section2 Advanced Configuration + +The D3D12 adaptation can keep multiple frames in flight, similarly to modern +game engines. This is somewhat different from the traditional render - swap - +wait for vsync model and allows better GPU utilization at the expense of higher +resource usage. This means that the renderer will be a number of frames ahead +of what is displayed on the screen. + +For a discussion of flip model swap chains and the typical configuration +parameters, refer to +\l{https://software.intel.com/en-us/articles/sample-application-for-direct3d-12-flip-model-swap-chains}{this +article}. + +Vertical synchronization is always enabled, meaning Present() is invoked with +an interval of 1. + +The configuration can be changed by setting the following environment variables: + +\list + +\li \c{QT_D3D_BUFFER_COUNT} - The number of swap chain buffers in range 2 - 4. +The default value is 3. + +\li \c{QT_D3D_FRAME_COUNT} - The number of frames prepared without blocking in +range 1 - 4. Note that Present will start blocking after queuing 3 frames +(regardless of \c{QT_D3D_BUFFER_COUNT}), unless the waitable object is in use. +Note that every additional frame increases GPU resource usage since geometry +and constant buffer data will have to be duplicated, and involves more +bookkeeping on the CPU side. The default value is 2. + +\li \c{QT_D3D_WAITABLE_SWAP_CHAIN_MAX_LATENCY} - When set to a value between 1 +and 16, the frame latency is set to the specified value. This changes the limit +for Present() and will trigger a wait for an available swap chain buffer when +beginning each frame. Refer to the article above for a detailed discussion. +This is considered experimental for now and the default value is 0 (disabled). + +\li \c{QT_D3D_BLOCKING_PRESENT} - When set to a non-zero value, there will be +CPU-side wait for the GPU to finish its work after each call to Present. This +effectively kills all parallelism but makes the behavior resemble the +traditional swap-blocks-for-vsync model, and can therefore be useful in some +special cases. This is not the same as setting the frame count to 1 because +that still avoids blocking after Present, and may block only when starting to +prepare the next frame (or may not block at all depending on the time gap +between the frames). By default blocking present is disabled. + +\endlist */ |