adaptations.qdoc

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// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\title Scene Graph Adaptations
\page qtquick-visualcanvas-adaptations.html
 
\section1 Scene Graph Adaptations in Qt Quick
 
In Qt 5.0, Qt Quick always relied on OpenGL (OpenGL ES 2.0 or OpenGL 2.0) to
parse the scene graph and render the results to a render target.
 
From Qt 5.8 onwards, Qt Quick also supports rendering in software, and with
OpenVG.  This is realized by having additional scene graph adaptations, either
in form of plugins (openvg) or built-in to the Qt Quick library
(software). The default adaptation continues to rely directly on OpenGL.
 
From Qt 5.14 onwards, the default adaptation gains the option of rendering via
a graphics abstraction layer, the Qt Rendering Hardware Interface (RHI),
provided by the \l [QtGui]{Qt GUI} module. When enabled, no direct OpenGL calls are made.
Rather, the scene graph renders by using the APIs provided by the abstraction
layer, which is then translated into OpenGL, Vulkan, Metal, or Direct 3D calls.
Shader handling is also unified by writing shader code once, compiling to
\l{https://www.khronos.org/spir/}{SPIR-V}, and then translating to the language
appropriate for the various graphics APIs.
 
Starting with Qt 6.0, the RHI-based rendering model is the default, and there is
no option to fall back to directly using OpenGL.
 
\target Switching Between the Adaptation Used by the Application
\section1 Switch Between Adaptations in Your Application
 
Unlike \c software, the RHI-based renderer is not an additional adaptation, and
is always built-in. As of Qt 6.0 it is always enabled. On platforms where
multiple graphics APIs are available, the scenegraph makes a platform-specific
choice. If this is not desired, applications can force a specified graphics API
by setting the environment variable \c{QSG_RHI_BACKEND} or via \l
QQuickWindow::setGraphicsApi() in combination with \l
QSGRendererInterface::GraphicsApi.
 
Switching to a different adaptation can be achieved in two ways:
 
\list
    \li Use an environment variable - Set the \c{QT_QUICK_BACKEND} or the legacy
        \c{QMLSCENE_DEVICE} environment variable before launching applications.
    \li Use a C++ API - Call QQuickWindow::setSceneGraphBackend() early on in the application's
        main() function.
\endlist
 
The following backends are supported:
 
\list
    \li Default - Request with the \c{"rhi"} string or a QSGRendererInterface::GraphicsApi enum value
        different than the ones listed below.
    \li Software - Request with the \c{"software"} string or the QSGRendererInterface::Software
        enum value.
    \li OpenVG - Request with the \c{"openvg"} string or the QSGRendererInterface::OpenVG enum
        value.
\endlist
 
To find out which backend is in use, you can enable basic scene graph information logging via the
\c{QSG_INFO} environment variable or the \c{qt.scenegraph.general} logging category. This results
in some information being printed onto the debug output, during application startup.
 
\note In Qt builds with both OpenGL and Vulkan disabled, the default adaptation
is \c software. This does not apply to Windows or \macos however, because these
platforms always have Direct 3D or Metal support, respectively, enabled.
 
\note Typically, adaptations other than the default one come with a set of
limitations as they are unlikely to provide a feature set that's 100%
compatible with OpenGL. However, these adaptations may provide their own
specific advantages in certain areas. For more information on the various
adaptations, refer to the sections below.
 
\section1 Default Adaptation
 
When using OpenGL directly, the default adaptation is capable of providing the
full Qt Quick 2 feature set. For more details, see
\l{qtquick-visualcanvas-scenegraph-renderer.html}{Default 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. For more details, see
\l{qtquick-visualcanvas-adaptations-software.html}{Software Adaptation}.
 
\section1 OpenVG
 
The OpenVG adaptation is an alternative renderer for \l{Qt Quick} 2 that renders the contents of
the scene graph using OpenVG commands to provide hardware-accelerated 2D vector and raster
graphics. For more details, see
\l{qtquick-visualcanvas-adaptations-openvg.html}{OpenVG 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 a hardware-accelerated 3D graphics API.
Consequently, some features and optimizations are not available. Most Qt Quick 2 applications can run
without any modification, but any attempts to use unsupported features are ignored.
By using the Software adaptation, it is possible to run Qt Quick 2 applications on hardware and
platforms that do not have \l{topics-graphics}{hardware-accelerated 3D graphics API} support.
 
The Software adaptation was previously known as the Qt Quick 2D Renderer. However, unlike the 2D
Renderer, this new, integrated version supports partial updates. This means that a full update
of the window or screen contents is now avoided; only the changed areas are flushed. Partial
updates can significantly improve performance for many applications.
 
\section2 Shader Effects
 
ShaderEffect components in QtQuick 2 cannot be rendered by the Software adaptation.
 
\section2 Particle Effects
 
It is not possible to render particle effects with the Software adaptation. Whenever possible,
remove particles completely from the scene. Otherwise, they will still require some processing,
even though they are not 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, compared to when using a hardware-accelerated
3D graphics API. The quality is similar to choosing \l [QML] {Text::renderType}{Text.NativeRendering}
with \l [QML] {Text} items.
*/
 
 
/*!
\title Qt Quick OpenVG Adaptation
\page qtquick-visualcanvas-adaptations-openvg.html
 
The OpenVG adaptation is an alternative renderer for \l{Qt Quick} 2 that renders the contents of
the scene graph using OpenVG commands to provide hardware accelerated 2D vector and raster
graphics. Much like the \l{qtquick-visualcanvas-adaptations-software.html}{Software Adaptation},
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.
 
\section2 EGL Requirement
 
Unlike the default OpenGL Renderer, there is no built-in support to acquire an OpenVG context.
This means that the renderer is responsible for requesting and managing the the current context.
To do this, you use EGL directly in the OpenVG renderer. Consequently, the OpenVG renderer can only
be used with platform plugins that support creating QWindows with support for
QSurfaceFormat::OpenVG. From this window, the renderer can get an EGLSurface which can then be used
with an EGLContext to render OpenVG content.
 
\section2 Renderer
 
The OpenVG Renderer uses the OpenVG API to send commands and data to a Vector GPU that renders the
scene graph in an accelerated manner, offloading graphics rendering from the CPU. Many operations
like the rendering of rectangles and font glyphs are ideal for OpenVG because they can be
represented as paths which are stroked and filled. Rendering scene graph items that would typically
involve textures are handled in the OpenVG renderer using VGImage. Additionally, when you render
to offscreen surfaces (like with Layers), the scene subtree is rendered to a VGImage which can be
reused in the scene.
 
\section2 Render Loop
 
The OpenVG Renderer mirrors the behavior of the Basic render loop and it runs all OpenVG commands
in a single thread.
 
For more information on render loops, see
\l{qtquick-visualcanvas-scenegraph.html}{Qt Quick Scene Graph}.
 
\section2 Shader Effects
 
ShaderEffect components in QtQuick 2 can't be rendered by the OpenVG adaptation. While it's
possible to use ShaderEffectSource and QML Item Layers (which are both offscreen surfaces), it's
not possible to apply shader effects to them via the ShaderEffect item. This is because OpenVG
lacks an API for applying per vertex and per fragment shader operations. However, you may be able
to take advantage of Image Filter operations in the OpenVG API to get effects that are similar to
what ShaderEffects provides in custom items. To integrate custom OpenVG rendering, use
QSGRenderNode in combination with QSGRendererInterface.
 
\section2 Particle Effects
 
It's not possible to render particle effects with the OpenVG adaptation. Whenever possible, remove
particles completely from the scene. Otherwise they'll still require some processing, even though
they are not visible.
 
\section2 Rendering Text
 
Text rendering with the OpenVG adaptation is based on rendering the glyph paths, and doesn't use
the distance fields technique, unlike with the OpenGL backend.
 
\section2 Perspective Transforms
 
The OpenVG API doesn't allow paths to be transformed with non-affine transforms, but it's
possible with Qt Quick. Consquently, when you render components using paths like Rectangles and
Text while applying perspective transforms, the OpenVG backend first renders to a VGImage before
applying transformations. This behavior uses more memory at runtime and takes more time; avoid it
if possible.
 
*/