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/*!
\title Scene Graph Adaptations
\page qtquick-visualcanvas-adaptations.html

\section1 Scene Graph Adaptations in Qt Quick

Originally Qt Quick always relied on OpenGL (OpenGL ES 2.0 or OpenGL 2.0) for parsing
the scene graph and rendering the results to a render target. From Qt 5.8 onwards
Qt Quick also supports rendering in software and with Direct3D 12.

\section1 Switching Between the Adaptation Used by the Application

The default rendering backend is still OpenGL, or - in Qt builds with disabled OpenGL support -
the software renderer. This 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.

\li OpenVG - Requested by the string \c{"openvg"} or the enum value QSGRendererInterface::OpenVG.

\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
adaptation 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. 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}

\section1 OpenVG

The OpenVG adaptation is an alternative renderer for \l {Qt Quick} 2 that will
renderer the contents of the scene graph using OpenVG commands to provide
hardware-acclerated 2D vector and raster graphics. The details for this
adaptation are available here:
\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 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.

\section2 Qt Graphical Effects Module
\l {Qt Graphical Effects} uses ShaderEffect items to render effects. If you use
graphical effects from this module, then you should not hide the source
item so that the original item can still be rendered.

\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
as when using OpenGL. The quality is similar to choosing \l [QML] {Text::renderType}
{Text.NativeRendering} with \l [QML] {Text} items.

\section2 Qt Multimedia VideoOutput
The VideoOutput item of the Qt Multimedia module is not supported with the software
adaptation.  This is because VideoOutput uses the QVideoRendererControl item which
requires custom QSGGeometryNode behavior that is only present in the default OpenGL
adaptation.


*/


/*!
\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

*/

/*!
\title Qt Quick OpenVG Adaptation
\page qtquick-visualcanvas-adaptations-openvg.html

The OpenVG adaptation is an alternative renderer for \l {Qt Quick} 2 that will
renderer the contents of the scene graph using OpenVG commands to provide
hardware-acclerated 2D vector and raster graphics. Much like the 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 defualt OpenGL Renderer, there is no built in support for acquiring
an OpenVG context.  This means that the renderer has the responsbility of
requesting and managing the the current context.  To do this EGL has to be used
directly in the OpenVG renderer.  This means that the OpenVG renderer is only
usable with platform plugins that support creating QWindows with support for
QSurfaceFormat::OpenVG. From this window, the renderer can get an EGLSurface
which can be used with an EGLContext to render OpenVG content.

\section2 Renderer
The OpenVG Renderer works by using the OpenVG API to send commands and data to
a Vector GPU which will render the scenegraph in an accelerated manner, offloading
graphics rendering from the CPU. Many operations like the rendering of rectangles
and fonts glyphs ideal for OpenVG because these can be represented as paths which
are stroked and filled. Rendering scenegraph items that would typically involve
textures are handled in the OpenVG renderer by using VGImage. In addition when
rendering to offscreen surfaces (like when using 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 will execute
all OpenVG commands in a single thread.

See the \l{qtquick-visualcanvas-scenegraph.html}{Scene Graph page} for more
information on render loops

\section2 Shader Effects
ShaderEffect components in QtQuick 2 can not be rendered by the OpenVG adaptation.
While it is possible to user ShaderEffectSource and QML Item Layers (which are both
offscreen surfaces), it is not actually 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.  It may be possible however to take
advantage of Image Filter operations in the OpenVG API to get similar effects to
what is provided by ShaderEffects in custom items. To integrate custom OpenVG
rendering, use QSGRenderNode in combination with QSGRendererInterface.

\section2 Qt Graphical Effects Module
\l {Qt Graphical Effects} uses ShaderEffect items to render effects. If you use
graphical effects from this module, then you should not hide the source
item so that the original item can still be rendered.

\section2 Particle Effects
It is not possible to render particle effects with the OpenVG 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 OpenVG adaptation is based on rendering the glpyh
paths, and does not use the distance fields technique used by the OpenGL backend.

\section2 Perspective Transforms
The OpenVG API does not allow paths to be transformed with non-affine transforms,
while it is possible with Qt Quick. This means that rendering components using
paths like Rectangles and Text, when applying perspective transforms the OpenVG
backend will first render to a VGImage before applying transformations. This uses
more memory at runtime and is a slower path so avoid doing this if necessary.

*/
