path: root/src/quick/doc/src/concepts/visualcanvas/scenegraph.qdoc

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/*!
\title Qt Quick Scene Graph
\page qtquick-visualcanvas-scenegraph.html

\section1 The Scene Graph in Qt Quick

Qt Quick 2 makes use of a dedicated scene graph based on OpenGL ES 2.0
or OpenGL 2.0 for its rendering. Using a scene graph for graphics
rather than the traditional imperative painting systems (QPainter and
similar), means the scene to be rendered can be retained between
frames and the complete set of primitives to render is known before
rendering starts. This opens up for a number of optimizations, such as
batch rendering to minimize state changes and discarding obscured
primitives.

For example, say a user-interface contains a list of ten items
where each item has a background color, an icon and a text. Using the
traditional drawing techniques, this would result in 30 draw calls and
a similar amount of state changes. A scene graph, on the other hand,
could reorganize the primitives to render such that all backgrounds
are drawn in one call, then all icons, then all the text, reducing the
total amount of draw calls to only 3. Batching and state change
reduction like this can greatly improve performance on some hardware.

The scene graph is closely tied to Qt Quick 2.0 and can not be used
stand-alone. The scene graph is managed and rendered by the
QQuickWindow class and custom Item types can add their graphical
primitives into the scene graph through a call to
QQuickItem::updatePaintNode().

The scene graph is a graphical representation of the Item scene, an
independent structure that contains enough information to render all
the items. Once it has been set up, it can be manipulated and rendered
independently of the state of the items. On many platforms, the scene
graph will even be rendered on a dedicated render thread while the GUI
thread is preparing the next frame's state.


\section1 Qt Quick Scene Graph Structure

The scene graph is composed of a number of predefined node types, each
serving a dedicated purpose. Although we refer to it as a scene graph,
a more precise definition is node tree. The tree is built from
QQuickItem types in the QML scene and internally the scene is then
processed by a renderer which draws the scene. The nodes themselves do
\b not contain any active drawing code nor virtual \c paint()
function.

Even though the node tree is mostly built internally by the existing
Qt Quick QML types, it is possible for users to also add complete
subtrees with their own content, including subtrees that represent 3D
models.


\section2 Nodes

The most important node for users is the \l QSGGeometryNode. It is
used to define custom graphics by defining its geometry and
material. The geometry is defined using \l QSGGeometry and describes
the shape or mesh of the graphical primitive. It can be a line, a
rectangle, a polygon, many disconnected rectangles, or complex 3D
mesh. The material defines how the pixels in this shape are filled.

A node can have any number of children and geometry nodes will be
rendered so they appear in child-order with parents behind their
children. \note This does not say anything about the actual rendering
order in the renderer. Only the visual output is guaranteed.

The available nodes are:
\annotatedlist{qtquick-scenegraph-nodes}

Custom nodes are added to the scene graph by subclassing
QQuickItem::updatePaintNode() and setting the
\l {QQuickItem::ItemHasContents} flag.

\warning It is crucial that OpenGL operations and interaction with the
scene graph happens exclusively on the render thread, primarily
during the updatePaintNode() call. The rule of thumb is to only
use classes with the "QSG" prefix inside the
QQuickItem::updatePaintNode() function.

For more details, see the \l {Scene Graph - Custom Geometry}.

\section3 Preprocessing

Nodes have a virtual QSGNode::preprocess() function, which will be
called before the scene graph is rendered. Node subclasses can set the
flag \l QSGNode::UsePreprocess and override the QSGNode::preprocess()
function to do final preparation of their node. For example, dividing a
bezier curve into the correct level of detail for the current scale
factor or updating a section of a texture.

\section3 Node Ownership

Ownership of the nodes is either done explicitly by the creator or by
the scene graph by setting the flag \l QSGNode::OwnedByParent.
Assigning ownership to the scene graph is often preferable as it
simplifies cleanup when the scene graph lives outside the GUI thread.


\section2 Materials

The material describes how the interior of a geometry in a \l
QSGGeometryNode is filled. It encapsulates an OpenGL shader program
and provides ample flexibility in what can be achieved, though most of
the Qt Quick items themselves only use very basic materials, such as
solid color and texture fills.

For users who just want to apply custom shading to a QML Item type,
it is possible to do this directly in QML using the \l ShaderEffect
type.

Below is a complete list of material classes:
\annotatedlist{qtquick-scenegraph-materials}

For more details, see the \l {Scene Graph - Simple Material}


\section2 Convenience Nodes

The scene graph API is very low-level and focuses on performance
rather than convenience. Writing custom geometries and materials from
scratch, even the most basic ones, requires a non-trivial amount of
code. For this reason, the API includes a few convenience classes to
make the most common custom nodes readily available.

\list
\li \l QSGSimpleRectNode - a QSGGeometryNode subclass which defines a
rectangular geometry with a solid color material.

\li \l QSGSimpleTextureNode - a QSGGeometryNode subclass which defines
a rectangular geometry with a texture material.
\endlist



\section1 Scene Graph and Rendering

The rendering of the scene graph happens internally in the
QQuickWindow class, and there is no public API to access it. There are
however, a few places in the rendering pipeline where the user can
attach application code. This can be to add custom scene graph
content or render raw OpenGL content. The integration points are
defined by the render loop.

For detailed description of how the scene graph renderer works, see
\l {Qt Quick Scene Graph Renderer}.


\section2 Threaded Render Loop

On many configurations, the scene graph rendering will happen on a
dedicated render thread. This is done to increase parallelism of
multi-core processors and make better use of stall times such as
waiting for a blocking swap buffer call. This offers significant
performance improvements, but imposes certain restrictions on where
and when interaction with the scene graph can happen.

The following is a simple outline of how a frame gets
composed with the threaded render loop.

\image sg-renderloop-threaded.jpg

\list 1

\li A change occurs in the QML scene, causing \c QQuickItem::update()
to be called. This can be the result of for instance an animation or
user input. An event is posted to the render thread to initiate a new
frame.

\li The render thread prepares to draw a new frame and makes the
OpenGL context current and initiates a blocks on the GUI thread.

\li While the render thread is preparing the new frame, the GUI thread
calls QQuickItem::updatePolish() to do final touch-up of items before
they are rendered.

\li GUI thread is blocked.

\li The QQuickWindow::beforeSynchronizing() signal is emitted.
Applications can make direct connections (using Qt::DirectConnection)
to this signal to do any preparation required before calls to
QQuickItem::updatePaintNode().

\li Synchronization of the QML state into the scene graph. This is
done by calling the QQuickItem::updatePaintNode() function on all
items that have changed since the previous frame. This is the only
time the QML items and the nodes in the scene graph interact.

\li GUI thread block is released.

\li The scene graph is rendered:
    \list 1

    \li The QQuickWindow::beforeRendering() signal is
    emitted. Applications can make direct connections
    (using Qt::DirectConnection) to this signal to use custom OpenGL calls
    which will then stack visually beneath the QML scene.

    \li Items that have specified QSGNode::UsePreprocess, will have their
    QSGNode::preprocess() function invoked.

    \li The renderer processes the nodes and calls OpenGL functions.

    \li The QQuickWindow::afterRendering() signal is
    emitted. Applications can make direct connections
    (using Qt::DirectConnection) to this signal to use custom OpenGL calls
    which will then stack visually over the QML scene.

    \li The rendered frame is swapped and QQuickWindow::frameSwapped()
    is emitted.

    \endlist

\li While the render thread is rendering, the GUI is free to advance
animations, process events, etc.

\endlist

The threaded renderer is currently used by default on Linux, Mac OS X
and EGLFS based QPA platforms, but this is subject to change. It is
possible to force use of the threaded renderer by setting \c
{QML_FORCE_THREADED_RENDERER=1} in the environment.


\section2 Non-threaded Render Loop

The non-threaded render loop is currently used by default on Windows
and non-EGLFS based embedded platforms. This is mostly a precautionary
measure, as not all combinations of OpenGL drivers and windowing
systems have been tested.

Even when using the non-threaded render loop, you should write your
code as if you are using the threaded renderer, as failing to do so
will make the code non-portable.

The following is a simplified illustration of the frame rendering
sequence in the non-threaded renderer.

\image sg-renderloop-singlethreaded.jpg


\section2 Mixing Scene Graph and OpenGL

The scene graph offers two methods for integrating OpenGL content:
by calling OpenGL commands directly and by creating a textured node
in the scene graph.

By connecting to the \l QQuickWindow::beforeRendering() and \l
QQuickWindow::afterRendering() signals, applications can make OpenGL
calls directly into the same context as the scene graph is rendering
to. As the signal names indicate, the user can then render OpenGL
content either under a Qt Quick scene or over it. The benefit of
integrating in this manner is that no extra framebuffer nor memory is
needed to perform the rendering. The downside is that Qt Quick decides
when to call the signals and this is the only time the OpenGL
application is allowed to draw.

The \l {Scene Graph - OpenGL Under QML} example gives an example on
how to use use these signals.

The other alternative is to create a FramebufferObject, render into it
and use the result as a textured node in the scene graph, for instance
using a QSGSimpleTextureNode. The \l {Scene Graph - Rendering FBOs}
and \l {Scene Graph - Rendering FBOs in a thread} examples show how
this can be done in an optimal manner.

A simple way of doing the same is to use a QQuickPaintedItem with
QQuickPaintedItem::FramebufferObject as render target and by calling
QPainter::beginNativePainting() before the OpenGL rendering and
QPainter::endNativePainting() after.

\warning When mixing OpenGL content with scene graph rendering, it is
important the application does not leave the OpenGL context in a state
with buffers bound, attributes enabled, special values in the z-buffer
or stencil-buffer or similar. Doing so can result in unpredictable
behavior.

\warning The OpenGL rendering code must be thread aware, as the
rendering might be happening outside the GUI thread.


\section2 Custom Items using QPainter

The QQuickItem provides a subclass, QQuickPaintedItem, which allows
the users to render content using QPainter.

\warning Using QQuickPaintedItem uses an indirect 2D surface to render
its content, either using software rasterization or using an OpenGL
framebuffer object (FBO), so the rendering is a two-step
operation. First rasterize the surface, then draw the surface. Using
scene graph API directly is always significantly faster.



\section1 Scene Graph Backend

In addition to the public API, the scene graph has an adaptation layer
which opens up the implementation to do hardware specific
adaptations. This is an undocumented, internal and private plugin API,
which lets hardware adaptation teams make the most of their hardware.
It includes:

\list

\li Custom textures; specifically the implementation of
QQuickWindow::createTextureFromImage and the internal representation
of the texture used by \l Image and \l BorderImage types.

\li Custom renderer; the adaptation layer lets the plugin decide how
the scene graph is traversed and rendered, making it possible to
optimize the rendering algorithm for a specific hardware or to make
use of extensions which improve performance.

\li Custom scene graph implementation of many of the default QML
types, including its text and font rendering.

\li Custom animation driver; allows the animation system to hook
into the low-level display vertical refresh to get smooth rendering.

\li Custom render loop; allows better control over how QML deals
with multiple windows.

\endlist

*/

/*!
  \title Qt Quick Scene Graph Renderer
  \page qtquick-visualcanvas-scenegraph-renderer.html

  This document explains how the scene graph renderer works internally
  so that one can write code that uses it in an optimal fashion, both
  performance-wise and feature-wise.

  One does not need to understand the internals of the renderer to get
  good performance.  However, it might help when integrating with the
  scene graph or to figure out why it is not possible to squeeze the
  maximum efficiency out of the graphics chip.

  \note Even in the case where every frame is unique and everything is
  uploaded from scratch, the default renderer will perform well.

  The Qt Quick items in a QML scene populates a tree of QSGNode
  instances.  Once created, this tree is a complete description of how
  a certain frame should be rendered. It does not contain any
  references back to the Qt Quick items at all and will on most
  platforms be processed and rendered in a separate thread. The
  renderer is a self contained part of the scene graph which traverses
  the QSGNode tree and uses geometry defined in QSGGeometryNode and
  shader state defined in QSGMaterial to schedule OpenGL state change
  and draw calls.

  If needed, the renderer can be completely replaced using the
  internal scene graph back-end API. This is mostly interesting for
  platform vendors who wish to take advantage of non-standard hardware
  features. For majority of use cases, the default renderer will be
  sufficient.

  The default renderer focuses on two primary strategies to optimize
  the rendering. Batching of draw calls and retention of geometry on
  the GPU.

  \section1 Batching

  Where a traditional 2D API, such as QPainter, Cairo or Context2D, is
  written to handle thousands of individual draw calls per frame,
  OpenGL is a pure hardware API and performs best when the number of
  draw calls is very low and state changes are kept to a
  minimum. Consider the following use case:

  \image visualcanvas_list.png

  The simplest way of drawing this list is on a cell-by-cell basis. First
  the background is drawn. This is a rectangle of a specific color. In
  OpenGL terms this means selecting a shader program to do solid color
  fills, setting up the fill color, setting the transformation matrix
  containing the x and y offsets and then using for instance
  \c glDrawArrays to draw two triangles making up the rectangle. The icon
  is drawn next. In OpenGL terms this means selecting a shader program
  to draw textures, selecting the active texture to use, setting the
  transformation matrix, enabling alpha-blending and then using for
  instance \c glDrawArrays to draw the two triangles making up the
  bounding rectangle of the icon. The text and separator line between
  cells follow a similar pattern. And this process is repeated for
  every cell in the list, so for a longer list, the overhead imposed
  by OpenGL state changes and draw calls completely outweighs the
  benefit that using a hardware accelerated API could provide.

  When each primitive is large, this overhead is negligible, but in
  the case of a typical UI, there are many small items which add up to
  a considerable overhead.

  The default scene graph renderer works within these
  limitations and will try to merge individual primitives together
  into batches while preserving the exact same visual result. The
  result is fewer OpenGL state changes and a minimal amount of draw
  calls, resulting in optimal performance.

  \section2 Opaque Primitives

  The renderer separates between opaque primitives and primitives
  which require alpha blending. By using OpenGL's Z-buffer and giving
  each primitive a unique z position, the renderer can freely reorder
  opaque primitives without any regard for their location on screen
  and which other elements they overlap with. By looking at each
  primitive's material state, the renderer will create opaque
  batches. From Qt Quick core item set, this includes Rectangle items
  with opaque colors and fully opaque images, such as JPEGs or BMPs.

  Another benefit of using opaque primitives, is that opaque
  primitives does not require \c GL_BLEND to be enabled which can be
  quite costly, especially on mobile and embedded GPUs.

  Opaque primitives are rendered in a front-to-back manner with
  \c glDepthMask and \c GL_DEPTH_TEST enabled. On GPUs that internally do
  early-z checks, this means that the fragment shader does not need to
  run for pixels or blocks of pixels that are obscured. Beware that
  the renderer still needs to take these nodes into account and the
  vertex shader is still run for every vertex in these primitives, so
  if the application knows that something is fully obscured, the best
  thing to do is to explicitly hide it using Item::visible or
  Item::opacity.

  \note The Item::z is used to control an Item's stacking order
  relative to its siblings. It has no direct relation to the renderer and
  OpenGL's Z-buffer.

  \section2 Alpha Blended Primitives

  Once opaque primitives have been drawn, the renderer will disable
  \c glDepthMask, enable \c GL_BLEND and render all alpha blended primitives
  in a back-to-front manner.

  Batching of alpha blended primitives requires a bit more effort in
  the renderer as elements that are overlapping need to be rendered in
  the correct order for alpha blending to look correct. Relying on the
  Z-buffer alone is not enough. The renderer does a pass over all
  alpha blended primitives and will look at their bounding rect in
  addition to their material state to figure out which elements can be
  batched and which can not.

  \image visualcanvas_overlap.png

  In the left-most case, the blue backgrounds can be drawn in one call
  and the two text elements in another call, as the texts only overlap
  a background which they are stacked in front of. In the right-most
  case, the background of "Item 4" overlaps the text of "Item 3" so in
  this case, each of backgrounds and texts need to be drawn using
  separate calls.

  Z-wise, the alpha primitives are interleaved with the opaque nodes
  and may trigger early-z when available, but again, setting
  Item::visible to false is always faster.

  \section2 Mixing with 3D primitives

  The scene graph can support pseudo 3D and proper 3D primitives. For
  instance, one can implement a "page curl" effect using a
  ShaderEffect or implement a bumpmapped torus using QSGGeometry and a
  custom material. While doing so, one needs to take into account that
  the default renderer already makes use of the depth buffer.

  The renderer modifies the vertex shader returned from
  QSGMaterialShader::vertexShader() and compresses the z values of the
  vertex after the model-view and projection matrices has been applied
  and then adds a small translation on the z to position it the
  correct z position.

  The compression assumes that the z values are in the range of 0 to
  1.

  \section2 Texture Atlas

  The active texture is a unique OpenGL state, which means that
  multiple primitives using different OpenGL textures cannot be
  batched. The Qt Quick scene graph for this reason allows multiple
  QSGTexture instances to be allocated as smaller sub-regions of a
  larger texture; a texture atlas.

  The biggest benefit of texture atlases is that multiple QSGTexture
  instances now refer to the same OpenGL texture instance. This makes
  it possible to batch textured draw calls as well, such as Image
  items, BorderImage items, ShaderEffect items and also C++ types such
  as QSGSimpleTextureNode and custom QSGGeometryNodes using textures.

  \note Large textures do not go into the texture atlas.

  Atlas based textures are created by passing
  QQuickWindow::TextureCanUseAtlas to the
  QQuickWindow::createTextureFromImage().

  \note Atlas based textures do not have texture coordinates ranging
  from 0 to 1. Use QSGTexture::normalizedTextureSubRect() to get the
  atlas texture coordinates.

  The scene graph uses heuristics to figure out how large the atlas
  should be and what the size threshold for being entered into the
  atlas is. If different values are needed, it is possible to override
  them using the environment variables \c {QSG_ATLAS_WIDTH=[width]},
  \c {QSG_ATLAS_HEIGHT=[height]} and \c
  {QSG_ATLAS_SIZE_LIMIT=[size]}. Changing these values will mostly be
  interesting for platform vendors.

  \section1 Batch Roots

  In addition to mergin compatible primitives into batches, the
  default renderer also tries to minimize the amount of data that
  needs to be sent to the GPU for every frame. The default renderer
  identifies subtrees which belong together and tries to put these
  into separate batches. Once batches are identified, they are merged,
  uploaded and stored in GPU memory, using Vertex Buffer Objects.

  \section2 Transform Nodes

  Each Qt Quick Item inserts a QSGTransformNode into the scene graph
  tree to manage its x, y, scale or rotation. Child items will be
  populated under this transform node.  The default renderer tracks
  the state of transform nodes between frames, and will look at
  subtrees to decide if a transform node is a good candidate to become
  a root for a set of batches. A transform node which changes between
  frames and which has a fairly complex subtree, can become a batch
  root.

  QSGGeometryNodes in the subtree of a batch root are pre-transformed
  relative to the root on the CPU. They are then uploaded and retained
  on the GPU. When the transform changes, the renderer only needs to
  update the matrix of the root, not each individual item, making list
  and grid scrolling very fast. For successive frames, as long as
  nodes are not being added or removed, rendering the list is
  effectively for free. When new content enters the subtree, the batch
  that gets it is rebuilt, but this is still relatively fast. There are
  usually several unchanging frames for every frame with added or
  removed nodes when panning through a grid or list.

  Another benefit of identifying transform nodes as batch roots is
  that it allows the renderer to retain the parts of the tree that has
  not changed. For instance, say a UI consists of a list and a button
  row. When the list is being scrolled and delegates are being added
  and removed, the rest of the UI, the button row, is unchanged and
  can be drawn using the geometry already stored on the GPU.

  The node and vertex threshold for a transform node to become a batch
  root can be overridden using the environment variables \c
  {QSG_RENDERER_BATCH_NODE_THRESHOLD=[count]} and \c
  {QSG_RENDERER_BATCH_VERTEX_THRESHOLD=[count]}. Overriding these flags
  will be mostly useful for platform vendors.

  \note Beneath a batch root, one batch is created for each unique
  set of material state and geometry type.

  \section2 Clipping

  When setting Item::clip to true, it will create a QSGClipNode with a
  rectangle in its geometry. The default renderer will apply this clip
  by using scissoring in OpenGL. If the item is rotated by a
  non-90-degree angle, the OpenGL's stencil buffer is used. Qt Quick
  Item only supports setting a rectangle as clip through QML, but the
  scene graph API and the default renderer can use any shape for
  clipping.

  When applying a clip to a subtree, that subtree needs to be rendered
  with a unique OpenGL state. This means that when Item::clip is true,
  batching of that item is limited to its children. When there are
  many children, like a ListView or GridView, or complex children,
  like a TextArea, this is fine. One should, however, use clip on
  smaller items with caution as it prevents batching. This includes
  button label, text field or list delegate and table cells.

  \section2 Vertex Buffers

  Each batch uses a vertex buffer object (VBO) to store its data on
  the GPU. This vertex buffer is retained between frames and updated
  when the part of the scene graph that it represents changes.

  By default, the renderer will upload data into the VBO using
  \c GL_STATIC_DRAW. It is possible to select different upload strategy
  by setting the environment variable \c
  {QSG_RENDERER_BUFFER_STRATEGY=[strategy]}. Valid values are \c
  stream and \c dynamic. Changing this value is mostly useful for
  platform vendors.

  \section1 Performance

  As stated in the beginning, understanding the finer details of the
  renderer is not required to get good performance. It is written to
  optimize for common use cases and will perform quite well under
  almost any circumstance.

  \list

  \li Good performance comes from effective batching, with as little
  as possible of the geometry being uploaded again and again. By
  setting the environment variable \c {QSG_RENDERER_DEBUG=render}, the
  renderer will output statistics on how well the batching goes, how
  many batches, which batches are retained and which are opaque and
  not. When striving for optimal performance, uploads should happen
  only when really needed, batches should be fewer than 10 and at
  least 3-4 of them should be opaque.

  \li The default renderer does not do any CPU-side viewport clipping
  nor occlusion detection. If something is not supposed to be visible,
  it should not be shown. Use \c {Item::visible: false} for items that
  should not be drawn. The primary reason for not adding such logic is
  that it adds additional cost which would also hurt applications that
  took care in behaving well.

  \li Make sure the texture atlas is used. The Image and BorderImage
  items will use it unless the image is too large. For textures
  created in C++, pass QQuickWindow::TextureCanUseAtlas when
  calling QQuickWindow::createTexture().
  By setting the environment variable \c {QSG_ATLAS_OVERLAY} all atlas
  textures will be colorized so they are easily identifiable in the
  application.

  \li Use opaque primitives where possible. Opaque primitives are
  faster to process in the renderer and faster to draw on the GPU. For
  instance, PNG files will often have an alpha channel, even though
  each pixel is fully opaque. JPG files are always opaque. When
  providing images to an QQuickImageProvider or creating images with
  QQuickWindow::createTextureFromImage(), let the image have
  QImage::Format_RGB32, when possible.

  \li Be aware of that overlapping compond items, like in the
  illustration above, can not be batched.

  \li Clipping breaks batching. Never use on a per-item basis, inside
  tables cells, item delegates or similar. Instead of clipping text,
  use eliding. Instead of clipping an image, create a
  QQuickImageProvider that returns a cropped image.

  \li Batching only works for 16-bit indices. All built-in items use
  16-bit indices, but custom geometry is free to also use 32-bit
  indices.

  \li Some material flags prevent batching, the most limiting one
  being QSGMaterial::RequiresFullMatrix which prevents all batching.

  \endlist

  If an application performs poorly, make sure that rendering is
  actually the bottleneck. Use a profiler! The environment variable \c
  {QSG_RENDER_TIMING=1} will output a number of useful timing
  parameters which can be useful in pinpointing where a problem lies.

 */