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Declarative Scenegraph CreationQt3D implements a scene-graph to make rendering more efficient in terms of both computation and space requirements. Loosely defined, a scene-graph is a tree-like structure which is used to represent a graphical scene logically. The variant of a scene-graph at the core of Qt3D's rendering is based on the QGLSceneNode class and breaks the contents of the scene down into nodes. Scene NodesNodes can represent several logical constructs used in rendering. A single node may include one or more of the following: GeometryGeometry describes the physical structure of an object in 3D space. It does this in terms of "vertices" which are joined together to form a triangle mesh. While many simple 3D objects can be represented by a single scene-node, complex objects such as those found in third party asset interchange formats (.obj, .3ds, etc) are comprised of multiple geometries stored across a multitude of scene-nodes. This allows for sub-sections of geometry to be animated, rendered with different materials and effects, and deconstructed as necessary. Materials/EffectsMaterials and effects describe how the surfaces of the geometry are rendered when the scene is drawn. They may describe simple parameters used to describe the object, such as color, texture images, alpha-blending, and other options. Alternatively the material may be described by a specific set of shader programmes in the GL shader-language which perform a custom implementation of rendering. Scene nodes can represent one or more materials which are then referenced when drawing the geometry of this nodes, or its child nodes. TransformationsTransformations in the scene-graph describe rotations around the x,y, or z axes, as well as scaling, translation, or other custom transformations in 3D space. A transformation node contains one or more transformations which are automatically applied to the current node, as well as all of its child nodes. Additional ParametersNodes can also contain additional parameters describing debug information, and other miscellaneous functions such as whether the node should be rendered, whether blending is enabled, and so on. Nodes can also represent picking information which is used for selecting 3D items with the mouse. Scene Graphs in C++When writing a C++ application the user can create one or more QGLSceneNode based scene-graphs which can then be rendered each frame by calling the QGLSceneNode::draw() function. Scene graphs can be constructed manually by the user one node at a time, or by using the QGLBuilder to assemble geometries. The Qt3D model loader plug-ins also create individual scene-graphs which can then be used separately, or inserted into an existing scene-graph. Model-loader scene-graphs represent all of the geometries, transformations, materials, and other elements, which are described in the model interchange file. These nodes can be manipulated at the code-level at any time to modify or adjust the model as needed. When the scenegraph is created, Qt3D optimises the drawing order of the scenegraph somewhat so that all materials of the same type are drawn first, and so on. These optimisations reduce the amount of context switching required by OpenGL when drawing, and can substantially improve performance in complicated scenes. QML 3D Scene Graphs in Qt3D for Qt 4.xWhile C++ allows users to manually construct and manipulate scene-graphs, Qt3D's extension into QtQuick does not currently allow the same degree of flexibility. Qt3D for Qt4.x generally used a single small scene-graph for each Item3D. These would only used shared geometry information if multiple Item3D instances referenced a single Mesh instance. While sufficient for simple 3D use cases, this method is inherently inefficient for drawing large/complex scenes. Without a single scenegraph we cannot optimise the entire 3D scene. Likewise in this implementation we have only a very limited ability to manipulate the content of a model file/meh instance. In Qt4.x users are able to specify that an Item3D contains a specific sub-node of a mesh's scenegraph. This removes the instance from the tree in the graph which owns it, thereby changing the structure of the originating model. This limits our ability to implement specific use-cases. These problems are addressed in Qt5. QML 3D Scene Graphs in Qt3D for Qt 5In Qt5 the QML implementation of Qt3D substantially improves this situation. Instead of creating individual scene-graphs for each Mesh/Item3D, the user now creates a single holistic scenegraph for each viewport. Addition of Item3D elements to a scene creates a hierarchy of scene-graph branches which are then optimised and drawn in a single call for the viewport. Components of a mesh can now be accessed with specific QQuickMesh functions which are exposed to QML. The sections below describe the various scenarios in which a user may wish to use sub-components of a model's scenegraph. Scenario 1: Simple Single-Model RenderingThis displays the model "as is" with no changes or alterations. Mesh {
id: model
source: "bike.3ds"
}
Item3D {
mesh: model
}
The output for this, as well as the scenegraphs it produces, can be seen below: This displays a sub-section of the model - in this case the wheel. The output for this, as well as the scenegraphs it produces, can be seen below: This displays an intermediate (sub)section of the graph but excludes specific nodes/ subtrees. The output for this, as well as the scenegraphs it produces, can be seen below: This displays the original model in one Item3D and a sub-tree of the model in another. The second item applies a transform to the item, which leaves the original model unchanged. The output for this, as well as the scenegraphs it produces, can be seen below: This displays the original model in one Item3D and a sub-tree of the model in another. The second item applies an effect to the item, which leaves the original model unchanged. The output for this, as well as the scenegraphs it produces, can be seen below: This displays an intermediate (sub)section of the graph to which an effect has been applied but excludes specific nodes subtrees. In a second Item3D it displays a sub-node/ tree which is not affected by the effect on the first item. The output for this, as well as the scenegraphs it produces, can be seen below: This displays an intermediate (sub)section of the graph but excludes specific nodes/ subtrees. In a second Item3D it displays a transformed sub-node/tree which does not affect the first item. The output for this, as well as the scenegraphs it produces, can be seen below: This displays disparate subtrees of the model as a single item. The output for this, as well as the scenegraphs it produces, can be seen below: |
