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/****************************************************************************
**
** Copyright (C) 2015 Klaralvdalens Datakonsult AB (KDAB).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the Qt3D module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL3$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see http://www.qt.io/terms-conditions. For further
** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPLv3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl.html.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or later as published by the Free
** Software Foundation and appearing in the file LICENSE.GPL included in
** the packaging of this file. Please review the following information to
** ensure the GNU General Public License version 2.0 requirements will be
** met: http://www.gnu.org/licenses/gpl-2.0.html.
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** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qt3d-overview.html
\title Qt3D Overview
\brief Qt3D provides C++ and QML APIs to incorporate 3D content into Qt
applications.
Qt3D provides a fully configurable renderer that enables developers to
quickly implement any rendering pipeline that they need. Further, Qt3D
provides a generic framework for near-realtime simulations beyond rendering.
Qt3D is cleanly separated into a core and any number of \e aspects that can
implement any functionality they wish. The aspects interact with
\e components and \e entities to provide some slice of functionality.
Examples of aspects include physics, audio, collision, artificial
intelligence (AI), and path finding.
\section1 Basic 3D Features
Qt3D is a 3D framework that enables the drawing of 3D shapes and moving
them around, as well as moving the camera. It supports the following basic
features:
\list
\li 2D and 3D rendering for C++ and Qt Quick applications
\li Meshes
\li Materials
\li Shadows
\li Shaders
\li Ambient occlusion
\li High dynamic range
\li Deferred rendering
\li Multitexturing
\li Instancing
\li Uniform Buffer Objects
\endlist
\section1 Configurable Renderer
To combine support for both C++ and QML APIs with having a fully
configurable renderer, the concept of a \e framegraph was introduced.
While a \e scenegraph is a data-driven description of \e what to render, a
framegraph is a data-driven description of \e how to render it.
A framegraph enables developers to choose between a simple forward renderer,
including a z-fill pass, or using a deferred renderer for example. It also
gives them control over when to render any transparent objects, and so on.
Since this is all configured purely from data, it is very easy to modify even
dynamically at runtime without touching any C++ code. It is possible to
extend Qt3D by creating your own framegraphs that implement custom
rendering algorithms.
\section1 3D Extensions
Beyond the essentials of displaying 3D content on the screen, Qt3D is
extensible and flexible enough to act as a host for following types of
extensions related to the 3D objects:
\list
\li Physics simulation
\li Collision detection
\li 3D positional audio
\li Rigid body, skeletal, and morph target animation
\li Path finding and other AI
\li Picking
\li Particles
\li Object spawning
\endlist
\section1 Performance
Qt3D is designed to perform well and scale up with the number of available
CPU cores, because modern hardware improves performance by increasing the
numbers of cores rather than base clock speed. Using multiple cores works
well, because many tasks are independent of each other. For example, the
operations performed by a path finding module do not overlap strongly with
the tasks performed by a renderer, except maybe when rendering debug
information or statistics.
\section1 Qt3D Architecture
The main use cases of Qt3D are simulating objects in near-realtime and
rendering the state of those objects onto the screen. The Space Invaders
example contains the following objects:
\image Space-invaders.jpg
\list
\li The player's ground cannon
\li The ground
\li The defensive blocks
\li The enemy space invader ships
\li The enemy boss flying saucer
\li The bullets shot by the enemies and the player
\endlist
In a traditional C++ design, these types of object would typically be
implemented as classes arranged in some kind of inheritance tree. Various
branches in the inheritance tree might add additional functionality to the
root class for features such as:
\list
\li Accepts user input
\li Plays a sound
\li Is animated
\li Collides with other objects
\li Is drawn on screen
\endlist
The types in the Space Invaders example can be classified against these
features. However, designing an elegant inheritance tree for even such a
simple example is not easy.
This approach and other variations on inheritance present a number of
problems:
\list
\li Deep and wide inheritance hierarchies are difficult to understand,
maintain and extend.
\li The inheritance taxonomy is set in stone at compile time.
\li Each level in the class inheritance tree can only classify upon a
single criteria or axis.
\li Shared functionality tends to \e {bubble up} the class hierarchy
over time.
\li It is impossible to predict what the developers will want to do.
\endlist
Extending deep and wide inheritance trees usually requires understanding,
and agreeing with, the taxonomy used by the original author. Therefore,
Qt3D places focus on aggregation instead of inheritance as the means of
imparting functionality onto an instance of an object. Specifically, Qt3D
implements an Entity Component System (ECS).
\section2 Using an ECS
In an ECS, an entity represents a simulated object but by itself is devoid
of any specific behavior or characteristics. Additional behavior can be
grafted onto an entity by having the entity aggregate one or more
components. Each component is a vertical slice of behavior of an object
type.
In the Space Invaders example, the ground is an entity with an attached
component that tells the system that the entity needs rendering and what
kind of rendering it needs. An enemy space invader ship is another entity
with attached components that cause the ship to be rendered, but also enable
it to emit sounds, be collided with, be animated, and be controlled by a
simple AI.
The player's ground cannon entity has mostly similar components to the enemy
space invader ship, except that it does not have the AI component. In its
place, the cannon has an input component to enable the player to move it
around and to fire bullets.
\section2 ECS Backend
\image ecs-2.png
The backend of Qt3D implements the \e system part of the ECS paradigm in
the form of \e aspects. An aspect implements the particular vertical slice
of the functionality provided to entities by a combination of one or more
of their aggregated components.
For example, the renderer aspect looks for entities that have mesh,
material, and optionally transformation components. If the renderer aspect
finds such an entity, it knows how to take that data and draw something nice
from it. If an entity does not have those components, the renderer aspect
ignores it.
Qt3D builds custom entities by aggregating components that provide
additional capabilities. The Qt3D engine uses aspects to process and
update entities with specific components.
For example, a physics aspect looks for entities that have some kind of
collision volume component and another component that specifies other
properties needed by such simulations like mass, coefficient of friction,
and so on. An entity that emits sound has a component that specifies it is
a sound emitter, as well as specifying when and which sounds to play.
Because ECS uses aggregation rather than inheritance, it is possible to
dynamically change how an object behaves at runtime simply by adding or
removing components.
For example, to enable a player to suddenly run through walls after a
power-up, that entity's collision volume component can be removed
temporarily, until the power-up times out. There is no need to create a
special one-off subclass for \c PlayerWhoRunsThroughWalls.
\section2 Qt3D ECS Implementation
Qt3D implements ECS as a simple class hierarchy. The Qt3D base class is
Qt3D::QNode, which is a subclass of QObject. Qt3D::QNode adds to QObject the ability to
automatically communicate property changes to aspects and an ID that is
unique throughout an application. The aspects exist in additional threads
and Qt3D::QNode simplifies the data transfer between the user-facing objects and
the aspects.
Typically, subclasses of Qt3D::QNode provide additional supporting data that is
referenced by components. For example, the QShaderProgram class specifies
the GLSL code to be used when rendering a set of entities.
\image ecs-1.png
Components in Qt3D are implemented by subclassing Qt3D::QComponent and adding the
data necessary for the corresponding aspect to do its work. For example, the
mesh component is used by the renderer aspect to retrieve the per-vertex
data that should be sent down the OpenGL pipeline.
Finally, Qt3D::QEntity is simply an object that can aggregate zero or more
Qt3D::QComponent instances.
\section1 Extending Qt3D
Adding functionality to Qt3D, either as part of Qt or specific to your
own applications to benefit from the multi-threaded back-end consists of the
following tasks:
\list
\li Identify and implement any necessary components and supporting data.
\li Register the components with the QML engine (only if you use the QML
API).
\li Subclass QAbstractAspect and implement the subsystem functionality.
\endlist
\section1 Qt3D Task-Based Engine
In Qt3D, aspects are asked in each frame for a set of \e tasks to execute
along with the \e dependencies between them. The tasks are distributed
across all the configured cores by a scheduler to improve performance.
\section1 Qt3D's Aspects
By default Qt3D provides the Qt3DRenderer and Qt3DInput aspects. The
components and other supporting classes provided by these aspects are
discussed in the documentation for those modules.
Additional aspects providing more capabilities will be added in future
versions of Qt3D.
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