// Copyright (C) 2015 Klaralvdalens Datakonsult AB (KDAB).
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \page qt3d-overview.html
    \title Qt 3D Overview
    \ingroup explanations-2dand3dgraphics
    \brief Qt 3D provides C++ and QML APIs to incorporate 3D content into Qt
    applications.

    Qt 3D provides a fully configurable renderer that enables developers to
    quickly implement any rendering pipeline that they need. Further, Qt 3D
    provides a generic framework for near-realtime simulations beyond rendering.

    Qt 3D 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

    Qt 3D 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 \l {Qt3DRender}{rendering} for C++ and Qt Quick applications
        \li \l{Qt3DRender::QMesh}{Meshes} and \l {Qt 3D Render Geometry}{Geometry}
        \li \l {Materials}
        \li \l {Shaders}
        \li \l {Shadow Mapping}{Shadow mapping}
        \li \l {Qt3DExtras::QMetalRoughMaterial::ambientOcclusion}{Ambient occlusion}
        \li High dynamic range
        \li \l {Deferred Renderer}{Deferred rendering}
        \li Multitexturing
        \li \l {Instanced Rendering}{Instanced rendering}
        \li \l {Uniform Buffer Objects}
        \li \l {Qt 3D Render Porting to RHI}{Porting to RHI}
        \li \l {Qt 3D Render Pro Tips}{Pro Tips}
    \endlist

    \section2 Materials

    Qt 3D has a robust and very flexible material system that allows multiple
    levels of customization. It caters for different rendering approaches on
    different platforms or OpenGL versions, enables multiple rendering passes
    with different state sets, provides mechanisms for overriding of parameters
    at different levels, and allows easy switching of shaders. All this from C++
    or using QML property bindings.

    The properties of a \l Material type can easily be mapped through to uniform
    variables in a GLSL shader program that is itself specified in the
    referenced effect property.

    For examples of using materials, see the following examples:
    \list
      \li \l {Qt 3D: Simple Custom Material QML Example}
      \li \l {Qt 3D: PBR Materials QML Example}
    \endlist

    \section2 Shaders

    Qt 3D supports all of the OpenGL programmable rendering pipeline stages:
    vertex, tessellation control, tessellation evaluation, geometry, and
    fragment shaders. Compute shaders are planned for a future release.

    For examples of using shaders, see the \l{Qt 3D: Wireframe QML Example}.

    \section2 Shadow Mapping

    Shadows are not directly supported by OpenGL, but there are countless
    techniques that can be employed to generate them. Shadow mapping is simple
    to use for generating good-looking shadows, while having a very small
    performance cost.

    Shadow mapping is typically implemented using a two pass rendering. In the
    first pass, the shadow information is generated. In the second pass, the
    scene is generated using a particular rendering technique, while at the
    same time using the information gathered in the first pass to draw the
    shadows.

    The idea behind shadow mapping is that only the closest fragments to the
    light are lit. Fragments \e behind other fragments are occluded, and
    therefore in shadow.

    Therefore, in the first pass, the scene is drawn from the point of view of
    the light. The information that is stored is simply the distance of the
    closest fragment in this \e {light space}. In OpenGL terms, this corresponds
    to having a Framebuffer Object, or FBO, with a depth texture attached to it.
    In fact, the \e {distance from the eye} is the definition of the depth,
    and the default depth testing done by OpenGL will actually store only the
    depth for the closest fragment.

    A color texture attachment is not even needed, because there is no need to
    shade fragments, only to calculate their depth.

    The following image displays a scene with a self-shadowed plane and trefoil
    knot:

    \image shadowmapping-qt3d.png {A 3D rendered scene showing a metallic knot
           hovering above a green plane and an orange plane flying in the
           background}

    The following image shows an exaggerated shadow map texture of the scene:

    \image shadowmapping-depth.png {Grayscale depth map of a 3D knot and
           airplane}

    The image indicates the depth stored when rendering the scene from the light
    point of view. Darker colors represent a shallow depth (that is, closer to
    the camera). In this scene, the light is placed somewhere above the objects
    in the scene, on the right side with respect to the main camera (compare
    this with the first screenshot). This matches with the fact that the toy
    plane is closer to the camera than the other objects.

    Once the shadow map has been generated, the second rendering pass is done.
    In this second pass, rendering is done using the normal scene's camera. Any
    effect can be used here, such as Phong shading. It is important that the
    shadow map algorithm is applied in the fragment shader. That is, the
    fragment that is closest to the light is drawn lit, whereas the other
    fragments are drawn in shadow.

    The shadow map generated in the first pass provides the necessary
    information about the distance of fragments to light. It then suffices to
    remap the fragment in light space, thereby calculating its depth from the
    light point of view, as well as where its coordinates are on the shadow map
    texture. The shadow map texture can then be sampled at the given coordinates
    and the fragment's depth can be compared with the result of the sampling. If
    the fragment is further away, then it is in shadow; otherwise it is lit.

    \section2 Instanced Rendering

    \e Instancing is a way of getting the GPU to draw many copies (instances) of
    a base object that varies in some way for each copy. Often, in position,
    orientation, color, material properties, scale, and so on. Qt 3D provides an
    API similar to the Qt Quick \l Repeater element. In this case, the delegate
    is the base object and the model provides the per-instance data. So whereas
    an entity with a \l Mesh component attached eventually gets transformed into
    a call to glDrawElements, an entity with a instanced component will be
    translated into a call to glDrawElementsInstanced.

    Instanced rendering is planned for a future release.

    \section2 Uniform Buffer Objects

    A Uniform Buffer Object (UBO) can be bound to OpenGL shader programs to make
    large amounts of data readily available. Typical use cases for UBOs are for
    sets of material or lighting parameters.

    \section2 Useful Tips

    Some very useful programming tips for 3D rendering can be found on this
    page: \l {Qt 3D Render Pro Tips}.

    \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 \l
    {Qt 3D Render Framegraph}{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 Qt 3D by creating your own framegraphs that implement custom
    rendering algorithms.

    \section1 3D Extensions

    Beyond the essentials of displaying 3D content on the screen, Qt 3D is
    extensible and flexible enough to act as a host for the 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

    Qt 3D 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 Qt 3D Architecture

    The main use cases of Qt 3D 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 {Screenshot of a space invaders gameplay}

    \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,
    Qt 3D places focus on aggregation instead of inheritance as the means of
    imparting functionality onto an instance of an object. Specifically, Qt 3D
    implements an Entity Component System (ECS).

    \section2 Using an ECS

    In an ECS, an entity represents a simulated object, but by itself it 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 {Diagram showing components of QEntity}

    The backend of Qt 3D 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.

    Qt 3D builds custom entities by aggregating components that provide
    additional capabilities. The Qt 3D 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 Qt 3D ECS Implementation

    Qt 3D implements ECS as a simple class hierarchy. The Qt 3D base class is
    Qt3DCore::QNode, which is a subclass of QObject. Qt3DCore::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 Qt3DCore::QNode simplifies the data transfer between the user-facing objects and
    the aspects.

    Typically, subclasses of Qt3DCore::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 {Diagram showing relation between QEntity, QComponent, QNode and QObject}

    Components in Qt 3D are implemented by subclassing Qt3DCore::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, Qt3DCore::QEntity is simply an object that can aggregate zero or more
    Qt3DCore::QComponent instances.

    \section1 Extending Qt 3D

    Adding functionality to Qt 3D, 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 Qt 3D Task-Based Engine

    In Qt 3D, 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 Qt 3D's Aspects

    By default, Qt 3D provides the Qt3DRender 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 Qt 3D.
*/
