Scenic is a system service that composes graphical objects from multiple processes into a shared scene graph. These objects are rendered within a unified lighting environment (to a display or other render target); this means that the objects can cast shadows or reflect light onto each other, even if the originating processes have no knowledge of each other.
Scenic's responsibilities are:
Composition: Scenic provides a retained-mode 3D scene graph that contains content that is independently generated and linked together by its clients. Composition makes it possible to seamlessly intermix the graphical content of separately implemented UI components.
Animation: Scenic re-evaluates any time-varying expressions within models prior to rendering each frame, thereby enabling animation of model properties without further intervention from clients. Offloading animations to Scenic ensures smooth jank-free transitions.
Rendering: Scenic renders its scene graph using Escher, a rendering library built on Vulkan, which applies realistic lighting and shadows to the entire scene.
Scheduling: Scenic schedules scene updates and animations to anticipate and match the rendering target's presentation latency and refresh interval.
Diagnostics: Scenic provides a diagnostic interface to help developers debug their models and measure performance.
Scenic and Fuchsia
Scenic's API allows any client to insert its UI into the global scene graph. Processes using the UI framework Flutter are one example; the lower layer of Flutter, called Flutter Engine, contains code responsible for communicating with Scenic.
Scenic has several internal subsystems. Gfx owns the scene graph and is responsible for rendering. Input is responsible for routing input events to clients, which also involves coordinating gesture recognition across clients. Anim is a yet-to-be created system for coordinating transitions across clients as well as offloading animations to Scenic.
Escher is a Vulkan-based rendering library used by the Gfx system.
Root Presenter is an independent service which is responsible for presenting the system's UI; using the Scenic API, it creates the root of a Scenic scene graph, embeds the root process's UI, and reads input events using its Input Reader library and continually forwards them to Scenic.
Scenic is a client of the Vulkan graphics driver and the system Display Driver.
Scenic FIDL protocol is Scenic's front door. Each instance of the
protocol represents a Scenic instance. Each Scenic instance is an isolated
rendering context with its own content, render targets, and scheduling loop.
Scenic protocol allows a client to create a
is the communication channel used to publish graphical content to this instance.
A single Scenic instance can update, animate, and render multiple
Scenes (trees of graphical objects) to multiple targets in tandem on the same
scheduling loop. This means that the timing model for a Scenic instance
is coherent: all of its associated content belongs to the same scheduling
domain and can be seamlessly intermixed.
In practice, there is one instance of Scenic and one Scene that is rendered to a target. However, creating separate Scenic instances can be useful for rendering to targets which have very different scheduling requirements or for running tests in isolation. Independent Scenic instances cannot share content and are therefore not coherent amongst themselves.
When a Scenic instance is destroyed, all of its sessions become inoperable and its rendering ceases.
Views typically do not deal with the Scenic instance directly; instead
they receive a Scenic
Session from the view manager.
Session FIDL protocol is the primary API used by clients of Scenic to
contribute graphical content in the form of
Resources. Each session has
its own resource table and is unable to directly interact with resources
belonging to other sessions.
Each session provides the following operations:
- Submit operations to add, remove, or modify resources.
- Commit a sequence of operations to be presented atomically.
- Awaiting and signaling fences.
- Schedule subsequent frame updates.
- Form links with other sessions (by mutual agreement).
When a session is destroyed, all of its resources are released and all of its links become inoperable.
Views typically receive separate sessions from the view manager.
Resources represent scene elements such as nodes, shapes, materials, and
animations which belong to particular
The list of Scenic resources is described by the API: //sdk/fidl/fuchsia.ui.gfx/resources.fidl
Clients of Scenic generate graphical content to be rendered by queuing and submitting operations to add, remove, or modify resources within their session.
Each resource is identified within its session by a locally unique id which
is assigned by the owner of the session (by arbitrary means). Sessions
cannot directly refer to resources which belong to other sessions (even if
they happen to know their id) therefore content embedding between sessions
is performed using
Link objects as intermediaries.
To add a resource, perform the following steps:
- Enqueue an operation to add a resource of the desired type and assign it a locally unique id within the session.
- Enqueue one or more operations to set that resource's properties given its id.
Certain more complex resources may reference the ids of other resources
within their own definition. For instance, a
Node references its
Shape must be added before the
Node so that the node may
reference it as part of its definition.
To modify a resource, enqueue one or more operations to set the desired properties in the same manner used when the resource was added.
The remove a resource, enqueue an operation to remove the resource.
Removing a resource causes its id to become available for reuse. However,
the session maintains a reference count for each resource which is
internally referenced. The underlying storage will not be released (and
cannot be reused) until all remaining references to the resource have been
cleared and until the next frame which does not require the resource has
been presented. This is especially important for
See also Fences.
This process of addition, modification, and removal may be repeated indefinitely to incrementally update resources within a session.
Node resource represents a graphical object which can be assembled into
a hierarchy called a
node tree for rendering.
Here is a walk-through on how Scenic internally manages the lifecycle of Node-like resources and embedded Views.
TODO: Discuss this in more detail, especially hierarchical modeling concepts such as per-node transforms, groups, adding and removing children, etc.
Scene resource combines a tree of nodes with the scene-wide parameters
needed to render it. A Scenic instance may contain multiple scenes but
each scene must have its own independent tree of nodes.
A scene resource has the following properties:
- The scene's root node.
- The scene's global parameters such as its lighting model.
In order to render a scene, a
Camera must be pointed at it.
Compositors are resources that come in two flavors:
ImagePipeCompositor; their job is to draw the content of a
into their render target. For
DisplayCompositor, the target display may
have multiple hardware overlays; in this case the compositor may choose
associate each of these with a separate layer, rather than flattening the
layers into a single image.
LayerStack resource consists of an ordered list of
Layers. Each layer
can contain either an
Image (perhaps transformed by a matrix), or a
Camera that points at a
Scene to be rendered (as described above).
Scenic Resource Graph
TODO: More Resources
Add sections to discuss all other kinds of resources: shapes, materials, links, memory, images, buffers, animations, variables, renderers etc.
Coordinate Frames and Units
Scenic manages a global scene graph in a three dimensional space. Some of the characteristics of this space are defined by Scenic itself, whereas some are defined by the root presenter or even other clients.
Units are configured by the root presenter. The default root presenter uses a device-independent scalable unit called "pips" for the root space. See Units and Metrics for details. What units are used for your view space depends on what transforms are applied to your view by your parent.
The Scenic world space is a right handed Cartesian space. It is configured by the root presenter which configures the view and projection parameters of the camera. The default root presenter will put the origin at the top left of the screen and make +X point right, +Y point down, and +Z point into the screen.
Ultimately the space of a given view depends on what transforms are applied to it by its parent View and the parent View's parent and so on. If no rotation transform is applied and all scale transforms are positive along all axes then the View's axes will align with the axes of the root presenter and the handedness will match.
The bounds of the root view are defined by a min and a max point as follows:
Views and Bounds
View Bounds shows how to set up your view bounds, how to debug them with wireframe rendering, and explains how view bounds interact with hit testing.
TODO(SCN-1228): Talk about synchronization.
Frame scheduling explains how the frame scheduling API work and contains examples of how to use it.
Examples of using Scenic
- bouncing_ball: Simple example that doesn't use any wrapper library.
- simplest_app: Draws a background color that changes with user input.
- spinning_square: Draws a square and animates it.
- tile: Simple window-manager-like app that embeds child views.
- Other examples:
Scenic client libraries
Scenic has convenience wrapper libraries for some languages. These can be used instead of using the FIDL API directly.
- C++ client library (see also examples: simplest_app, spinning_square, simplest_embedder, tile)
- Rust client library
The following files define and document the collection of FIDL protocols that make up Scenic.
Scenic top-level protocols (
Talk about how to get started using Scenic, recommended implementation strategies, etc.
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Last updated 2020-02-12.