RFC-0162: Flatland API

RFC-0162: Flatland API
  • Graphics

Describes Flatland, the 2D composition API for Fuchsia.

Gerrit change
Date submitted (year-month-day)2021-05-24
Date reviewed (year-month-day)2021-05-30

Former API Design Document

This RFC was previously submitted as an API design document and converted to an RFC afterwards when the API design doc template was deprecated.


This document proposes a 2D API for Fuchsia graphics clients. Flatland offers a display-controller-like functionality to clients where the resources are defined in a 2D world.

Goals and use cases

Scenic currently provides graphics clients with a 3D API under the fuchsia.ui.gfx namespace (the "gfx api"). This 3D API provides clients with a scene model similar to video game engines or other 3D graphics programs. Drawing order is handled by Z depth and opacity is handled via alpha blending based on depth. Unfortunately the gfx API is no longer suitable for the demands being placed on Scenic, both from a product standpoint and a performance standpoint. Features like group opacity are impossible, because how to handle shadows for a group of translucent content is unclear.

From the product standpoint, our current customers are 2D products. There is no concept of depth and draw order. They are simply in the order that different batches of draw geometry are submitted in. Because there is no depth, transparency effects are also dictated by draw order. The "2D" clients (via Flutter, Chromium, and session framework) have to do extra work to resolve the mismatch between Scenic's 3D scene representation and the 2D representation experienced by the user.

From the performance standpoint, modern Video Display Controller (VDC) hardware provides acceleration features such as multiple display planes and hardware overlays that Scenic would like to leverage in the future to lower power consumption and GPU usage. The hardware operates in a strictly 2D paradigm however, and only understands rectangular layers that can be positioned in X/Y. Scenic's current 3D API allows and encourages clients to submit content that doesn't fit into this paradigm and frustrates optimization attempts.

By exposing a true 2D API we intend to make our 2D clients life better:

  • 2D API matches the 2D client's expectations more closely.
  • Less GPU usage by delegating work to the VDC where possible.
  • A more lightweight Scenic optimized for 2D rectangular layers only.


Our suggestion is to write a 2D protocol from scratch that doesn't reuse parts of fuchsia.ui.gfx.

The current state of the proposed 2D API is in the fuchsia.ui.scenic.internal.flatland library being submitted as part of this review. Please take a look at and drop comments.

Here are explanations for some high level decisions made around Flatland API:

  • Flatland closely follows and provides functionality similar to the display controller API defined on fuchsia.hardware.display/Controller.
    • The most performant scenario is when the Flatland implementation in Scenic passes its resources to the display without compositing. Therefore, there are common resources with the display controller API that are defined in the same manner. I.e., zx.handle:EVENT that are used as fences that signal when it is safe to access resources, are transported across these protocols and carry the same meaning and purpose.
  • Flatland aims to offer deterministic CPU cost to the clients. There is one deadline-scheduled render thread. Each Flatland session runs on their own dispatchers, which are on their own threads in the current configuration.
    • There may be multiple Flatland sessions where each is a channel speaking to Flatland for the purpose of rendering to a rectangular layer on the display. These sessions may not affect each other's presentation or performance flow.
  • Image allocations are enforced to be done through Scenic's Allocator protocol defined under fuchsia.ui.composition/Allocator.
    • All Image usages are zero-copy as the client and Scenic agrees on the allocation and formats before allocation happens using Sysmem.
    • Allocator allows Images to be used across multiple Flatland sessions.
  • Flatland does not offer a command union pattern. Present() call is the marker for processing the enqueued commands, which are individual methods.
    • Flatland strictly enforces limits on how many times the clients may call Present(). This is communicated through the response OnPresentProcessed(). This is used as a throttling mechanism.
    • Flatland may return an Error in OnPresentProcessed() callback to inform clients about an illegal operation. The Flatland channel is closed following this.
  • Flatland expects clients to define and keep track of unique resource identifiers. Type safety is enforced by the structs defined around these identifiers, such as TransformId and ContentId.
  • Flatland uses hanging-gets to notify clients about changes in the link structure or properties. These are emitted through a separate protocol.


Flatland design might evolve in some areas to better fit the clients needs.

  • The clients may expect the compositor to handle some 2D effects that are not handled by the display hardware, such as blur. These operations are more expensive. We are planning to make this distinction clear by decoupling these two categories of operations. The current status of Flatland only supports the operations that can be handled by the display.
  • Present and feedback flow evolves around our clients’ feedback. There were three iterations on gfx API's presentation flow before defining the current Flatland flow. Therefore, PresentArgs is defined as a table to allow some flexibility for the future changes.
    • We focus on the low-latency client and the high-throughput client in our design and presentation feedback. OnPresentProcessed() notifies the client when it is a good time to start work for the next frame. OnFramePresented() informs the advanced client about the time when the presented frame made it to the screen.


Please take a look at out in-tree tests on flatland_unittest to find an extensive set of examples of Flatland API usage.

Below is a figure that explains how a connected graph that links multiple Flatland sessions would work. This is a more complex but common use case.

Figure 1 - Flatland This figure presents the linking between Flatland


Flatland is currently defined as an internal API and testing is provided by in-tree unit tests. Since there is no other way to run Flatland code other than tests, we have been extensively covering every code with an automated test. We are planning to maintain this test coverage and quality.

We are planning to eventually convert every graphics example to Flatland. We do not have any clients that strictly need to live in a 3D world. We are currently working on having a Flatland presenter to be the basis for the addition and migration of in-tree integration tests https://fxbug.dev/42156206. This will prepare us towards the future migration of some critical external clients, such as Flutter and Chromium.

Flatland is designed to allow running business logic without dependency on hardware capabilities, such as Vulkan and display. Vulkan can be swapped by the null renderer implementation, which skips compositing. Display can be swapped the existing fake display implementation.

Additional Flatland API integration tests will be provided under Compatibility Test Suite.

Performance considerations

Flatland has been designed to allow many clients to operate at the same time without affecting each other’s work.

  • Each method is a one-way FIDL method representing a client's command to render to a rectangular portion of the display. Present() signals the end of the command sequence coming from the client and the start of Scenic's processing for the next display update.
  • Most clients make at least one call every vsync when actively updating their content. That is ~16ms for 60 fps display scenario. In each vsync interval:
    • Clients may make N calls to modify scene graph followed by one Present() call.
    • Flatland emits OnPresentProcessed() to inform the client that the operation has been queued and the client should begin producing its next frame. Currently, our clients have no idea when it's the ideal time to begin work and rely on hardcoded offsets to avoid overlaps and OnPresentProcessed() fixes that. This feedback also gives hints about the ideal future Present() call to the client, as well as informing them about their Present() allowance.
    • The client may ignore the timing and hints given in OnPresentProcessed() and keep presenting according to their internal clock. This is still valid, but there is no guarantee on avoiding resource contention.
    • Flatland emits OnFramePresented() to inform the client that the content has actually been displayed on the screen. This feedback is necessary for synchronization, such as audio/video, for the advanced client.
  • Flatland stops a malicious client from queuing too many Present() calls by explicitly defining num_presents_returned. The client may not call Present() more times than it is allowed by OnPresentProcessed(). Each client starts with one present allowance.
  • Remember that Flatland runs each channel connection on its own dispatcher. These feedback mechanisms are asynchronous.

Security considerations

Flatland users are isolated from each other. Each of them connect to Scenic through their own channels. Unique resource identifiers are defined only within the scope of their channel. They can only target the portion of the screen that is defined by their parent Flatland session.

Each error case causes Flatland channel closure. In these error states, it is not clear what should be drawn on the screen, so we don’t see any point to allow the client to keep presenting.

Privacy considerations

Flatland does not expose any device identifiers or privacy-sensitive information. The client does not interact with the hardware directly.

Flatland allows the client to set an identifiable debug string through SetDebugName(). This is used as a prefix when printing detailed system logs about errors to help the client distinguish what is theirs. The client has full control over what to set here, and if nothing is set, there is no prefix in system logs.

Drawbacks and alternatives

What we learned from the existing 3D api under fuchsia.ui.gfx is the basis for the decision made for the 2D API of Flatland. We took into account all the user feedback, bugs and lessons when making design decisions.

  • We could opt for the evolution of 2D API under the existing 3D API. However, there are some fundamental differences that would make this unnecessarily complicated for the client as well as the implementation.
  • Flatland could use a command union pattern like fuchsia.ui.gfx did. Currently, each Flatland command is mapped as a FIDL method. This decision was made because of the negatives observed with the existing command pattern in 3D API. We had to provide and maintain wrappers in different languages for the clients. However, there are some negatives about mapping each comment as a method. This design prevents batching until support for multiple messages in single write becomes available. However, we don’t expect the clients to manipulate the scene graph often and don’t consider this costly.

Future work

There are some areas that we plan to work on for improving Flatland API:

  • Each Flatland instance will have a ViewRef associated with it, and methods to grab a ViewRef of yourself and your children. See https://fxbug.dev/42159888.
  • A "factory function" to bind input protocols to a specific Flatland instance, thus limiting their scope to the instance's view sub-tree. See https://fxbug.dev/42159922.
  • Size and metrics flow one way, from parent to child, but the parent (and the mediating server) doesn't know which frame those size and metrics take effect, except for the very first frame. This has knock-on effects: user can see imperfect frames, and other APIs are exposed to latency in client logic. See https://fxbug.dev/42156345.
  • Synchronizing Present() across multiple instances is considered but not yet solved. See https://fxbug.dev/42159935.
  • Flatland will live under fuchsia.ui.composition namespace along with its dependencies. fuchsia.scenic.allocation and fuchsia.scenic.scheduling are also moving under there. See https://fxbug.dev/42158797.