Advanced Graphics and Animations for iOS Apps

Description: Creating a responsive UI requires an understanding of Core Animation and how mobile GPUs work. Learn about the iOS rendering pipeline in Core Animation, the new UIVisualEffectView and how it utilizes the GPU. Find out about the available tools for profiling UI performance. See how to identify and fix performance issues on a variety of devices.

Core Animation pipeline

  • It all starts in the application, which builds a view hierarchy.
  • This view hierarchy is defined indirectly via UIKit or directly with Core Animation.
  • The application process does not do the actual rendering work for Core Animation. Instead, this view hierarchy is committed to the render server, which is a separate process, and this render server has a server side version of Core Animation that receives the view hierarchy.
  • The view hierarchy is then rendered with Core Animation with OpenGL or metal. This is ran in the GPU, hence it's GPU accelerated.
  • Once the view hierarchy is being rendered, we can finally display it to the user.

How does this look like time wise within the application?

Note: the chart assumes a refresh rate of 60 hertz, hence each cycle lasts 16.67ms.

  • First the app receives an event (e.g., a touch) and the application decides that it wants to update a view hierarchy.
  • The actual update of the view hierarchy happens in a phase called the Commit Transaction phase.
  • Once this Commit Transaction phase ends, the view hierarchy is encoded and sent to the render server.
  • Then the render server decodes this view hierarchy and wait for the next resync, in order to wait for buffers to get back from a display that they can actually render to. Once the resync stars, the render server starts issuing Draw Calls for the GPU (OpenGL or metal).
  • Once the Draw Calls complete, the GPU starts doing its rendering work (as soon as its resources are available).
  • Ideally this rendering work finishes before the next resync, because then we can swap in the frame buffer and show the new view hierarchy to the user.

All the steps/phases above span over multiple frames, however the pipeline overlay all the steps so that they can run in parallel, hence the application will be able to refresh at a higher refresh rate:

Commit Transaction

The commit transaction is separated in four phases:

  1. Layout - Set up the views
  2. Display - Draw the views
  3. Prepare - Additional Core Animation work
  4. Commit - Package up layers and send them to render server

Layout phase

  • layoutSubviews overrides are called
  • This phase is where we add layers to the view hierarchy with addSubview
  • Populate content and do some lightweight database lookups (e.g., for localized strings - as they're needed for the content to be placed).
  • Usually CPU and I/O bound

Display phase

  • drawRect overrides are called
  • String drawing
  • Apple uses core graphics for this rendering via CGContext
  • Usually CPU or memory bound

Prepare phase

  • Image decoding (if there are any images in the view hierarchy)
  • Image conversion (if there are any images that are on a format that is not supported by the GPU

Commit phase

  • Package up layers and send to render server
  • Recursive
  • Expensive if layer tree is complex

Keep the layer tree as flat as possible to make sure that this phase is as efficient as it can be.

Animation

An animation is a three-stage process:

  1. (Application) Create animation and update view hierarchy (e.g., via UIView.animate(withDuration:animations:completion:))
  2. (Application) Prepare and commit animation (this is the coming transaction that we've seen above, where layoutSubviews and drawRect: are called)
  3. (Render server) Once the animation is committed, we give it to the render server that will render each frame appropriately (without the need for inter-process communication between application and render server)

Rendering concepts

Tile Based Rendering

  • Screen is split into tiles of NxN pixels
  • Each tile fits into the SoC cache
  • Geometry is split in tile buckets
  • Rasterization can begin after all geometry is submitted

Example, render of an app icon (a CALayer):

  • The CALayer in CoreAnimation is two triangles.
  • If you look at the two (blue) triangles (in the image above), they are spanning multiple tiles.
  • The GPU will start splitting up those (blue) triangles in each tile (forming the red triangles), so that each tile can be rendered individually.

Rendering pass

  • Once the render server decoded the committed view hierarchy, it needs to render it and it will use OpenGL or metal.
  • The render server will submit a command to the GPU.
  • The GPU will receive this command buffer and start doing its work.
  • Tiler stage: vertex processing is where the vertex shader runs, this transforms all vertices into screen space, so that we can then do the second stage, which is the actual tiling
  • the output of the Tiler stage is called Parameter Buffer
  • the GPU will wait until either:
    • all geometry is processed and sits in the parameter buffer
    • the parameter buffer is full (this is a performance hit because we need to flush the buffer and stall the vertex processing)
    • Renderer stage: where the pixel processing is done
    • the output of the Renderer stage is called Renderer buffer

Rendering examples

Note: these passes are what are usually referred to "offscreen passes".

Masking:

  • In the first pass we render the layer mask to a texture
  • In the second pass we render the layer content to a texture
  • in the last pass, called the compositing pass, we apply the layer mask texture to the content texture and composite to the reside to screen and end up with this light blue camera icon

UIBlurEffect:

  • In the first pass we render the content
  • in the second pass we downscale the rendered content, this downscale depends on the hardware
  • in the third and fourth passes the horizontal and vertical blur is applied, these two passes could have been done together, but it saves a lot of memory if done one by one
  • in the last pass we upscale the blurred image and tint it

Performance-wise:

  • the first and last passes are the ones that require more GPU time (the content rendering and upscale + tint)
  • between passes, there will be a small GPU idle time for content switching, the more passes we have, the more idle time we will get. These idle times are about 0.1-0.2 ms each, meaning that applying a UIBlurEffect will have around 0.4-0.8ms of GPU idle time. This starts to be significant when we want to accomplish everything within 16.67ms.
  • the darker the UIBlurEffect, the more performant.

UIBlurEffect + UIVibrancyEffect:

  • the first five passes are used for the blur
  • then we render the layer content to a texture
  • then we compose the blur output with the layer texture with a filter

Performance-wise:

  • like before the first and last passes of the blur phase are expensive
  • like the fist blur pass, the rendering of the layer content phase can also be expensive (depending on the content size)
  • lastly the filer pass is the most expensive pass of all
  • these two extra passes add even more GPU idle time for context switching, which now adds up to 0.6-1.2ms of GPU idle time

Tips:

  • apply vibrancy only to small parts of the screen
  • enable rasterization (CALayer's shouldRasterize), used to composite to image once with the GPU.
    • extra offscreen passes are added when updating the content, hence enable it only for static content
    • don't over use it, as its cache is limited to 2.5x of the screen size
  • rasterized images evicted from cache if unused for more than 100ms
  • always disable CALAyer's allowsGroupOpacity property on CALayer

Performance Investigation Mindset

  • What is the frame rate? Core Animation or OpenGL ES Driver instrument
  • CPU or GPU bound? OpenGL ES Driver and Time Profiler instrument
  • Any unnecessary CPU rendering? Time Profiler instrument
  • Too many offscreen passes? Core Animation instrument
  • Too much blending? Core Animation instrument
  • Any strange image formats or sizes? Core Animation instrument
  • Any expensive views or effects? Xcode View Debugger
  • Anything unexpected in hierarchy? Xcode View Debugger

Missing anything? Corrections? Contributions are welcome 😃

Written by

Federico Zanetello

Federico Zanetello

iOS Engineer with strong passion for Swift, minimalism, and design. When he’s not busy automating things, he can be found writing at FIVE STARS and/or playing with the latest shiny toys.