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Creating natural stylus experiences with low latency graphics and movement prediction APIs
With the rising reputation of tablets and foldable gadgets, customers are adopting new behaviors with their gadgets. The bigger display screen space allows new experiences and powers productiveness duties beforehand reserved for laptops and desktops.
Handwriting, drawing and sketching with a stylus unlock exact and natural workflows, similar to we may do within the analog world. Nevertheless, the delay between gestures and rendering can stop the pure feeling of writing on a bit of paper, that is known as latency.
I am completely satisfied to share that the Android workforce has launched two new libraries: Low Latency Graphics and Movement Prediction, presently alpha02. The libraries tackle stylus enter latency on Android and ChromeOS.
The purpose of the Low Latency Graphics Library is to scale back the processing time between stylus enter and rendering on the display screen. The movement prediction library leverages a strong algorithm to foretell the place the subsequent stylus transfer will probably be. Implement each libraries collectively to supply the bottom doable perceived latency.
On this article, we will discover how these libraries work and how one can implement a best-in-class pen expertise in your app. Whereas we concentrate on the stylus expertise, these two libraries work with any pointer, finger, stylus, capacitive stylus, and mouse.
However earlier than we get into the main points, let’s check out simply how good this expertise will be:
With out latency optimizations, the system has to undergo many steps to render the pixels on the display screen. The Low Latency Graphics Library minimizes latency by profiting from entrance buffered rendering. The result’s quicker rendering on the display screen.
Perceive conventional rendering
Conventional rendering makes use of a number of buffers. This ensures a fluid consumer expertise with out visible tearing, however the trade-off is extra latency between the consumer’s interplay with the display screen and when the content material is seen on the display screen. To be taught extra about rendering and buffering, see the Mission Butter presentation.
With a multi-buffered rendering method, the show buffer incorporates information that the consumer presently sees on the display screen, and the rendering buffers are utilized by the graphics engine to render the information (in reminiscence).
After a render is full, the show buffer is swapped with the render buffer in order that the consumer can view the outcome on the display screen. The render buffer turns into the show buffer, and the show buffer is freed for future use, the place the rendering now happens.
That is doable because of SurfaceFlinger and WindowManager. The next animation exhibits the method.
Entrance Buffer Rendering – On to Display
To attain the entrance damping method, entrance and double damping layers are launched. The entrance layer is designed to rapidly render small parts of the display screen. The layer is clear and ephemeral, as we’ll see in a second.
Inputs from the entrance layer are processed within the double buffered layer to protect work. After which the entrance buffer is hidden.
However you is likely to be questioning how entrance buffering manages to render quicker than the conventional double buffering method and what are the caveats.
The low latency graphics library writes on to the entrance finish buffer, in different phrases, on to the display screen. This works as a result of solely a small space of the display screen is modified (solely a small a part of the stylus stroke is modified every body).
As soon as the stylus is lifted, common rendering with the double-buffered method resumes and the stroke is maintained.
When to make use of the low latency graphics library
The entrance buffer method works greatest for particular use instances, significantly handwriting, drawing, and sketching. These actions modify solely a small a part of the display screen, which is the place the library shines. If we have been to switch bigger areas, this might create tears and artifacts.
The low latency graphics library will not be meant for rendering a full display screen, for instance a recreation, or when altering massive areas akin to panning or zooming. Android has different instruments, akin to Android Recreation Growth and common OpenGL rendering, that assist builders in these circumstances.
The low latency graphics library is offered for Android 10 (API degree 29) and better and ChromeOS gadgets working Android 11 and better.
Earlier than we take a look at what the code seems like, you may wish to take into consideration your utility and assess what job needs to be processed within the entrance buffer (brief job that impacts a small a part of the display screen, akin to a stroke) and what needs to be processed within the entrance buffer. double buffer layer (bigger areas, akin to pan and zoom).
Let’s begin coding! The
GLFrontBufferedRenderer works with its personal information kind, which might usually comprise x and y coordinates, but in addition coloration, brush kind, stress, and so forth.
The strategy requires two callbacks the place the OpenGL code will probably be executed, one to render to the entrance buffer (delta of the stroke) with
onDrawFrontBufferedLayerand the opposite for double buffering (persistence) with
I like to recommend the Stylus on your app presentation, from my colleague Nader Jawad, in addition to the Android Graphics presentation launched on the Android Developer Summit 2022.
It’s also possible to confer with the Low Latency Graphics Library documentation.
Now that you understand how to scale back latency within the graphics layer, let us take a look at how we will additional enhance responsiveness by lowering perceived latency with the movement prediction library.
The purpose of the movement prediction library is to go one step forward of rendering and additional scale back perceived latency by creating synthetic factors based mostly on the Kalman filter algorithm powered by mathematical prediction. Movement prediction improves the consumer expertise by saving extra milliseconds and contributing to the natural feeling of ink flowing from the stylus.
Prediction Methods: The Kalman Filter Algorithm
With out going too deep into how the Kalman filter algorithm works, it is necessary to know that the algorithm depends on course, pace, stress, and time to foretell the place the subsequent level will probably be.
The algorithm predicts in a short time by leveraging a data-poor set of equations and by understanding the variance and uncertainty of the enter.
This prediction technique is quick and reminiscence environment friendly. The algorithm is used to trace planes, satellites, and now the motion of the pen on Android. Different prediction strategies could also be used on completely different gadgets, relying on the OEMs.
Movement prediction on Android
The movement prediction library will get enter from actual customers as MotionEvent objects. MotionEvent incorporates details about x and y coordinates, stress, time… all harnessed to feed the movement predictor and predict the subsequent MotionEvent.
As now we have seen above, the contact sampling price can differ from system to system. The upper the contact sampling price, the quicker information factors will be offered to the movement predictor, the quicker the algorithm can precisely predict. The movement prediction library can detect the sampling price and use it to gather info quicker and enhance prediction accuracy.
When to make use of movement prediction
The movement prediction library relies on MotionEvent, so so long as you are working with MotionEvent objects, you may make the most of the library.
Predicted movement occasions are synthetic and inaccurate by definition. Use the expected occasion to scale back perceived latency, however make sure that the expected information is changed with the precise information as soon as it’s obtained.
The movement prediction library is offered from Android 4.4 (API degree 19) and better and ChromeOS gadgets working Android 9 and better.
The movement occasion predictor provides two strategies,
report that takes
MotionEvent objects and
predict which returns a
See the movement prediction library documentation for extra particulars.
Efficiency immediately associated to the traits of the display screen
For contact screens to precisely register pointer inputs, the system should incessantly examine that the display screen is touchable. The contact or pen pattern price is how usually the system checks the display screen. Most screens have a contact pattern price that’s twice the display screen refresh price. For instance, on a typical 60Hz show, the pattern price could be 120Hz. In different phrases, the show will register contact enter each 8ms.
The pattern price of the stylus will be even increased; on ChromeOS, some gadgets have a stylus pattern price of as much as 400 Hz. On these shows, the consumer expertise is even higher because the display screen is extra responsive.
On this article, we discover two methods to enhance the stylus expertise and scale back latency on the subject of drawing, sketching, or handwriting. The low latency graphics library Jetpack permits functions to render quicker. As well as, the Jetpack movement prediction library predicts the subsequent
MotionEvent to all the time be one step forward of the rendering engine.
Mixed, these modern libraries make it easier to obtain best-in-class pen experiences.
We wish to hear from you
The Low Latency Graphics Library and Movement Prediction Library can be found as alpha at present, so you can begin integrating and testing them in your functions. We hope to include your beneficial early suggestions as we proceed to enhance the libraries in future releases.
Please share your suggestions on the low latency graphics library in androidx.graphics suggestions and the movement prediction library in androidx.enter suggestions. Thanks a lot for studying. Joyful coding!
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