Stylus Low Latency. Creating natural Stylus experiences… | by Cedric Ferry | Android Builders | Dec, 2022 | App Tech

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Creating natural stylus experiences with low latency graphics and movement prediction APIs

With the rising recognition of tablets and foldable gadgets, customers are adopting new behaviors with their gadgets. The bigger display space permits new experiences and powers productiveness duties beforehand reserved for laptops and desktops.

Handwriting, drawing and sketching with a stylus unlock exact and natural workflows, identical to we might do within the analog world. Nevertheless, the delay between gestures and rendering can stop the pure feeling of writing on a chunk of paper, that is known as latency.

I am completely satisfied to share that the Android staff has launched two new libraries: Low Latency Graphics and Movement Prediction, at the moment alpha02. The libraries handle stylus enter latency on Android and ChromeOS.

The aim of the Low Latency Graphics Library is to cut back the processing time between stylus enter and rendering on the display. The movement prediction library leverages a robust algorithm to foretell the place the following stylus transfer will likely 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 may be:

Sluggish Movement Exhibiting Low Latency Positive factors – Machine: Samsung Galaxy Tab S7 + S-Pen

With out latency optimizations, the system has to undergo many steps to render the pixels on the display. The Low Latency Graphics Library minimizes latency by profiting from entrance buffered rendering. The result’s sooner rendering on the display.

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 and when the content material is seen on the display. To study extra about rendering and buffering, see the Mission Butter presentation.

With a multi-buffered rendering method, the show buffer comprises information that the consumer at the moment sees on the display, 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 end result on the display. 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 due to SurfaceFlinger and WindowManager. The next animation reveals the method.

Animation exhibiting how multi-buffer rendering works

To attain the entrance damping approach, entrance and double damping layers are launched. The entrance layer is designed to rapidly render small parts of the display. 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 may be questioning how entrance buffering manages to render sooner than the traditional 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. This works as a result of solely a small space of ​​the display 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 approach resumes and the stroke is maintained.

Animation exhibiting entrance buffer rendering copies strokes to reflect buffer after completion

The entrance buffer approach works greatest for particular use instances, significantly handwriting, drawing, and sketching. These actions modify solely a small a part of the display, which is the place the library shines. If we have been to change bigger areas, this might create tears and artifacts.

The low latency graphics library is just not supposed for rendering a full display, for instance a recreation, or when altering giant areas equivalent to panning or zooming. Android has different instruments, equivalent to Android Recreation Improvement and common OpenGL rendering, that assist builders in these circumstances.

The low latency graphics library is obtainable for Android 10 (API stage 29) and better and ChromeOS gadgets operating Android 11 and better.

Earlier than we take a look at what the code appears to be like like, you would possibly wish to take into consideration your utility and assess what job must be processed within the entrance buffer (quick job that impacts a small a part of the display, equivalent to a stroke) and what must be processed within the entrance buffer. double buffer layer (bigger areas, equivalent to pan and zoom).

Do’s and Don’ts: Use low latency for small display space updates, do not use it for full display rendering equivalent to zooming, panning, gaming

Let’s begin coding! The GLFrontBufferedRenderer works with its personal information sort, which might usually comprise x and y coordinates, but in addition colour, brush sort, stress, and so on.

The strategy requires two callbacks the place the OpenGL code will likely be executed, one to render to the entrance buffer (delta of the stroke) with onDrawFrontBufferedLayerand the opposite for double buffering (persistence) with onDrawDoubleBufferedLayer.

I like to recommend the Stylus in your app presentation, from my colleague Nader Jawad, in addition to the Android Graphics presentation launched on the Android Developer Summit 2022.

You may also confer with the Low Latency Graphics Library documentation.

Now that you understand how to cut 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 aim of the movement prediction library is to go one step forward of rendering and additional scale back perceived latency by creating synthetic factors primarily based 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.

With out going too deep into how the Kalman filter algorithm works, it is essential to know that the algorithm depends on course, pace, stress, and time to foretell the place the following level will likely 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 methodology 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 totally different gadgets, relying on the OEMs.

The movement prediction library will get enter from actual customers as MotionEvent objects. MotionEvent comprises details about x and y coordinates, stress, time… all harnessed to feed the movement predictor and predict the following MotionEvent.

As we have now seen above, the contact sampling price can differ from system to system. The upper the contact sampling price, the sooner information factors may be supplied to the movement predictor, the sooner the algorithm can precisely predict. The movement prediction library can detect the sampling price and use it to gather info sooner and enhance prediction accuracy.

The movement prediction library is predicated on MotionEvent, so so long as you are working with MotionEvent objects, you may reap the benefits of the library.

Predicted movement occasions are synthetic and inaccurate by definition. Use the anticipated occasion to cut back perceived latency, however be certain that the anticipated information is changed with the precise information as soon as it’s acquired.

The movement prediction library is obtainable from Android 4.4 (API stage 19) and better and ChromeOS gadgets operating Android 9 and better.

The movement occasion predictor gives two strategies, document that takes MotionEvent objects and predict which returns a MotionEvent .

See the movement prediction library documentation for extra particulars.

For contact screens to precisely register pointer inputs, the system should ceaselessly examine that the display is touchable. The contact or pen pattern price is how usually the system checks the display. Most screens have a contact pattern price that’s twice the display refresh price. For instance, on a typical 60Hz show, the pattern price can be 120Hz. In different phrases, the show will register contact enter each 8ms.

The pattern price of the stylus may be even greater; 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 is extra responsive.

On this article, we discover two methods to enhance the stylus expertise and scale back latency with regards to drawing, sketching, or handwriting. The low latency graphics library Jetpack permits purposes to render sooner. As well as, the Jetpack movement prediction library predicts the following MotionEvent to all the time be one step forward of the rendering engine.

Mixed, these progressive libraries provide help to obtain best-in-class pen experiences.

The Low Latency Graphics Library and Movement Prediction Library can be found as alpha in the present day, so you can begin integrating and testing them in your purposes. We hope to include your worthwhile early suggestions as we proceed to enhance the libraries in future releases.

Please share your suggestions on the low latency graphics library in suggestions and the movement prediction library in androidx.enter suggestions. Thanks a lot for studying. Completely happy coding!

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Stylus Low Latency. Creating organic Stylus experiences… | by Cedric Ferry | Android Developers | Dec, 2022