B2G Performance Polish: Unlock Animation (Part 2)

Our destination

Our starting point is a 200ms unlock delay, and an uniform ~25 FPS animation. Our aim should be a 0ms unlock delay and a uniform 60 FPS (or whatever vsync is). The former we will minimize as much as we can but the latter is entirely possible.

Let’s talk about how we should design a lock screen animation in the optimal case. When we go and apply it in practice we often hit some requirements and constraint that make it impossible to behave like we want but let’s ignore that for a second and discuss we want to get to.

In the ideal case we would have the lockscreen rendered offscreen to a GPU texture(s) set. We would have the background app ready in another GPU texture(s) set. These are ‘Layers’. We place the background app behind the lockscreen. When the transition begins we notify the compositor to start fading out the lockscreen. Keep them around costs memory but if we keep the right thing around we can reduce or eliminate repaints entirely.

Our memory requirement is what’s required by the background app + about one fullscreen layer required for the lockscreen. This should be fine for even low end of B2G phones. Our overdraw should be about 200%-300%, again enough for mobile GPUs to keep up at 60 FPS/vsync.

Ideal Lockscreen Layer Tree

Now let’s look at what we hope our timeline for our Main Thread and our Compositor Thread to look like:

Ideal Unlock Timeline

We want to use Off-Main-Thread-Animation to perform the fade entirely on the Compositor. This will be initiated on the main thread and will require a style flush to set a CSS transform transition. If done right we don’t expect to have to reflow or repaint any part of the page if we properly built the layer like shown in the first figure. Note that the style flush will contribute to the unlock delay (and so will the first composite time as incorrectly shown in the diagram). If we can keep that style flush + first composite in under say 50ms and each composite in 16ms or less then we should have a smooth unlock animation.

Next up let’s look at what’s actually happening in the unlock animation in practice…

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