Experiment: don't fade in line labels showing for the first time

[ChrisLoer]

Reviewing PR #10436 called our attention to a way masters behavior might be superior. When you cross an integer zoom level, most line labels will move to slightly different positions, so they don't get detected as "duplicates" by the CrossTileSymbolIndex. The old versions will immediately disappear and the new ones will fade in. On master, because there's no fading, and because placement happens before a tile is first loaded, the symbols never disappear, they just instantly shift location. The overall effect is that viewport-collision looks more flickery.

One solution we've discussed is to hold onto the old symbols for a little while so we can do a cross-fade.

This PR experiments with a different approach -- try to replicate masters behavior by disabling fade animations for line labels that are being placed for the first time (e.g. they're part of a higher-res tile that just loaded). There are some kinks I would need to iron out, but after playing with it a little bit I think I'll probably abandon this approach. The problem is that even with the fade animation removed, there's still a gap between when a new tile first loads and when the next placement happens. Depending on how you hit the timing of placements, it can feel more or less flickery.

master behavior

viewport-collision behavior

This PR behavior

/cc @ansis @kkaefer @jfirebaugh @nickidlugash

[kkaefer]

I think the behavior in master/this PR is superior to the current vanish + fade-in. As you mentioned, I think a crossfade behavior would also work.

In the GIF you posted, I'm noticing that there are some frames that don't have any street labels (just neighborhood labels). It looks like there are some situations in which the new labels aren't ready yet, but we no longer show the old labels either. Master continues showing the old labels until it has new labels to display in that position.

[ChrisLoer]

It looks like there are some situations in which the new labels aren't ready yet, but we no longer show the old labels either. Master continues showing the old labels until it has new labels to display in that position.

Yeah, on master placement is tied to the tile, so it gets updated at the same time the tile is swapped in. On viewport-collision, placement happens independently of when the tile loads, and that's why the labels disappear for a bit until the next placement happens. In order to keep showing the old labels, we'd have to be rendering symbol buckets from both tiles simultaneously (at which point it would be easy to do a cross-fade).

The current GL JS approach is a little different -- because it requires running a full/synchronous placement every time a tile is added to/removed from the tile pyramid, there's no gap in placement timing (although the fade animation is still an issue: but basically if we did the equivalent to this PR on GL JS, we could replicate masters behavior).

[ChrisLoer]

There are two closely-related "flicker" situations for point labels:

• Zoom-specific label changes (e.g. "CALIF." -> "CALIFORNIA") vanish/fade-in. We sort of assumed from the start that using the label text was a necessary part of identifying duplicates, but I wonder if it would be worth trying location alone?
• There's a tendency towards point label flicker during fast zoom out operations because we'll remove the high zoom tiles from the render tree before low zoom tiles load. (In contrast, when we're zooming in, we'll overzoom the heck out of a low zoom tile while we wait for high zoom tiles to load).

[ChrisLoer]

PR #10468 is looking like a more promising approach.