Overzoom cached/offline tiles

[incanus]

Something that would boost perceived performance, at least on raster, is to consider a TileData::state::cached somewhere between invalid and loaded. It's odd because it's logic branching, not serial, but with this we could consider loading and parsing/decoding for use in child/parent tile scenarios.

To put it another way, this wouldn't require the user to have ever zoomed to a parent or child tile zoom level in the current run of the app in order to make use of those tiles for drawing. If I launch the app at z8 but have one or more z7 tiles cached, they could get drawn until I get all the z8 tiles I need for optimum resolution.

There's a tradeoff between spending time pulling these from cache vs. spending resources in getting the z8 tiles, but it might be worthwhile.

[kkaefer]

Yeah, we should definitely add a caching layer, but I'm not sure how it should look. Right now, I'm thinking of adding one in the "tile load" step, e.g. a sort of URL/file cache that caches the downloaded tiles. The other cache layer that I want to introduce, is caching the tessellation for a downloaded tile. This will have a huge impact on reparsing speed because this is where we spend 80-90% of our time on.

[incanus]

I'm not sure we're talking about the same thing. I'm talking about consulting whatever disk-based caching we do have and drawing those to screen as parent/child tiles even if those zooms haven't been visited in the current session.

[jfirebaugh]

Implemented in #5143.