Support overzoom

[aptogo]

I'm displaying vector tiles from a OSM mbtiles country extract using MapboxGL that has data up to zoom level 14. When I overzoom past the supported zoom level then I just see a blank map. Tessera is returning empty tiles rather than a '404 tile not found'.

It would be great if Tessera could respond with a 404 instead because this would allow you to zoom all the way in (albeit with reduced resolution).

[mojodna]

This is related to #49 / #51, but we didn't consider the overzoom angle.

@ramunasd can you weigh in?

@aptogo a workaround for you in the meantime is to install 0.8.x (which does return 404s): npm install tessera@~0.8.0

[aptogo]

Thanks for the quick response. Just FYI Klokantech's tileserver-php returns 404s in this instance.

[ramunasd]

@aptogo good catch, i will check this

[ramunasd]

tiles outside allowed zoom range is served as 404 errors - https://github.com/mojodna/tessera/blob/master/lib/app.js#L161

@aptogo can You show your config? Does overzoom works only when maxzoom in tilejson is greater than actual?

[aptogo]

I'm using this UK extract: https://osm2vectortiles-downloads.os.zhdk.cloud.switch.ch/v1.0/extracts/united_kingdom.mbtiles

and serving it using this command:
tessera mbtiles:///data/tileserver/www/united_kingdom.mbtiles -p 8899

The file has tiles covering the UK up to zoom 14 and the rest of the world up to about zoom 6-8 (varies on region). Hit the URLs below to see the results:
http://tileserver.emudataproducts.net/14/8185/5449.pbf UK zoom 14 - 200 response OK
http://tileserver.emudataproducts.net/15/16373/10897.pbf UK zoom 15 - no tile at this zoom but a 200 reponse is given with zero content length.

With tileserver-php a 404 response is given for the second URL and MapboxGL renders the map correctly at all zoom levels, ignoring the 404 responses and overzooming tiles to fill in the gaps.

[ramunasd]

So, as i see, metadata is correct and mbtiles module returns correct info. The problem is in the way how tessera returns http status.

It needs to refactor a little bit callback function.

[tephenavies]

This can be fixed by moving the logic for suppressing 404 errors into the code that identifies what the type of error is. See #55.