Hi Steve,
Thanks!, I put this back on the list so other can benefit from your
thoughts.
Tiles at various scales is not intuitive to me either. If you don't
simplify the geometries, then all you are getting it more features in
each tile as you zoom out. But your point about degenerate geometries
play right into this and you can't really simplify them except via
clustering which makes more sense on the client side.
I was thinking of using this for serving point features to an OL5
application. Since my case was so simple, I ended up writing a trivial
PHP handler to do this using a bbox strategy, but I want to revisit that
using mapcache and MVT because I think it would be good to learn and I
think it would be more efficient on both the client and server, not that
I need to worry that much about that.
-Steve
On 2/12/2019 11:56 AM, Lime, Steve D (MNIT) wrote:
I like the OL example as well. Years ago I created an initial run at that
format using the CGI-based options against a common MapFile. It worked ok for
that use case because all the differences between examples were configured
completely on the client.
1) You actually don't need to define an output format - there's one set by
default if the support is compiled in. You can override of course.
2) Configuration for MVT mirrors that of WMS (strangely) and WFS at least in
terms of determining which feature attributes are exposed, so OWS_INCLUDE_ITEMS
and OWS_[item name]_TYPE are about all you care about. This applies to either
MapCache or mode=tile requests. GeoJSON and KML drivers behave similarly.
3) If using MapCache the requests are made using WMS so that protocol needs to
be enabled in the MapFIle.
4) The other option is using the CGI and mode=tile. You can set the format
either in the mapfile via IMAGETYPE or via URL using map_imagetype=mvt
5) The MVT format is layer-based so it maps nicely to MapServer. You can
certainly mix geometry types across layers. I don't know that the format
supports mixed geometry types within layers - MapServer doesn't so it's a moot
point anyway.
6) MapServer is generating binary protobuf files so there isn't really a way to
show sample outout.
7) The rendering process does respect MIN/MAXSCALEDENOM values
8) Features are converted from map coordinates to tile coordinates much the
same way as you would for drawing in an images. Features are clipped and then
sampled based on the tile extent and resolution. I think it's helpful to think
of MVT features as almost pre-processed features suitable for drawing-only.
9) One difference from the vector->raster conversion and the vector->mvt conversion
is that I was finding that the Mapbox GL client was pretty sensitive to degenerate
geometries - specifically triangles that trend towards a point (area < 1) or a line
(triangle height along the longest size < 1 pixel). The geometries are valid according
to a few MVT utilities out there but Mapbox GL stops rendering the geometry as soon as it
reaches a part like this. I've got a feeling this is related to Web GL itself. Anyway,
MapServer 7.2.2 has some additional filtering in place to remove this portions of a
geometry. I suspect we'll run into other issues like this as we gain experience.
One thing I don't have good handle on is strategies to optimize tiles based on
scale. For example, it's certainly possible to send full-resolution features at
a certain zoom level and it would seem to me that you wouldn't want to keep
sending them since the client already has as good a data as it can get. This is
all probably heavily dependent on the behavior of the client and until folks
get a chance to use this functionality we won't be able to provide much
guidance.
--Steve
-----Original Message-----
From: Stephen Woodbridge [mailto:[email protected]]
Sent: Thursday, February 07, 2019 9:06 PM
To: Lime, Steve D (MNIT) <[email protected]>; [email protected]
Subject: Re: [mapserver-dev] Time for a 7.2.2 release?
Steve L,
I like what Openlayers has in the way of examples but that would be a
major undertaking so I think it is beyond the scope of things. I have
found the Mapserver OGC pages to be very help as an example. For this
specific case of MVT tiles, a simple mapfile with the appropriate bits
commented. And for mapcache, and a sample mapcache.xml referencing the
sample mapfile should do it. I would think this could all be done on a
single page and added to the documentation.
For the mapfile, I presume you need to setup an output format, and then
there is probably some metadata that needs to be defined to tell what
properties should be included with the data. Is this on each layer? What
happens if multiple layers are requested with different geometry types?
you get back a FeatureCollection? What does a request look like, is
this a standard WMS or WFS request? I'm guessing a WMS because that is
what mapcache expects. A sample geojson output might be helpful. I'm
just tossing out ideas here that I think might be helpful to someone not
familiar with it and trying to make use of it.
If you want to send me some info on/off list, I will try to organize it
and put together markdown page and send that back to you.
-Steve W
On 2/6/2019 5:45 PM, Lime, Steve D (MNIT) wrote:
I'll update the RFC. At a minimum it's missing the voting history.
As for other documentation we’d need to create a new page I guess that would be more
task-based (e.g. I want to serve vector tiles using MapServer and MapCache). We don't do
much of that and perhaps we should. There really aren't any new configuration options,
heck the MVT output format is built in so as long as the support is compiled in there
isn't much to it. Steve, maybe you could put together an outline of what your ideal
"how to" might contain?
--Steve
-----Original Message-----
From: mapserver-dev [mailto:[email protected]] On Behalf Of
Stephen Woodbridge
Sent: Wednesday, February 06, 2019 9:31 AM
To: [email protected]
Subject: Re: [mapserver-dev] Time for a 7.2.2 release?
It would be nice if RFC 119 on MVT Tiles was updated to indicate it is
release. I'm not sure if there is any docs, I did go looking and the
only thing I found was references to SteveL's github stuff, but it would
be nice is there was an example showing how to setup mapserver and
mapcache serving MVT Tiles.
Thanks,
-Steve W
On 2/6/2019 9:47 AM, Lime, Steve D (MNIT) wrote:
I had mentioned a mid-January release a while back so we’re a tad
late. There a couple of MVT-related bug fixes, among other things,
that would be nice to release. Anything folks were hoping to see included?
Otherwise I’ll offer a +1 for a 7.2.2 release in the next week. Jeff
has graciously offered to help...
—Steve
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