MS RFC 126: Port to PROJ 6 API

Date

2019-10-01

Author

Even Rouault

Contact

even.rouault@spatialys.com

Status

Adopted

Last update

2019-10-10

Version

MapServer 8.0

Overview

Since March 1st 2019, PROJ 6.0 has been released. This version deprecates the „traditional“ API available in proj_api.h, in favor or the new API available in proj.h. For now, traditional API is still usable by defining the ACCEPT_USE_OF_DEPRECATED_PROJ_API_H macro, but starting with PROJ 7.0, proj_api.h will no longer be available.

This RFC thus is about the changes needed in MapServer to use proj.h new API. Support for traditional proj_api.h is still kept for now, when MapServer is built against PROJ 4.x or PROJ 5.x.

Proposed solution

Most of the interface with PROJ is contained in mapproject.c, so most of the changes are done there, and the projectionObj object is the abstraction used consistently in the code base to model a coordinate reference system. There were just a number of direct calls to pj_is_latlong() that have been abstracted by a new msProjIsGeographicCRS() function.

However, during testing of the initial implementation, it appeared quickly that other changes were needed to preserve MapServer performance. PROJ 6 new mechanisms to compute coordinate operations are quite powerful, using a „late binding“ approach trying to find the most direct transformations between the source and target coordinate reference systems, instead of going systematically through WGS84, with the well-known +towgs84= or +nadgrids= PROJ keyword. Going through that WGS84 pivot can lead to bad results, for example when transforming between 2 plate-fixed CRS such as GDA94 and GDA2020 which differ by 1.8m, but where the transformation between each of those and WGS84 is a zero-shift transformation.

That new capability has however a computation price. Whereas with PROJ 4.x, pj_transform() can be used without any problem to transform a single coordinate tuple, using naively the equivalent calls proj_create_crs_to_crs() + proj_trans() would be a major performance killer, since proj_create_crs_to_crs() can take a time in the order of 100 milliseconds in the most complex situations. There is consequently a need to cache the PJ object resulting from proj_create_crs_to_crs() and reuse it when transforming between the same (source_crs, target_crs).

The solution is thus to have a global pool of projection contexts, through a projectionContext opaque structure, which in the case of PROJ 6 holds a PJ_CONTEXT as well as a cache of PJ* transformation objets between several (source_crs, target_crs) pairs. The mapObj structure is extended to store tha projectionContext, and acquires one from the global pool in loadMap(). It assigns it to the mapObj.projection member, and all call sites where a projectionObj is created in MapServer are modified to inherit thie projection context. When the mapObj is disposed, its projection context is returned to the global pool, so it can be reused by another request (FastCGI or MapScript loop typically).

To avoid researching for an existing PJ* object for a (source_crs, target_crs) in the cache of the current projection context, which has a price by itself, a variant API for a number of functions like msProjectPoint(), msProjectShape() or msProjectLine() have has been created to directly take a reprojectionObj* pointer instead of the (projectionObj in, projectionObj *out) tuple. That reprojectionObj is mostly a holder for the underlying PJ* coordinate transformation object. This can also help in the PROJ 4.x case, since msProjectPoint() had to do a number of checks on the nature of the CRS for each coordinate transformation, whereas reprojectionObj can cache them.

So to sum up, the new functions are:

projectionContext* msProjectionContextGetFromPool(void);
void msProjectionContextReleaseToPool(projectionContext* ctx);
void msProjectionContextPoolCleanup(void);

reprojectionObj* msProjectCreateReprojector(projectionObj* in, projectionObj* out);
void msProjectDestroyReprojector(reprojectionObj* reprojector);

int msProjectPointEx(reprojectionObj* reprojector, pointObj *point);
int msProjectShapeEx(reprojectionObj* reprojector, shapeObj *shape);
int msProjectLineEx(reprojectionObj* reprojector, lineObj *line);

void msProjectionInheritContextFrom(projectionObj *pDst, projectionObj* pSrc);
void msProjectionSetContext(projectionObj *p, projectionContext* ctx);

Continuous integration changes

The PHP 7.3 configurations is modified to use PROJ 6 API, whereas other targets still use the PROJ 4.x API. To make maintaince of the test suite easier, all the Travis configuration are modified to use PROJ 6.1.1. For the configurations which are not using the PROJ 6 API, the proj.h file is simply deleted, and thus MapServer will use proj_api.h. The reason to align all configurations on the same PROJ version is that, independantly of the API which is used, there have been changes in PROJ that make the result of coordinate reprojection slightly different, which affected a number of expected results (this is mostly due to PROJ 6 defaulting to the enhanced transverse mercator method, instead of the traditional algorithm used previously). That way, the expected results are the same when using proj_api.h or proj.h (except one single case where there is a difference at the micrometer level in a WCS GeoTIFF geokeys, since proj_api.h and proj.h use a slightly different algorithm for geographic to geocentric conversion)

Vagrant is also modified to use PROJ 6.1.1, and it creates 2 builds: build_vagrant against the PROJ 4.x API, and build_vagrant_proj6 against the PROJ 6 API.

Implementation Details

The following files are modified:

  • .travis.yml: change name of the PHP 7.3 config

  • CMakeLists.txt: take into account removal of mapprojhack.c

  • kerneldensity.c: create a reprojectionObj object, and use it with msProjectShapeEx

  • mapchart.c: same as above

  • mapcluster.c: same as above

  • mapcopy.c: share context in msCopyProjectionExtended()

  • mapdraw.c: use a reprojectionObj object and msProjectShapeEx(). Use msProjIsGeographicCRS()

  • mapfile.c: move msInitProjection(), msFreeProjection() and _msProcessAutoProjection() to mapproject.c. Context sharing in initMap() and initLayer()

  • mapgml.c: use a reprojectionObj object and msProjectShapeEx()

  • mapgraticule.c: use a reprojectionObj object and msProjectShapeEx(). Use msProjIsGeographicCRS()

  • mapkmlrenderer.cpp: context sharing. Use msProjIsGeographicCRS()

  • maplayer.c: use a reprojectionObj object and msProjectShapeEx()

  • mapmvt.c: same as above

  • mapobject.c: release projection context to the projection pool in msFreeMap()

  • mapogcfilter.c: context sharing

  • mapogcfilter.h: modify prototype of FLTDoAxisSwappingIfNecessary() to accept a mapObj* for context sharing

  • mapogcfiltercommon.c: context sharing

  • mapogcsos.c: use a reprojectionObj object and msProjectShapeEx()

  • mapogr.cpp: context sharing, and avoid lossy step through PROJ string in msOGRSpatialRef2ProjectionObj() when the CRS is a EPSG code

  • mapogroutput.c: use a reprojectionObj object and msProjectShapeEx()

  • mapows.c: context sharing. Use msProjIsGeographicCRS()

  • mapproject.c: lots of changes !

  • mapproject.h: new functions and structures

  • mapquery.c: use a reprojectionObj object and msProjectShapeEx()

  • mapresample.c: some PROJ 6 specific code paths, as there are direct calls to PROJ API

  • mapserver.h: projContext member added to mapObj, and 2 reprojector objects stored in layerObj

  • mapservutil.c: use of msProjIsGeographicCRS()

  • mapshape.c: use a reprojectionObj object and msProjectShapeEx()

  • mapshape.h: store a reprojector object in msTiledSHPLayerInfo

  • maptemplate.c: use a reprojectionObj object and msProjectShapeEx()

  • mapunion.c: same as above

  • maputil.c: call msProjectionContextPoolCleanup() in msCleanup()

  • mapwcs.c: context sharing. Use msProjIsGeographicCRS()

  • mapwcs11.c: context sharing.

  • mapwcs20.c: context sharing. Use msProjIsGeographicCRS()

  • mapwfs.c: context sharing

  • mapwms.c: context sharing

  • Vagrantfile: call a proj6.sh script

  • ci/travis/before_install.sh: install curl

  • ci/travis/install.sh: download and build PROJ 6. modify PHP 7.3 target to use it

  • cmake/FindProj.cmake: detect PROJ 6 API

  • msautotest/pymod/testlib.py: logic to test for an alternative expected result

  • msautotest/wxs/expected/: a number of file changed due to PROJ 6 enhanced transverse mercator

  • scripts/vagrant/mapserver.sh: PROJ 6 related buid changes

  • scripts/vagrant/packages.sh: install curl and sqlite3

The following files are added:

  • scripts/vagrant/proj6.sh: download and build PROJ 6

The following files are deleted:

  • mapprojhack.c: merged into mapproject.c

Backwards Compatibility Issues

None on the Mapfile syntax. There might be slight changes in coordinate reprojections due to PROJ changes.

Security implications

None.

Performance implications

Using PROJ 6 API is slower, especially for single-time invocations of the MapServer CGI binary. This shows up in the runtime of msautotest, which consists in a lot of such single-time invokations. Typical runtimes showed on Travis-CI show about 9 min for a PROJ 4 API build vs 15 min 30s for a PROJ 6 API build. There isn’t much that can be done about that.

For use cases involving FastCGI or WSGI using MapScript where a process is long/longer lived, the projection context pooling strategy helps a lot to alleviate that cost.

We have used the following simple MapScript python code and mapfile to bench the performance.

def test():
    map = mapscript.mapObj('bench.map')
    mapscript.msIO_installStdoutToBuffer()
    request = mapscript.OWSRequest()
    request.loadParamsFromURL('service=WMS&version=1.1.1&request=GetMap&layers=grey&srs=EPSG:4269&bbox=-180,-90,180,90&format=image/png&width=80&height=40')
    status = map.OWSDispatch(request)
    return mapscript.msIO_getStdoutBufferBytes()
MAP
    NAME TEST
    STATUS ON
    SIZE 80 40
    EXTENT -180 -90 180 90

    PROJECTION
        "init=epsg:4326"
    END

    OUTPUTFORMAT
        NAME png24_t
        DRIVER "GDAL/PNG"
        IMAGEMODE RGBA
    END

    WEB
    METADATA
        "ows_enable_request" "*"
        "wms_srs" "EPSG:4326 EPSG:4269"
        "ows_http_max_age" "86400"
    END
    END

    LAYER
        NAME grey
        TYPE raster
        STATUS default
        DATA ../gdal/data/grey.tif
    END
END

This involves a reprojection from WGS84 to NAD83, which With the same GDAL 2.4 and PROJ master (7.0dev) builds, looping 500 times over test() leads to a 0.9 ms runtime per iteration using PROJ 4 API, whereas 1.1 ms using PROJ 6 API.

If doing single iteration, on hot runs, with PROJ 4 API, the runtime is 19 ms, and with PROJ 6 API, 30 ms.

This case is a bit extreme, in that the rendering is extremely fast, and thus coordinate operation setup is non neglectable. For use cases where the request takes longer, it is expected that the time difference is proportionnaly lower.

MapScript implications

None.

Documentation needs

None in theory since there is no syntax change. However some words will be added to https://mapserver.org/mapfile/projection.html to recommand the use of the init=epsg:XXXX syntax instead of PROJ 4 CRS strings to benefit from the more accurate late-binding coordinate transformations.

Voting history

+1 from PSC members Even Rouault, Mickael Smith, Seth Girvin, Daniel Morissette and Jukka Rahkonen.

Credits

Thanks to funding from the French Ministry of Defense.