Unnecessary reprojection in drawing when processing WMS 1.3.0 GetMap request with CRS=EPSG:4326

[qianxiaoming]

When processing WMS 1.3.0 GetMap request, the CRS and extent in parameters will be assigned to mapObj. WMS 1.3.0 states that “EPSG:4326” refers to WGS 84 geographic latitude, then longitude. That is, in this CRS the x axis corresponds to latitude, and the y axis to longitude. This would cause there are 2 arguments in projectionObj of drawing map:

• init=epsg:4326
• +epsgaxis=ne

At this time, even the CRS of the layer is EPSG:4326, but since it has only one argument, it is judged to be different with the CRS of drawing map. So there are unnecessary reprojections in msLayerIsVisible and drawing each shape object.

Steps to reproduce the problem.

mapserv -nh "QUERY_STRING=SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&BBOX=18.1632471876408,73.4510046356224,53.5319431522177,134.976797646506&CRS=EPSG:4326&WIDTH=600&HEIGHT=345&LAYERS=country&FORMAT=image/png&TRANSPARENT=TRUE&MAP=/path/to/map/file.map"
The map file has no real impact on this issue.

Operating system

Ubunut 20.04.3 64bit

MapServer version and installation method

The 7.6.4 version built from source.

[jratike80]

What this test should show me?

[qianxiaoming]

The result of test is correct, this issue is about performance of map drawing. Because of additional argument(“+epsgaxis=ne“) in map’s projection, every shape to be drawn has to be reprojected even though the map and layers in it are both EPSG:4326.

[jratike80]

Could you show metrics about the severity of the issue? Perhaps it could be tested by making same requests with WMS 1.1.1 and 1.3.0. Naturally with the required changes to BBOX order and CRS/SRS. I am not sure if Mapserver 1.3.0 supports &CRS=CRS:84, that would probably be the most direct way to test.

[qianxiaoming]

@jratike80
I test MapServer 7.6.4 build on Ubuntu 20.04 with gcc 9.3.0 in EC2 instance(x86_64) on AWS. The test data is about 400000 points stored in PostgreSQL 12.4. Here is the result for WMS 1.1.0 and WMS 1.3.0 GetMap request with same parameters:
WMS1.1.0 GetMap execution time:

• 3.134s
• 3.115s
• 3.091s
• 3.105s

WMS1.3.0 GetMap execution time:

• 3.718s
• 3.702s
• 3.698s
• 3.667s

All points are drawed with simple circle symbol. Hope this test result will be helpful.

[sdlime]

I thought this was resolved (at least in main), can someone confirm?