Software environment

• Mapserver 8.2.0
• Mapcache 1.14.0
• Gdal V3.8.4
• Alma Linux V8

Problem description

(see discussion on mapserver-users)
WMS Getmap responses depend on version V1.1.1 vs V1.3.0:

• SRS/CRS : epsg:4326,
• source data : GPKG, with InspireCRS84Quad tiling scheme.

For example:

• WMS V1.1.1 request:
  https://xxx/layers/baselayers/wms?service=WMS&version=1.1.1&request=GetMap&Layers=osm_4326&Styles=&SRS=EPSG:4326&Format=image/png&BBOX=45,22.5,67.5,45&Width=1024&Height=1024
  Labels are easy to read: mapcache/mapserver is selecting the right zoom level in the GPKG file.

• WMS V1.3.0 request:
  https://xxx/layers/baselayers/wms?service=WMS&version=1.3.0&request=GetMap&Layers=osm_4326&Styles=&CRS=EPSG:4326&Format=image/png&BBOX=22.5,45,45,67.5&Width=1024&Height=1024
  Labels are much too small and not readable. It seems that mapcache/mapserver is selecting a higher zoom level than expected and down-sample the image

Some explanations

(see https://www.mail-archive.com/[email protected]/msg29229.html, thanks to @jratike80)

• WMS 1.1.0 is using method "msDrawRasterLayerGDAL()"

msDrawRasterLayerGDAL(): Entering transform.
msDrawRasterLayerGDAL(): src=0,0,4096,2048, dst=0,10,994,498

• WMS 1.3.0 probably believes that is must do warping and it selects to use

method "msDrawGDAL(Image_Layer)"
msResampleGDALToMap in effect: cellsize = 0.181087
msDrawGDAL(Image_Layer): using RAW_WINDOW=0 0 4096 2048, dst=0,0,1988,994

The Apache error log shows that depending on the WMS version, GDAL selects different overview levels from the GeoPackage, either "overview 1024 x 512" or "overview 2048 x 1024"

Workarounds

Adding some processing directives in the LAYER definition:
PROCESSING "OVERSAMPLE_RATIO=1.0"

An other option is to modify CRS value in the WMS/V1.3.0 request (instead of EPSG:4326):
CRS=CRS:84

How to reproduce

Here is the mapserver file:

MAP
    NAME "Services_WMS"
    STATUS ON

    PROJECTION
        'init=epsg:4326'
    END

    EXTENT -180.000000 -90.000000 180.000000 90.000000
    MAXSIZE 1000

    LAYER
        NAME "Image_Layer"
        STATUS ON
        TYPE RASTER
        DATA "osm_3_levels.gpkg"

        PROJECTION
            'init=epsg:4326'
        END

        # paramètres de connection
        PROCESSING "CLOSE_CONNECTION=DEFER"
        PROCESSING "GAMMA=0.5"

        METADATA
            "wms_name" "Image_Layer"
            "wms_title" "Couche image"
            "wms_abstract" "Affichage de la couche image"
        END
    END #LAYER

END

The GPKG file, containing OSM up to ZoomLevel=3 can be found on the following link:
https://drive.google.com/file/d/1XjduR5L6aoSRuKEKYb9hBNB07Gv7_-P2/view?usp=drivesdk