OpenLayers HIDPI often causes GeoServer cache to fail.

[jesade-vbg]

Turning on HIDPI changes the way OpenLayer requests tiles etc.

You can specify diffrent dpi:s in geoserver, 90, 180 etc.
But OpenLayers requests the exact dpi as the device has, for example 72, 113, 155, 189 etc etc.
Confirmed in docs: HiDPI >

Use the ol/Map#pixelRatio value when requesting the image from the remote server.

If you're lucky, it requests 180 as it's set up in GeoServer.

For example, OpenLayers adds this to the Url:

WIDTH=538
HEIGHT=538
FORMAT_OPTIONS=dpi:189

Resulting in this header from geoserver:
geowebcache-cache-result: MISS
=== slow

Could we somehow force OpenLayers to request (as an example)?:

FORMAT_OPTIONS=dpi:90
WIDTH=256
HEIGHT=256

or double resolution for hidpi devices:

FORMAT_OPTIONS=dpi:180
WIDTH=512
HEIGHT=512

Everyone wants fast maps and sharp "crisp" maps. It would be nice to be able to deliver both.

[jesade-vbg]

Spontaneous thought:
Combine custom imageLoadFunction (or similar) with window.devicePixelRatio

[jesade-vbg]

Quick test: Turned off HiDPI and Forced 180dpi 512x512 using tileLoadFunction on the left. (Chrome device emulator).

[Hallbergs]

I think the idea of forcing specific DPI:s is good! Would 90 and 180 be enough (one for ordinary devices and one for highDPI-devices)? Or do we want more?

[jesade-vbg]

Maybe it could be configured like (devicePixelRatio: dpi):
{ "1.0": 90, "2.0": 180, }

[Hallbergs]

Ahh very nice! I like it!

[jesade-vbg]

POC
https://github.com/hajkmap/Hajk/tree/feature/915-custom-hdpi

[Hallbergs]

The code looks great, nicely commented and easy to understand. Will give it a go next week!

[jesade-vbg]

Thanx! Don't forget to add useCustomHdpi: true in the layers config.
I just noticed I forgot the first 'I' in hidpi :)

[jesade-vbg]

@jesade-vbg: Note: dpi can't be float, round up.

[jesade-vbg]

Added this to wms layer admin

[jesade-vbg]

Problems occur when having pixelRatio 3. ratio 1 and 2 seems to be working fine but 3 causes cache MISS in geoserver.
geowebcache-cache-result:MISS
geowebcache-miss-reason:request does not align to grid(s)

This also happens with the hidpi = true setting when requesting pixelRatio 3 270dpi, so its not my code.

I've found out that alot of people is having the exact same problem with openlayers + geoserver + pixelRatio 3.
Feels like a rounding error. Still investigating........

If anyone has any idea, please let me know.