wms: no way to set width and height, but no error message when beyond max

[mlennert]

In the current implementation of the wms client, there is no way to set a max width and height. Trying to access a server that limits requests to a max width and height fails when these values are too high. Apparently they are determined by the map window size, as downsizing this window makes the map appear.

So, there are two problems here:

1) No way to set max values other then by resizing window.

2) If the window size is too large, the layer is simply not shown. What is weird in my test case [1] is that I see the map in its full extension, but when I try to zoom, the map window remains empty. Which is obviously an unexpected behaviour, unless you understand the whole issue of max sizes and how these are determined.

So, this is somewhere between a bug (no error message explaining why the map doesn't show up) and an enhancement wish for the possibility of indicating max values.

Moritz

[1] Belgian National Geographical Institute's test WMS server: ​http://www.ngi.be/testbed/ (only in French or Dutch); imposes a 1000X1000 limit

[mlennert]

As images speak more than words, here's a ​screencast of the problem.

As the behaviour is really not as expected, I set this to bug instead of enhancement.

Moritz

[mhugent]

1) No way to set max values other then by resizing window.

the min-max pixel values are a property of the server, not of the WMS client. For QGIS as a WMS client, I don't see any other reasonable way of requesting the pixel resolution (WIDTH and HEIGHT parameters) of the QGIS map window.

I still need to check the WMS specs. As far as I remember, the max width height of the server is not given in all WMS version capabilities. Also, if the server does return only an empty image and no exception, QGIS cannot know that it is an error.

​http://www.ngi.be/testbed/

Please provide directly a wms url

[mlennert]

Replying to mhugent:

1) No way to set max values other then by resizing window.

the min-max pixel values are a property of the server, not of the WMS client. For QGIS as a WMS client, I don't see any other reasonable way of requesting the pixel resolution (WIDTH and HEIGHT parameters) of the QGIS map window.

GRASS allows you to give row+col max values and then calculates the bounding boxes of all the tiles it has to download to respect both those max values and the bounding box of the total region. But I imagine that this is not as easy for simple viewing as in QGIS.

I still need to check the WMS specs. As far as I remember, the max width height of the server is not given in all WMS version capabilities. Also, if the server does return only an empty image and no exception, QGIS cannot know that it is an error.

​http://www.ngi.be/testbed/

Please provide directly a wms url

The problem is that you have to login to get access to their WMS server. Contact me off-trac and we should be able to find a way to get you access if you do not want to sign up (it's free, but only in French or Dutch).

Moritz

[lutra]

Replying to mhugent:

1) No way to set max values other then by resizing window.

the min-max pixel values are a property of the server, not of the WMS client. For QGIS as a WMS client, I don't see any other reasonable way of requesting the pixel resolution (WIDTH and HEIGHT parameters) of the QGIS map window.

I still need to check the WMS specs. As far as I remember, the max width height of the server is not given in all WMS version capabilities. Also, if the server does return only an empty image and no exception, QGIS cannot know that it is an error.

​http://www.ngi.be/testbed/

Please provide directly a wms url

I confirm this behaviour. If the map canvas is small enough the layer shows normally and is possible to zoom/pan with no problems.

A URL you can use as test is

​http://www.ngi.be/testbed/wms/top50r_l72_fr

on the service page is stated:

"les images envoyées ont une limitation en taille de: 1000px X 1000px"

[pcav]

Still true?

[lutra]

Replying to pcav:

Still true?

yes, easily replicable with

​http://www.ngi.be/testbed/wms/top50r_l72_fr

[jef]

error reporting improved in r15457.

[lutra]

the patch available here #3768 also solves this problem