[mapserver-users] Problems with non-square pixels

[Pfeiffer Michael]

Hi,

thx for your answers.
It seems that possibility 2 solves the problem.
Although I can't understand wherefore this is only a problem for portrait and not for landscape.
Now we have the problem that we do need exactly this BBOX because we want to print a user given scale.
So is it possible to send a float width and height to mapserver or do we have to change the logic in mapwms.c?

thx

best regards

Michael


>-----Ursprüngliche Nachricht----- 
>Von: Steve Lime [mailto:Steve.Lime at dnr.state.mn.us] 
>Gesendet am: Freitag, 17. Juli 2009 00:21 
>An: Pfeiffer Michael; Frank Warmerdam 
>Cc: mapserver-users at lists.osgeo.org 
>Betreff: Re: [mapserver-users] Problems with non-square pixels 
> 
>The nonsquare_enabled check is occurring *before* the WMS bbox (in map->extent) is converted 
>to the MapServer extent model so I believe the code is correct. I'd lean towards option 2 below. 
> 
>Steve 
> 
>>> On 7/16/2009 at 2:40 PM, in message <4A5F8230.2080704 at pobox.com>, Frank 
Warmerdam <warmerdam at pobox.com> wrote: 
>> Michael Pfeiffer wrote: 
>>> Try this wms request 
>>>  
>>>  
>> http://www.sogis1.so.ch/cgi-bin/sogis/sogis_print_test.wms?SERVICE=WMS&VERSION=  
>> 1.1.1&REQUEST=GetMap&FORMAT=image/jpeg&LAYERS=Orthofoto&STYLES=&TRANSPARENT=true&BB 
>> OX=605035.8790650383,224864.94050038885,609110.4623983715,230528.7877226111&WI 
>> DTH=6417&HEIGHT=8921&SRS=EPSG:21781 
>>> A0 portrait  
>>>  
>>> This request needs over 1 minute. 
>>  
>> Michael, 
>>  
>> There seems to be a modest difference in the aspect ratio of the BBOX 
>> and the SIZE in the given request. The logic in mapwms.c that tests whether 
>> non-square mode needs to be used looks like this: 
>>  
>>    /* Check whether requested BBOX and width/height result in non-square  
>> pixels 
>>     */ 
>>    nonsquare_enabled = msTestConfigOption( map, "MS_NONSQUARE", MS_FALSE ); 
>>    if (!nonsquare_enabled) 
>>    { 
>>        double dx, dy, reqy; 
>>        dx = MS_ABS(map->extent.maxx - map->extent.minx); 
>>        dy = MS_ABS(map->extent.maxy - map->extent.miny); 
>>  
>>        reqy = ((double)map->width) * dy / dx; 
>>  
>>        /* Allow up to 1 pixel of error on the width/height ratios. */ 
>>        /* If more than 1 pixel then enable non-square pixels */ 
>>        if ( MS_ABS((reqy - (double)map->height)) > 1.0 ) 
>>        { 
>>            if (map->debug) 
>>                msDebug("msWMSLoadGetMapParams(): enabling non-square  
>> pixels.\n"); 
>>            msSetConfigOption(map, "MS_NONSQUARE", "YES"); 
>>            nonsquare_enabled = MS_TRUE; 
>>        } 
>>    } 
>>  
>> I suspect the asymmetry is just enough to amount more than one pixel at 
>> this size.  I didn't run all the numbers but I suspect at smaller paper 
>> sizes (smaller SIZE values) the assymmetry amounts to less than one pixel. 
>> I'm not sure why it would affect portrait and not landscape.  I only ran 
>> the ratios on the case you mentioned was slow. 
>>  
>> So there are a couple of possibilities here. 
>>  
>>   1) It is possible the above computation is wrong.  I vaguely recall there 
>>      is an issue with map->extent actually being from the center of edge 
>>      pixel to the center of edge pixel, not the outer edges, in which case 
>>      the calculation should perhaps be: 
>>        reqy = MS_ABS(((map->width - 1) * dy / dx) + 1); 
>>      which might or might not make a difference in this case. 
>>  
>>   2) Perhaps you just need to generate your WMS requests with more precise 
>>      BBOXes corresponding to the aspect ratio of the SIZE requested. 
>>  
>> Well, I'm sure there could be other possibilities too. 
>>  
>> Best regards,