[Qgis-user] Small maths problem!

[Richard McDonnell]

Andreas,
Thanks for getting back, I have looked at similar ways to those outlined 
by your good self, the only issue being, they all require the creation 
of additional data or fields.
I will keep tipping away at it, once again, thanks for your help!
Regards,

Richard.



On 23/07/2015 10:01, Neumann, Andreas wrote:
>
> Hi Richard,
>
> It would be interesting to have this as a built-in feature of the 
> "line pattern fill" renderer in QGIS: being able to specify an angle 
> relative to the longest line segment of a polygon.
>
> I have the same issue for our black and white base map - see 
> http://webgis.uster.ch/wsgi/webgisLauncher.wsgi?uuid=a6634a18-d37f-4997-b3d8-d4c3aed51716
>
> There we would like the line pattern fill angle to be 30 or 45 degrees 
> relative to the longest side of a building. Currently all of these 
> hatch-pattern fills are manually rotated ;-(
>
> Meanwhile - until such a feature is available in QGIS - you could 
> preprocess your data with a Python script be decomposing your polygon 
> data to individual line segments, find the longest ones and get their 
> angles. I would probably save this in a data-column to speed up 
> rendering. But you said you can't do this - so maybe you really have 
> to calculate this on the fly.
>
> For data in Postgis one could write a plsql function to calculate 
> these angles.
>
> Sorry that I can't be of better help here. Just showing that I also 
> have in interest in such a feature.
>
> Andreas
>
> On 2015-07-23 10:10, Richard McDonnell wrote:
>
>> Hi,
>> I am in need of some guidance, I am trying to create an "active 
>> hatch/Fill" as part of a style im working on. What I want is for a 
>> hatch to rotate depending on the orientation of a Polygon.
>> For example, here is the same entity rotated, with the hatch rotating.
>>
>>
>> The issue is I am working on a database, onto which I cant add any 
>> data (additional columns) to the tables, I would like to do this from 
>> within the styling.
>> You can use an expression to denote the angle, so I was hoping to do 
>> it that way.
>> I was looking to utilize the geometry of a bounding box, and 
>> calculate the angle of the diagonal, using something like
>>
>> 180*atan((ymax - ymin)/(xmax - xmin))/$pi + 90
>>
>> But I am not having any luck....If there is no way at present to do 
>> this, Might I put it forward as a possible new feature?
>>
>> Regards,
>>
>> Richard.
>>
>>
>>
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