[Gdal-dev] hatches rendering issues

[Brian Hamlin]

On Nov 6, 2007, at 8:11 AM, Frank Warmerdam wrote:
> Eric Lemoine wrote:
>>> It would be helpful to see the results, but generally it is hard to
>>> downsample repeating patterns without odd aliasing effects.  The best
>>> is to be very certain that exact factor of 2 downsamplings are done 
>>> in
>>> which case the algorithms should sample the data in a consistent way.
>> Thanks for your response Franck. Would you mind elaborate a bit more
>> on your last sentence?
>> <http://dev.camptocamp.com/grandlyon_plu/problem_images/>
>> Three screenshots at 1/20000, 1/10000, and 1/5000.
>
> I've looked at the three images.  The 1/5000 is the original, right?  
> Is this
> really the *original*?  It seems the hatching spacing is not exactly 
> uniform
> and darkness of the hatch is not uniform.  So if this is what you are 
> working
> from you are basically doomed to relatively poor and inconsistent 
> results.
>
> The hatching does not follow on nice power of 2 boundaries, so nearest
> neighbour downsampling will certainly produce inconsistent results 
> (some
> hatches will be completely missed for instance).  Are the 1/10000 and
> 1/20000 samples you provide produced with averaging?  With MapServer or
> gdaladdo?
> ...
> Basically raster hatching effects put through sampling processes suck,
> and at most you can hope for not-too-bad in my opinion.
>

people use Gaussian blur and others at that point, y?