[mapserver-users] differing image size with different mapserver versions

Mon May 17 07:53:26 PDT 2021

Hi,

Mapniks *-gamma-method defaults to "power" which means the transform pow(x, gamma) is applied to the original pixel value. I'd guess this is done before merging the color value to draw and the value currently stored in the pixel buffer. It's described as "producing slightly smoother line". I think technically this results in color values that are more nearby which might mean they computationally collapse to the same quantization value or are at least better compressable and therefore result in smaller image files. But might be wrong...

Greets, Andreas 

-----Ursprüngliche Nachricht-----
Von: Rahkonen Jukka (MML) <jukka.rahkonen at maanmittauslaitos.fi> 
Gesendet: Montag, 17. Mai 2021 16:14
An: Eichner, Andreas - SID <Andreas.Eichner at sid.sachsen.de>; Mapserver-Users (mapserver-users at lists.osgeo.org) <mapserver-users at lists.osgeo.org>
Betreff: Re: [mapserver-users] differing image size with different mapserver versions

Hi,

Does it mean that Mapnik is using the many settings called *-gamma with range 0-1 for a shortcut to adjust both the gamma and corresponding gamma method by the same http://mapnik.org/mapnik-reference/?

The goal of this thread is not actually to turn of antialiasing but to create smaller files and decreasing the antialiasing effect could probably do that. The original sample image "mapserv64.png" is antialiased to my eyes. Do you think that adjusting the gamma value alone could have an effect on the file size?

-Jukka Rahkonen-

-----Alkuperäinen viesti-----
Lähettäjä: Eichner, Andreas - SID <Andreas.Eichner at sid.sachsen.de> 
Lähetetty: maanantai 17. toukokuuta 2021 16.53
Vastaanottaja: Rahkonen Jukka (MML) <jukka.rahkonen at maanmittauslaitos.fi>; Mapserver-Users (mapserver-users at lists.osgeo.org) <mapserver-users at lists.osgeo.org>
Aihe: AW: [mapserver-users] differing image size with different mapserver versions

Hi Jukka,

as a result of the discussion IMO Erik created a patch to be able to switch AGGs anti-aliasing on and off using the ANTIALIAS keyword. This was merged by Even Rouault in https://github.com/MapServer/MapServer/pull/6225
It originally worked IMHO only for lines styles. But GAMMA is __not__ the way to turn anti-aliasing off.

Greets, Andreas

-----Ursprüngliche Nachricht-----
Von: mapserver-users <mapserver-users-bounces at lists.osgeo.org> Im Auftrag von Rahkonen Jukka (MML)
Gesendet: Montag, 17. Mai 2021 15:26
An: Rahkonen Jukka (MML) <jukka.rahkonen at maanmittauslaitos.fi>; Mapserver-Users (mapserver-users at lists.osgeo.org) <mapserver-users at lists.osgeo.org>
Betreff: Re: [mapserver-users] differing image size with different mapserver versions

Hi,

Here is a long thread about turning off antialias http://osgeo-org.1560.x6.nabble.com/Draw-roads-WITHOUT-anti-aliasing-td5338614.html <http://osgeo-org.1560.x6.nabble.com/Draw-roads-WITHOUT-anti-aliasing-td5338614.html> .

I had the same wrong idea about gamma=0 back then but perhaps gamma=0.01 could work.

-Jukka Rahkonen-

Lähettäjä: mapserver-users <mapserver-users-bounces at lists.osgeo.org> Puolesta Rahkonen Jukka (MML)
Lähetetty: maanantai 17. toukokuuta 2021 16.06
Vastaanottaja: Mapserver-Users (mapserver-users at lists.osgeo.org) <mapserver-users at lists.osgeo.org>
Aihe: Re: [mapserver-users] differing image size with different mapserver versions

Hi,

(Re-sent as edited to mailing list because the body was originally too long.

Are you sure about “I actually found that 6.4 produced small, aliased images with both the GD and AGG drivers”.  Originally the AGG library could only do anti-aliasing and that affected also Mapserver 5 https://lists.osgeo.org/pipermail/mapserver-users/2007-September/025467.html <https://lists.osgeo.org/pipermail/mapserver-users/2007-September/025467.html> .

I guess that Mapnik folks have since that added support for controlling anti-aliasing with gamma setting http://mapnik.org/mapnik-reference/.

I can see that gamma can be used also with Mapserver https://www.mapserver.org/mapfile/outputformat.html

Have a try with gamma=0. However, Even wrote that turning off antialiasing is possible only in master, so perhaps gamma=0 does not work.

The documentation for Mapserver 5 tells that “All ANTIALIAS keywords are now ignored". But the keyword still appears in quite a many places:

https://mapserver.org/search.html?q=antialias

Would it be time to remove them as well as now not useful references to GD renderer (removed by RFC 99 in 2013)?

-Jukka Rahkonen-

Lähettäjä: mapserver-users <mapserver-users-bounces at lists.osgeo.org <mailto:mapserver-users-bounces at lists.osgeo.org> > Puolesta Richard Greenwood
Lähetetty: maanantai 17. toukokuuta 2021 15.26
Kopio: mapserver-users at lists.osgeo.org <mailto:mapserver-users at lists.osgeo.org> 
Aihe: Re: [mapserver-users] differing image size with different mapserver versions

Thanks for the suggestions. The consensus seems to be that anti-aliasing is causing the increased size. The documentation here <https://mapserver.org/output/antialias.html>  says "Since version 6.0, MapServer will produce aliased output for the “gd/” drivers, and antialiased output for the “agg/” and “cairo/” ones" and it's also mentioned here <https://lists.osgeo.org/pipermail/mapserver-users/2012-April/072011.html>  in Thomas Bonfort's reply. I actually found that 6.4 produced small, aliased images with both the GD and AGG drivers and regardless of my ANTIALIAS true/false setting. 

Would anyone consider re-enabling the ANTIALIAS true/false option?

Thanks,

Rich