MapServer
Undocumented Mapfile COLOR options
It is possible to user negative color values in Mapserver. This is not documented. The most common way is to use -1 as the red value which makes a layer transparent. However some other effects can also be achieved. I documented some of those in my Mapserver Easter Egg findings: https://lists.osgeo.org/pipermail/mapserver-users/2018-March/080547.html Those are also documented in the GIST: https://gist.github.com/LarsSchy/df91553f1fda02196ba7881a129b4c64 This issue was rasied lately by a question on the dev-list by Seth: https://lists.osgeo.org/pipermail/mapserver-dev/2020-July/016246.html
Lars, This is clearly in "undefined behaviour" land and whereas using -1 for color was used by the GD render until mapserver 5 this isn't the case anymore. I suspect the funny colours you're getting using negative values is the effect of unsigned char wrapping and not something that has anything intentional to it. This should clearly not be documented anywhere as there is no reason this behaviour would remain in a subsequent (even minor) release. Regards, Thomas
Le jeu. 23 juil. 2020 à 18:56, Lars Schylberg [email protected] a écrit :
It is possible to user negative color values in Mapserver. This is not documented. The most common way is to use -1 as the red value which makes a layer transparent. However some other effects can also be achieved. I documented some of those in my Mapserver Easter Egg findings: https://lists.osgeo.org/pipermail/mapserver-users/2018-March/080547.html Those are also documented in the GIST: https://gist.github.com/LarsSchy/df91553f1fda02196ba7881a129b4c64 This issue was rasied lately by a question on the dev-list by Seth: https://lists.osgeo.org/pipermail/mapserver-dev/2020-July/016246.html
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Thomas, I tend to agree with You, that this should not be documented. Well since Jeff asked me, I filed the ticket. But I think this could really confuse people, so this possibility should really be removed. That is was why I wrote about this as a novel Easter Egg hidden feature and didn't sugest to document it in the first place two years ago. Regards /Lars
@LarsSchy @tbonfort - thanks for the feedback.
I was trying to get a list of valid options for all Mapfile properties, and while testing validation came across -255 -255 -255 and -1 -1 -1 in Mapfiles in msautotest (see https://lists.osgeo.org/pipermail/mapserver-dev/2020-July/016246.html).
It seems to make sense to not document these and assume any negative values are invalid colours. @tbonfort - any opinion on removing the values from msautotest Mapfiles?
Is there code to be removed as well?
From: Jeff McKenna [mailto:[email protected]] Sent: Thursday, July 23, 2020 2:08 PM To: mapserver/docs [email protected] Cc: Subscribed [email protected] Subject: Re: [mapserver/docs] Undocumented Mapfile COLOR options (#339)
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Thanks Thomas for clarifying. I'm +1 to removing these from msautotest.
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