[mapserver-users] How to make label background transparent?

[Schylberg Lars]

Hi again,

What happens if You skip the IMAGECOLOR statement ?
I did a project a year ago with a lot of transparent layers.   
But I can't see that I used any IMAGECOLOR statements in that case.
I did however use predefined 256 color palettes for another reason.

/Lars

-----Original Message-----
From: mapserver-users <mapserver-users-bounces at lists.osgeo.org> On Behalf Of Stephen Woodbridge
Sent: den 11 mars 2019 16:25
To: lars.schylberg at blixtmail.se; mapserver-users <mapserver-users at lists.osgeo.org>
Subject: Re: [mapserver-users] How to make label background transparent?

Hi Lars,

Thank you for the feedback on this. Let me clarify what I'm trying to do. I have a shape file with contour lines in it. I drawing this as an overlay image so the background is transparent. In the mapfile the IMAGECOLOR sets the background color and this is the color that is identified as transparent. So in a PNG image the background color is set in the first slot of the color table and marked as transparent. In my case this is color #ffffff, but if I create labelpoly and assign it as #ffffff then another slot in the color table is allocated and assigned the color #ffffff but not the first slot that was allocated for the background, so it shows up as white.

In my image I have a contour line drawn and when it is labeled the contour line goes through the text of the label. If I use the labelpoly then a white background for the label is shown hiding the contour line. 
This is what I want to happen only I want the labelpoly color to be the same color slot as the background color to it hides the contour line behind the label like the white case but is transparent instead of white. IE: it needs to write the label poly with the first slot color, not the new white color slot.

I think, setting the COLOR -1 -1 -1 was intended to do this, but maybe not.

Attached are two small images:

contour.png - shows the contour line through the label text with labelpoly color #FFFFFF00 or color -1 -1 -1 contour-white.png - show labelpoly color #FFFFFF

-Steve W

On 3/11/2019 5:25 AM, lars.schylberg at blixtmail.se wrote:
> Hi Steve,
>
> I am not sure what your use of IMAGECOLOR has to do here.  Normally I 
> have some layers in between the background and the elevation lines.
>
> But there are two more ways do get the color to be transparent in Mapserver:
>
> STYLE
>    GEOMTRANSFORM 'labelpoly'
>    COLOR "#ffffff00"
> END
>
> or
>
> STYLE
>    GEOMTRANSFORM 'labelpoly'
>    COLOR 255 255 255
>    OPACITY 0
> END
>
>
> But those are doing the same thing as your solution below with COLOR 
> -1 -1 -1
>
> Maybe You are after some of the potential techniques that Thomas 
> talked about a couple of years ago with layer blending with the label 
> cache. Those were some future techniques that could be added to the layer composite pipeline if anyone was interested.
>
> Best regards
>
> Lars Schylberg



---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus