AGG, paletted image and transparency (Mapserver 5.0 beta 5)

[thomas bonfort]

quantization does not seem to support transparent backgrounds
(regardless of gd or agg)
I'll file a bug about this but I don't know if it will be fixed for 5.0

tb

On 8/30/07, Gilles Bassière <gilles.bassiere at makina-corpus.com> wrote:
> Hi List,
>
> I run Mapserver 5.0 beta5 as a WMS server and have some troubles with
> background transparency :(
>
> In a first attempt, I just replaced my GD OUTPUTFORMAT definition by:
>
> OUTPUTFORMAT
>    NAME             "png"
>    DRIVER           "AGG/PNG"
>    MIMETYPE         "image/png"
>    IMAGEMODE        "rgba"
>    EXTENSION        "png"
>    FORMATOPTION     "transparent=on"
>    # TileCache can't use meta-tiling on interlaced PNG files...
>    FORMATOPTION     "interlace=off"
> END
>
> Changed my config to have my OpenLayers pointing directly at Mapserver
> 5.0, launch my browser and... Wow, it looks great !!!
>
> Then, I tried some optimization and I added to my OUTPUTFORMAT element
> (as seen in "AGG rendering specifics" howto) :
>    FORMATOPTION     "quantize_force=on"
>    FORMATOPTION     "quantize_dither=off"
>    FORMATOPTION     "quantize_colors=256"
>
> Well, the line work is still beautiful but I now have 2 problems :
> 1) I lost background transparency. My tiles appear with a white
> background, I tried to change IMAGECOLOR from 255 255 255 to -1 -1 -1,
> also to remove this element but it doesn't change anything.
> 2) Fonts and symbols looks very crappy, they are mixed black and white
> where they should be filled with black with a white outline.
>
> On a random tile, the image size fall from 102KB down to 25KB. Such
> ratio is not representative at all since it's not an average and it's
> only about vector data but still, quantize option seems very interesting
> and I don't want to give up because of transparency problems.
>
> Trying to solve the first problem, I found a workaround for the second.
> When I set the TRANSPARENT parameter of the WMS request to 'false' (or
> any value different of 'true'), then fonts and symbols are displayed
> normally. It could have been ok if TRANSPARENT was not an optional
> parameter... The map should looks good in the default case.
>
> Does someone could give more explanation about the quantize option,
> especially regarding transparency management ?
>
> If possible, a broader answer about the FORMATOPTION element would help
> a lot. Indeed, it seems like most optimizations are possible through
> output format tuning. Unfortunately, I have not been able to find any
> reference document on this...
>
> Regards,
>
> --
> Gilles Bassiere
> MAKINA CORPUS
> 30 rue des Jeuneurs
> FR-75011 PARIS
> http://www.makina-corpus.com
>