[OpenLayers-Dev] WMS should default to GIF instead of JPG

[Tim Schaub]

> For WMS layers, which are usually placed over a photo basemap 
> like GoogleMaps?, GIF is a better format. GIF is lossless and 
> works best for images with a fixed number of colors (as is 
> usually returned by WMS layers). GIF allows transparency.

I don't know if it's true that WMS layers are used as overlays more often
than basemaps.

To get a WMS layer to behave as an overlay, you have to send it either the
isBaseLayer:false option or the transparent:'true' parameter.  As long as
you're constructing your WMS overlay with one of these, it doesn't seem like
too much of a hassle to add format: 'image/gif' or format:'image/png' and
transparent:'true'.

> And for the record, PNG has similar properties to GIF, but 
> older versions of IE have problems rendering PNG layers, so 
> is not a suggested format.

Actually, regular old transparency in PNG is supported in the most common IE
setup (IE6 on Windows).  It is alpha transparency in 24 bit PNG that is a
headache in pre-IE7 (this is for partially transparent pixels instead of a
single color indexed as transparent).

Also, not all WMS servers will give you a GIF with transparency just by
asking for GIF.  Compare these two images (first one GIF without a
transparent background, second one PNG that does give you transparency).

(GIF that doesn't have a transparent background)
http://maps.customweather.com/image?LAYERS=earthquakes&FORMAT=image%2Fgif&TR
ANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&EXCEPTIONS=a
pplication%2Fvnd.ogc.se_inimage&SRS=EPSG%3A4326&BBOX=-180%2C-90%2C180%2C270&
WIDTH=256&HEIGHT=256

(PNG that does have a transparent background)
http://maps.customweather.com/image?LAYERS=earthquakes&FORMAT=image%2Fpng&TR
ANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&EXCEPTIONS=a
pplication%2Fvnd.ogc.se_inimage&SRS=EPSG%3A4326&BBOX=-180%2C-90%2C180%2C270&
WIDTH=256&HEIGHT=256

Note that this site is serving up PNG with alpha transparency (32 bit), so
this is a problem for IE.  The point is that just because you ask for a GIF,
you may not get a transparent background.

> So I suggest that the default format offered by the 
> Openlayers WMS is GIF format instead of JPG as is currently the case.

Whether GIF or JPEG is the default doesn't make much difference to me.
Since transparent is not 'true' by default (in the WMS.js parameters), I
think your best off if you're explicit about asking for what you want.

Tim

> --
> Cameron Shorter
> http://cameron.shorter.net
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