Lack of good transparency support in Silverlight and what it means to online mapping apps
The support for transparency in PNG images in Silverlight is to say the least really bad. Here is what the Microsoft docs says about PNG transparency support in Silverlight
Silverlight does not support all possible color depths that are included in the PNG specification. The following are the PNG color depths supported in Silverlight:
- Indexed color: 1-bit, 4-bit, or 8-bit color depth (per channel).
- Truecolor: 24-bit color depth, or 32-bit color depth (per channel) for truecolor plus alpha.
Notably, gray scale (with or without alpha), and 64-bit truecolor, are not supported in Silverlight.
Note that they don’t mention anything about the support for 1, 2, 4 and 8 bit PNGs. This creates some problems when using the ESRI Silverlight API and you might what to keep your eye out for it and pay good attention to how your map caches are being generated. Look at the results when I overlay ArcGIS Online’s Transportation layer on top of the Imagery layer in the ESRI Silverlight maps.
<esri:Map x:Name=“MyMap” Grid.Row=“1” Grid.Column=“2”>
And here it what the map look like. As you can see, the the tile areas outside the continental US are grayed out.