From World Wind Wiki
Multi-layer transparency is a new feature in World Wind 1.3.3, and it is 'a doozie,' as my grandmother used to say. This page is begun with great intentions, but for now we will start with some basics.
World Wind is a lot of things to a lot of different people. Some just like to zoom (once there was a page on this topic, but you'll just have to trust us on this), while others want to spin. Many of us focus on the scientific capabilities of World Wind (which are truly impressive), but most people probably mainly just want to zoom (there's that 'zoom' thing again).
But one use of World Wind has probably rarely crossed the average user's mind. Well, make that at least two related uses. The first (and unusual) use would be that of a canvas, allowing the informed and artistically-inclined user to 'paint' his or her very own worlds/maps, etc. The second (and very much related, but likely also more apparent) is the use of WW for actual cartographic purposes. With multi-layer transparency, these uses for WW become very accessible, even to the novice.
 Why is this important?
This feature opens up many new possibilities for using World Wind as more than just a viewer.
Here are some recently submitted hotspots I posted with transparency info contained in the images:
and even more recent (11-16-05) - The Demis series
Please come back again soon so we can give you more examples and ideas.
 What is multi-layer transparency?
Multi-layer transparency is the ability to set various characteristics and levels of opacity for each layer separately, and to prioritize to some extent the position of individual layers in the transparency 'stack'. Layers are blended with those above and below, creating a complex view of different data types. It is also known as alpha compositing, or alpha blending.
 How does multi-layer transparency work?
Multi-layer transparency works by uh, magic :-). Actually, it utilizes RGB and Alpha channels to blend multiple layers based on chosen settings. It is useful to have some understanding of image blending (or 'compositing'), and alpha channels (see links above), but is by no means necessary for the use of this relatively easy and very powerful feature of World Wind.
Visit the forum page where I recently attempted to explain it - maybe it will make sense to you...
Here are links to some discussions on the technical side of things - I will at some point attempt to translate these concepts into plain English. For now - give a read:
To make things easier to get started, I will be posting more (see above) screenshots with relevant settings soon, which will provide you with basic settings for an area, which can then be easily adjusted for similar conditions elsewhere. Stay tuned!
 Where and how do I adjust the settings?
- In World Wind 1.3.3, click View and select Layer Manager.
- With Layer Manager open, click on each main heading, exposing submenus. Any submenus within these will show up with arrows to the left of the layer name - click those to expand submenus completely. Each name which does not have an arrow to its left may be considered a layer for this discussion. Right click on the layer you wish to adjust settings for, and then click Properties. A dialog box will display, with a number of settings. This is what you must play with, until we get better documentation up. Do not worry, layer settings at this point cannot be saved (that will most likely be available in the not-too-distant future), and they revert back to original settings once that layer (or WW) is turned off. Therefore, you can mess with these all you want, and it won't hurt your settings . . .
 Which layers work the best?
Well, that depends on what you want to accomplish. For now, let's just say that the ones you are able to blend successfully are the best ones. You will likely have the most luck if you start with something like the USGS Topographical map layer and the NLT Visible Landsat7 layer - these complement each other very well, and are fairly conducive towards experimentation.
Note: The Demis Worldmap Add-On is a good international replacement for the USGS Topo Maps layer, and it adds some potential of its own, one of which is not enabled by default - the simultaneous display of both Demis layers. This is useful only in certain situations, and can be very difficult to take advantage of, due to the sometimes conflicting blending behavior of the two layers.
In order to view both Demis layers simultaneously, you must edit the DemisWorldMap.xml file found in your WorldWind Config\Earth directory - this is very simple - at the top, you will see this line:
<LayerSet Name="Demis Worldmap" ShowOnlyOneLayer="true" ShowAtStartup="false" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="LayerSet.xsd">
You simply need to change this part - ShowOnlyOneLayer="true" - to: ShowOnlyOneLayer="false" - and resave the file. That's it. Also, please don't use this excessively, as it places unneeded load on their servers and is only occasionally useful.
Note that in the hotspot examples above in which I used Demis, I also changed both layers to use this setting:
You are advised to backup any and all xml files before editing them! Do this always, or please refrain from complaining when you mess up your own files - thank you.
 Who can help me, since I can't get it to work?
Unfortunately, this feature is not supported on all video cards, even those which are otherwise supported by WW. If your card is less than 2 years old, it will most likely work, particularly if it provides full support for DirectX 8.0 and above (note that just because you use DirectX 9c without apparent problems does not mean that your card fully supports the feature set). Please see this forum thread for more information and to post which video cards are/are not supported from your experience. In the near future, we will try and compile a list of what is known to work, and specifically what needs to be supported by your video card to use this feature.
Keep in mind also that these settings are provided as guidelines, and may not produce exactly the same results on your system, depending on video/etc. However, if you take your time and work out several of the examples, it should give you enough familiarity with how your system responds to various settings, and then you could adjust accordingly. Also VERY IMPORTANT - location is key. These settings are not likely to produce optimal results anywhere other than the specific place where they were originally made. Even if the terrain/groundcover, etc appears the same, there are many factors which can affect blending. To find location, look to upper right corner.
 When is this documentation going to be done?
Very good question. Maybe never. But it is being worked on currently. And will be worked on some more soon.