Windows XP introduced the ability to provide icon images which contain an 8-bit alpha channel. Up until this point, you had only a 1-bit alpha channel, represented by a mask.
The representation of an alpha-blended image in your ICO file is pretty straightforward. Recall that the old ICO format supports 0RGB 32bpp bitmaps. To use an alpha-blended image, just drop in a ARGB 32bpp bitmap instead. When the window manager sees a 32bpp bitmap, it looks at the alpha channel. If it's all zeroes, then it assumes that the image is in 0RGB format; otherwise it assumes it is in ARGB format. Everything else remains the same as for the non-alpha version.
Note carefully that everything else remains the same. In particular, you are still required to provide a mask. I've seen some people be a bit lazy about providing a meaningful mask and just pass in all-zeroes. And everything seems to work just fine, until you hit a case where it doesn't work. (Read on.)
There are basically three ways of drawing an alpha-blended icon image.
DrawIcon(DI_: This is by far the most common way icons are drawn. In the alpha-blended case, this is done by blending the image with the destination according to the alpha channel.
DrawIcon(DI_: This draws the image portion of the icon image, completely overwriting the destination.
DrawIcon(DI_: This draws only the mask portion of the icon image, completely overwriting the destination.
DI_ flags let an application draw just one of the two images contained in an icon image. Applications do this if they want finer control over the icon-drawing process. For example, they might ask for the mask so they can build a shadow effect under the icon. The mask tells them which parts of the icon are opaque and therefore should cast a shadow.
If you understand this, then you can see how people who set their mask image to all-zeroes managed to get away with it most of the time. Since most programs just use
DI_ to draw icons, the incorrect mask is never used, so the error never shows up. It's only when the icon is used by a program that wants to do fancy icon effects and asks for
DI_ (or calls
GetIconInfo and looks at the
hbmMask) that the incorrect mask results in an ugly icon.
The ironic thing is that the people who incorrectly set the mask to all-zeroes are probably the same people who will then turn around and say, "When I try to use alpha-blended icons, the result is hideously ugly under conditions X and Y. Those Microsoft programmers are such idiots. More proof that Windows is a buggy pile of manure." What they don't realize is that the hideous ugliness was caused by their own error.