The evolution of the ICO file format, part 3: Alpha-blended images

Raymond Chen

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.

1. Draw­Icon(DI_NORMAL): 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.
2. Draw­Icon(DI_IMAGE): This draws the image portion of the icon image, completely overwriting the destination.
3. Draw­Icon(DI_MASK): This draws only the mask portion of the icon image, completely overwriting the destination.

The DI_IMAGE and DI_MASK 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_NORMAL 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_MASK (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.

Raymond Chen

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