Friday, November 12, 2010

Extracting Icons with the Windows SDK

This is an article about the technical aspects of dealings with Windows Icons, including icon resources and HICON handles. But first, let me share with you how I feel about icons. I hate icons. I hate creating them. I hate working with them. I hate rendering them. I hate converting them. I hate dealing with transparency. About the only thing I like about icons is the artistry, and that's sadly lacking lately with the one and two letter icons now popularized with Adobe, Microsoft, Pandora, and Facebook. And let's not even talk about the new dark blue Visual Studio icon, that's pretty much invisible on the task bar.
Life got a little better about four years ago when I bought Axialis IconWorkshop. That's one handy piece of software, with free upgrades for life. If you are in the business of writing software, IconWorkshop will save you a lot of headaches.

But I digress.

Let me start out by pointing out the "official" documentation on icons. See the article on MSDN titled simply, Icons. Note the date of the article: 1995. Things have changed a lot in the last 15 years.

In the beginning, there were 16-color (4-bit) icons, with one color reserved for transparency, so there were really 15 colors available for use. As computers because more powerful, Microsoft introduced 256 color icons, then 24-bit truecolor icons, then 32-bit icons with 24-bit color and 8-bit transparency. Then, as a final coup-de-grace, Vista introduced 32-bit icons sized at 256x256 pixels. These took up so much space that they were stored in a completed different format (PNG). Ouch!

A good case study is the application icon for Outlook Express/Windows Mail in the file MSOERES.DLL, which has existed for most of the life of Microsoft Windows and has evolved as operating system support (and video card support) for icons has improved. 
  • Vista and Windows 7: eight different sizes (256 pixel, 64, 48, 40, 32, 24, 22, 16) at three bit depths (32-bit, 8-bit and 4-bit.)
  • Outlook XP SP3: three sizes (48 pixel, 32, 16) at at three bit depths (32-bit, 8-bit and 4-bit.)
  • Windows 2000 SP4: three sizes (48 pixel, 32, 16) at at three bit depths (32-bit, 8-bit and 4-bit.)
  • Windows 95: two sizes (32 pixel and 16 pixel) at one bit depth (4-bit.)

The only surprise for me in this list is that the Windows 2000 DLL included 32-bit icons, which would have transparency. However, that DLL was part of Service Pack 4 in 2002 and so was probably shared with Windows XP. I'm not aware that Windows 2000 was able to handle transparent icons. Also, you might wonder why I list "dead" operating systems like Windows 95. The reason is that there are some APIs that are stuck in the days of Win9x and never improved, notably the toolbar code in MFC. The common controls also behave differently depending on whether the Common Controls 6 is enabled, either explicitly or with a manifest.
Here's an important point about dealing with icons that took me quite a while to understand. First, there's an icon resource with a DLL or EXE that contains the icon in multiple sizes and multiple bit depths. On the other hand, an HICON contains only a single one of those formats. If you want a different size or a different bit depth, you need to load a new HICON.

The most basic icon-handling function is LoadIcon. It loads an icon from a particular HINSTANCE whose "size conforms to the SM_CXICON and SM_CYICON system metric values." So let's say you call this function and get an icon back. What bit depth? I have no idea. Presumably the same as your desktop, but even that's a hazy concept, because your bit depth can change on the fly, especially when you start a Remote Desktop session. And there's no such thing as a 32-bit desktop - there's no transparency on your desktop. So a 32-bit icon kind of/sort of matches a 24-bit and a 16-bit desktop. But if the icon matches the bit depth of your desktop, how would the icon keep its transparency bits? Again, I don't know.

The documentation for LoadIcon says that "This function has been superseded by the LoadImage function."  The LoadImage function takes the desired x and y size as parameters. That's helpful, now I can load those 256x256 bitmaps. But there's still no documentation on the bit depth.

What lead me down the path to this point is that I'm trying to extract some application icons for a web page that is being created for the local user. Those icons needs to be in PNG format to preserve transparency, so the goal was to load them from the original EXE as 32-bit icons, then save them as compressed PNG files.

Let me summarize my success for this project: Total Fail.

The first task was to get the icon for the particular file extension, such as ".doc".  This problem is easy to solve and well documented - use SHGetFileInfo, which returns the HICON in either small or large (however those happen to be defined.) I spent several hours trying to convert that icon to a PNG file. The popular strategy seemed to be using OleCreatePictureIndirect, as described by DanRollins in the comments in this article and by neilsolent in this article The problem I saw was that my PNG file was always 16 colors. In later comments of that article, neilsolent said that he was seeing a similar problem, which no one was able to provide an answer to. I wasn't able to solve that problem.

Note that the algorithm offered in the initial question in that article is utterly wrong. An HICON is not an HGLOBAL and you can't access it using GlobalLock.

My fallback strategy was to just write a .ico file instead of automatically converting to a .png file. With a .ico file, converting to .png is trivial using IconWorks.

This was nowhere near as simple as I'd hoped. Remember that an icon resource contains multiple sizes and multiple bitdepths? A .ico file is really a container for manager all of those formats, and there is no Windows API for writing that file. Some documentation is given in that 1995 article I mentioned earlier, but it's a non-trivial problem to solve. There is a sample from Microsoft called IconPro (run the executable, open the .chm help file, look for IconPro, and it will give you the option to extract file sample code.) IconPro hasn't been updated in fifteen years. It knows how to write .ico files, so I was hoping that I could make some minor modifications and feed it the HICON from SHGetFileInfo. No dice. IconPro only reads the original EXE or DLL file.

No problem. I'd just retrieve to pointer to the original DLL or EXE and pass that information to the appropriate routine in IconPro.

Unfortunately, it's not that easy. In fact, it's really ugly. There's no Windows API that will automatically tell you the filename and index to retrieve an icon. SHGetFileInfo only returns an index to the system image list (aka the Icon Cache.) There's no hint of information about where the icon came from. I was surprised because I thought that this problem was simple - just look up the DefaultIcon key in the registry. Turns out that this is just one way of specifying icons. To even begin to understand the other methods, you have to delve into the shell namespace and the IExtractIcon interface. I didn't care that much.

I spent several more hours searching for a method that could take an HICON and write it to a .ico file. I found numerous other people asking this same question, but I found no answers that worked and would create a .ico file with 32-bit color depth.

At this point I gave up on an automated solution. I'll use IconWorks to manually extract the resources that I need from the relevant EXE and DLL files.


  1. "And there's no such thing as a 32-bit desktop - there's no transparency on your desktop" Layered windows supports alpha transparency (Tool tips and fading menus added to 2000 uses it, and the dropshadow window class style)

    Raymond Chen recently did a series about icons on his excellent blog;

    If you look at the list of shell functions on MSDN you should find something that will tell you the filename of system image list icons. (This shell stuff used to be undocumented, but MS has been forced to document some of it over the years) SHMapPIDLToSystemImageListIndex, and Shell_GetCachedImageIndex for the reverse.

  2. Thanks WndSks. I'll take a look at Raymond's blog.

    I'd forgotten about the window transparency, I did some work with that last year. However, I'm guessing that those are special cases - your desktop is still considered 24-bit. For example, you can't have a child window set to be transparent, only a top level window and all of its children.

    Also, if you try to use the AlphaBlend API, you discover that you have to do most of the hard work yourself. You can't just feed Windows a bitmap with transparency. It doesn't work that way.

    SHMapPIDLToSystemImageListIndex takes an IShellFolder, which is also required to use IExtractIcon. Every time I've had to use PIDLs in the past, it has cost me at least two days of tearing my hair out. All of which is why I didn't want to take the time to pursue that path.

  3. SHGetDesktopFolder gives you the root IShellFolder, calling IShellFolder->ParseDisplayName on the path gives you the full pidl (Or use SHILCreateFromPath) Then pass the full pidl to SHBindToParent, this will give you the IShellFolder and child pidl required for SHMapPIDLToSystemImageListIndex

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