Question

I have an icon with partial alpha (alpha values between 0 and 255) that I want to display using GDIPlus. When using the Bitmap constructor of GDI+ that takes the direct filename, the file displays properly. However, when loading from resource, it has a problem recognizing alpha. I looked on MSDN, and there are problems with alpha: http://msdn.microsoft.com/en-us/library/windows/desktop/ms536318.aspx. By retrieving the ICONINFO structure from the Icon, I can get rid of the fully transparent pixels, however, the partially transparent pixels still appear either as fully opaque or fully transparent.

I wanted to know how to create a Win32 Bitmap from an Icon in resource with the partial alpha values.

Answer 1

You can use LoadResource to get a pointer to the icon and and its image data. You can pass the pointer to the image data to the appropriate Bitmap constructor. This is a bit of a chore because icons have a peculiar resource format.

If possible, it would be simpler to store your image as a transparent (i.e. 32bpp argb) bitmap. In this case you can use LoadImage with LR_CREATEDIBSECTION.

Update

Apparently LoadIcon does load the alpha correctly. It would appear that the problem is GdiPlus not respecting the alpha when you construct a GdiPlus::Bitmap from an HICON. What you could do is:

• Use LoadIcon to load the icon.
• Use GetIconInfo to get the ICONINFO. hbmColor is the handle of the transparent bitmap.
• Use GetDIBits to get the bitmap bits from hbmColor.
• Pass the data to the Bitmap constructor that takes bits and understands alpha.