Can a HBITMAP contains alpha channel information?

Question

I notice ICONINFO contains two bitmap. If I understand correctrly, hbmMask is for the icon's outline which at the same time specifies which part should be transparent, and hbmColor is for the icon's actual color HBITMAP .

• https://msdn.microsoft.com/en-us/library/windows/desktop/ms648052(v=vs.85).aspx
• https://social.msdn.microsoft.com/Forums/vstudio/en-US/490fb15a-61a5-4a5d-a1a0-fed675975293/hbmmask-hbmcolor-in-iconinfo?forum=vcgeneral

So My question is, why the hbmMask is necessary? Why not just put the alpha channel's information inside hbmColor ? Is it able to combine them into a single bitmap?

Besides, Gdiplus::Bitmap has a constructor which accepts a HICON . Does it mean this GDI+ bitmap is able to handle alpha channel information?

Thanks:)

Answer 1

why the hbmMask is necessary? Why not just put the alpha channel's information inside hbmColor?

An alpha channel is only possible for 32 bpp bitmaps (8 bit alpha channel, 8 bit red, green and blue channels each). Icons with 1 to 8 bpp bitmaps therefore have to store the transparency information separately from the color bitmap in the hbmMask bitmap.

Actually, even 32 bpp bitmaps that have an alpha channel, are required to include the hbmMask , unless the PNG format is used . In the latter case, the mask is derived automatically from the alpha channel.

Besides, Gdiplus::Bitmap has a constructor which accepts a HICON . Does it mean this GDI+ bitmap is able to handle alpha channel information?

As the documentation doesn't state otherwise, you are safe to assume that the constructor converts a 32 bpp HICON into a 32 bpp bitmap. A GDI+ bitmap certainly is able to store an alpha channel if the pixel format is appropriate (in the case of an icon PixelFormat32bppARGB is required).