Extracting and displaying icons with VB.NET

• Download Source and Demo - 91.4 KB

Introduction

One of the first 'bugs' that users of the .NET framework find is that lack of support for 32bit Icons with an alpha channel. This has to be one of the most annoying bugs because 32bit icons add that extra swishness and professionalism to user interfaces thanks to their semi-transparency enabling them to blend with whatever background you or the end user of your application may have. This article explains how icons are stored in .ico files and provides code that is able to take 4, 8, 24 and 32 bit Icons and display them properly with an alpha channel on 32bit.

Further Reading

Further information on icons can be found:

• In an article from 1995 by John Hornick on MSDN
• In an article by Narb M on CodeProject about how to get 32bit icons with the Windows API
• In an article by Joshua Flanagan on CodeProject about converting bitmaps to icons

Using the code

Including In Your Application

The link above provides you with Source Code and an example application testing the code. You can either reference Icons.dll in your project, add the Icons Project to your solution or add IconFile.vb, IconImage.vb and IconImageCollection.vb to a project. The default namespace for the Icons project and Icons.dll is System.Drawing, so the namespaces are kept nice and neat and tidy.

Overview of Objects

The project consists of five classes: three Public and two Friend. The Public Classes are:

• IconFile
  Represents an icon file. Takes either a filename, FileInfo or Stream as constructor parameters from which icon information is loaded. Includes a GetIcon function, which returns the icon most closely matching the specified size and pixelformat. Also includes an Entries property that returns an IconImageCollection.
• IconImage
  Represents an individual Icon Image within an icon file. Uses a Friend constructor that takes the stream used to create the parent IconFile. Exposes an Icon Property that returns the Icon Image as a Bitmap so it can be edited easily by your application if necessary. IconImages should not be instantiated from your code.
• IconImageCollection
  A collection of IconImages in an IconFile. Normal collection methods, functions and properties are exposed.

Friend Classes are:

• SizeCollection
  A collection of sizes used to find the best icon in IconFile.GetIcon
• PixelFormatCollection
  A collection of pixelformats used to find the best icon in IconFile.GetIcon

Points of Interest

I want to take the time to explain the Icon File Format, because I only found the information that let me complete the project in one place (despite numerous intensive searches in Google), and I can't find it anymore. Each Icon File has a header, the IconDir structure, which contains the following members:

As mentioned above, for each icon there is a corresponding IconDirEntry which specifies size and where in the file the Icon is:

Once information for all of the icons has been specified, the file moves straight in to the first icon. Each Icon has a BitmapInfoHeader, a colour table and an XOR and an AND mask. For anyone who knows anything about the Bitmap file format this may look familiar, but for the unacquainted I will explain:

We then move into the RGB Table, XOR and AND masks. Here a number of different things happen depending upon the BitCount of the icon Image.

• In a 4bpp Icon and an 8bpp Icon, the RGB Table has a number of different colours and then the XOR table provides an array of numbers that are indicies to the colour in the RGB Table that that pixel should be.
• In a 24bpp Icon, there is effectively no RGB Table and the XOR numbers are the Blue, Green and Red values of the colour that the corresponding pixels should be.
• In a 32bpp Icon, there is effectively no XOR mask and the RGB Table provides separate RgbQuads for each pixel.

The RGB Table uses RgbQuads to specify it's colours:

The AND mask specifies which pixels show colours and which pixels are transparent. It is basically a 1bpp XOR mask of opaque and transparent. This originally gave me problems because I couldn't get the AND mask to line up on 16x16 and 48x48 icons. This is because the indices in the XOR and AND masks are padded out so that each row ends on a 32bit boundary. For the XOR mask this is usually fine because the rows end up ending on a 32bit boundary (16x4=64, which divides exactly into 32).

Once the AND mask is read, you have reached the end of the icon and are ready to convert the bytes into their correct index numbers and use them to build a bitmap of the icon.

Reading in the file is simply done by using a Binary Reader. Converting bytes into four bit blocks is done by converting the byte into two digit hexadecimal and using the first digit for the first pixel and the second digit for the second pixel.

Conclusion

This code now allows developers to enhance the professionalism of their applications while maintaining cross-platform capability by enabling them to use 32bit icons in their work.