ArcObjects: Setting image of a command button to a partially transparent bitmap

Question

In an ArcMap 10 add-in, I am attempting to create a toggle button of sorts whose image changes based on the button's state.

This answer describes how to set an add-in button's image programmatically and I am using the same technique. The problem is that this method does not appear to support full alpha channel transparency, only single-color transparency, so those fancy PNG icons they added at ArcGIS 10 don't look quite right.

The button's image is set initially by the add-in framework (using a 32-bit PNG file with alpha channel) -- the image looks as you would expect with proper partial transparency.

To create the image toggling behavior, in the button's OnClick method, I check the state of a boolean variable and set the button's image (using the technique in the above-mentioned answer) to one of two bitmaps stored as resources in the add-in assembly. This all works fine except that the button's image is given a gray background instead of a transparent background.

I have searched around a bit and come to the conclusion that ESRI's GetIPictureDispFromBitmap method converts from a GDI+ bitmap to a GDI bitmap and in doing so loses the alpha channel of the original bitmap.

I have found a solution to this issue and will post it as an answer shortly.

Answer 1

To maintain partial alpha channel transparency, an IPictureDisp object must be created using some unmanaged code, described in this post on Ryan Gregg's blog.

Copy the C# (mirror) or VB.NET (mirror) code to a new code file and add it to your project. Then, instead of the technique in the previously-mentioned answer, set ICommandItem.FaceID to the return value of PictureDispConverter.ToIPictureDisp, passing in your original bitmap.

For example:

UID uid = new UIDClass();
uid.Value = ThisAddIn.IDs.MyButton;
ICommandItem cmdItem = ArcMap.Application.Document.CommandBars.Find(uid, true, true);
cmdItem.FaceID = PictureDispConverter.ToIPictureDisp(Resources.MyButtonEnabledState);

The partially-transparent areas of the original bitmap should be maintained.