This is a simple Alpha Composition Plugin for ImageJ.
- Create a new folder named AlphaComposition (or similiar) in your ImageJ/plugins directory
- Copy all files contained in the zip archive into that folder
- Install the Plugin in ImageJ
- Open your images (it is advised to open the alpha channel of your images as seperate images) with ImageJ
- Go to Plugins -> AlphaComposition -> Alpha Blending
- Select the operator (e.g. A over B)
- Select which image should be used as A, B, alpha_A, alpha_B
- Hit Compose!
Note: The resulting image will be stored in your ImageJ folder as alpha_composition_result.png to retain the alpha channel.