Photoshop in Blender, Part 3: Creating an Alpha Mask

In the previous tutorial, Part 2, we saw how the alpha mask of the scene was used so it could work as the foreground image in the AlphaOver node, to combine a foreground with a background. In this tutorial, I will show you how to use The Gimp to create an alpha mask for my locomotive JPEG image, which does not have an alpha channel. In a future tutorial, I will show you how to create an alpha mask in Blender.
The first step is to separate the locomotive from its background, using the various selection tools the Gimp (and Photoshop as well) offer. The Gimp has 7 different selection tools - it’s a time consuming and detail oriented process - and of course the more detailed the selection the better the result. I don’t own Photoshop. I suspect there may be even more selection tools in it. Regardless, the process is the same.