Rendering an Alpha Channel to Merge Images in Photoshop

These are the three elements - the main render, an alpha channel and a background.
Once the foreground image was rendered normally, I clicked on "Render transparency" to get the alpha channel.
In Photoshop, I pasted the background over the main image, and clicked on the mask icon. (It is the grey rectangle with the little circle in the middle at the bottom of the palette.) By Alt-clicking the the mask thumbnail, it will open on the Photoshop screen as a plain white blank. Copy and paste the alpha channel image in place - thus.
Merge all layers and it is done. Very simple. If by any chance you reverse the images, actually putting the background image on the background - which seems logical to me - invert the alpha channel to a negative. [Ctrl+I].
©2007 Larry N.Bolch