Using GIMP to copy an image layer into a channel

In GIMP, it is possible to copy a colored image layer into a single grayscale channel. Here's how to do the job.

Create an empty channel

1. In GIMP, open up the image file.
   
   
   
   
2. Click the Channels tab on the bottom right.
   
   
   
   
3. Then click the Create a new channel icon.
   
   The New Channel dialog box appears.
   
   
   
4. Optional. In the Channel name field, type in a name, e.g. flats. In the Fill opacity field, slide to 50.0.
    
   
5. Click OK.
   
   A new channel is created.
   
   

Copy and paste the colored image into the channel

1. Click the Layers tab. Click the image layer, e.g. bored-tiger.jpg to make sure it is active.
   
   
   
   
   
2. In the menu, choose Select | All. Then choose Edit | Copy.
   
   The layer is copied to the clipboard.
   
   
3. Click the Channels tab. Click the previously created channel, e.g. flats.
   
   
   
   
4. In the menu, select Edit | Paste in Place. 
   
   
   
   
5. Then click the Layers tab.
   
   A Floating Selection (Pasted Layer) is visible in the Layers list.
   
   
   
   
6. Click the green Anchor the floating layer icon.
   
   The pasted layer is added to the active channel.
   
   
7. Optional. Click the Channels tab.
   
   Note the channel thumbnail has been updated with a grayscale image of the pasted layer.