Re: [Gimp-user] change background in photo to grey scale

[Gary Aitken <gimp dreamchaser org>]

On 2/5/20 8:08 AM, PeterJWhite wrote:
> I'm brand new to GIMP. I photographed a bronze sculpture on an
> off-white paper background in natural light. The shadow areas of the
> background range from white to a peach color. I'd like to change the
> background to greyscale and then adjust it separately from the
> sculpture. I've made a quick mask but now can't figure out how to
> make the background a separate layer from the sculpture, so that I 
> can color correct the two layers independently. The sculptor wants to
> keep the shadows in the background.

0. Make sure the Layers dockable dialog is visible:
     Windows/Dockable Dialogs/Layers
1. Duplicate the original layer: Layer/Duplicate Layer
2. Rename the duplicated layer Bkg (dbl click in layers dialog)
3. Move the duplicated layer below the original
4. Select the original layer
5. Add an alpha channel: Layer/Transparency/Add Alpha Channel
6. Convert your mask to a selection / select the background or foreground
7. If your mask/selection is for the foreground, invert it: selection/Invert
8. Edit/Clear  This will make the background parts transparent

Your foreground is now in the top layer, and the background in the one
underneath labelled Bkg.