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.

I had to fake a photograph up a little for the background. However.

A quick mask is just a means to a selection. Depending which way you painted, it might look like (1)

Toggle the quickmask off, (Shift-Q), to show the crawling ants and if necessary invert the selection ( Select -> Invert )

Duplicate the layer (Layer -> Duplicate layer) and on the duplicated layer create a layer mask (Layer -> Mask -> Add layer mask). Choose the 'Selection' Option (2)

The layer mask will be 'active' so click on the icon to the left (ie **not** the mask layer icon. Active icon will show with a white border. ) Invoke Colors -> Desaturate on the layer (3)

Then you can mess about on the canvas touching up either layer as required. You can not make the image true grayscale (8 bit) and keep the original layer in colour. Keep the image as RGB.

Attachments: * https://www.gimpusers.com/system/attachments/1356/original/mask.jpg

rich404 (via www.gimpusers.com/forums)

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.