Layer Masks in GIMP the Free Photo Editor

So you understand Layers, and you’re having fun with Layer Blend Modes. Wonderful! If you really want more precision and control over which parts of which layers you see in your final image you’ll need to add one last element to this trifecta. Enter the Layer Mask. This is a seriously cool tool. Layer masks give you so much fine tuned control over your images you’ll find yourself using them all the time.

Layer Masks hide Layers. When a layer is hidden, you can see the layer below it just like when you use a Transparency or the Opacity Slider.

The cool part is that  you can hide or show any part of a layer by painting on it’s layer mask.  Layer masks don’t use color, only black and white and shades of gray. This makes them really simple to work with. Black on a layer mask completely hides the layer, white completely shows the layer and shades of gray are as transparent as they are light or dark. Dark grays show a little of the layer (like using a low Opacity) while light grays show a lot of it (like using a high Opacity).

In this example I’ll show you how to easily make a photo black and white with select areas of color, like in my barn photo.

Duplicate the Background Layer

Open the image you want to work with. Make sure the Layers Dialogue (that’s the Layers menu) is open. (Go to Windows > Dockable Dialogues > Layers in the main GIMP window to open the Layers Dialogue.)

Right click (two-finger tap on Mac) the Background layer icon and choose Duplicate Layer. We will be working with the Background Copy layer so make sure  it’s highlighted in blue.

Black and White with Desaturate

Go to Color in the main window and select Desaturate. I set the Desaturate options window to Luminance (but you should choose the option that gives the best black and white image) and click OK. Background Copy is now black and white. Background is still in color.

Add a Layer Mask

Right click (two-finger tap on Mac) the Background Copy icon and select Add Layer Mask.  An options menu will pop up. Choose White so we can still see all of the Background Copy layer.

There will now be a white rectangle icon next to the Background Copy icon. That’s the Layer Mask. It’s not hiding, or masking, anything yet  because it’s white. We’ll change that in a second. First make sure that the layer mask is selected and not the layer itself. Notice that the Background icon and the Background Copy icon both have a black border around them. This means they are NOT selected. The Layer Mask icon has a white border around it, meaning it IS selected. It’s hard to tell there is a white border on a white Mask, so if you want to be sure it’s selected click on the Mask icon.  Now we’re ready to paint.

Paint on the Layer Mask with Black

Choose Black as the foreground color. There are two overlapping rectangles in the Toolbox window. The default setting is for the top (Foreground) color to be white, and the bottom (Background) color to be black. When you paint, you’re typically painting with the foreground color. It could be made any color, but for now we  just want the black and white reversed so we can paint with black. There is a double sided arrow at the top right corner of the foreground/background color icons. Clicking the arrow switches the two colors. Black will be on top and white on the bottom.

Select the paintbrush tool and choose a hard edge circle. Leave the Blend Mode set to Normal. Leave everything else the same. Paint on the layer mask over the area that you want to change back to full color. Don’t worry too much about the edges. Those will get fixed in a minute.

Fix Edges of the Layer Mask with White

We’ll use a white paintbrush to clean up the edges. Click the double sided arrow again to switch the paint brush colors back to a white foreground and a black background.  Make the brush smaller by pressing the left square bracket key ” [ “.  I like to zoom in a lot for this so I can really see what I’m doing. Paint over any areas that shouldn’t be in color. If you’re having trouble “staying in the lines” try using the Pen Tool to make a Selection.

To Make Some Areas a Little Less Colorful

I think the wood in the barn is a little too colorful and looks kind of funny next to the black and white. There are a few ways I could desaturate all of the color in the picture but I’d  like the rust and the green panel to stay really saturated. I want to desaturate just the wood so I’ll brush in some dark gray. That will make just a little of the black and white layer visible through the mask on only the parts I paint with gray.

Hmmm…If I just paint randomly, I’ll have to be very careful of the edges I just cleaned up. I don’t want to mess up all of the work I just did refining the edges of my Layer Mask. To make sure I only work on the color parts of my picture I’ll Select those parts. Right now it’s as if the whole layer mask is selected. I can paint over the whole thing if I want to. Making a smaller Selection will limit the parts of the Mask where my tools will work.  Now that we have a mask to work with GIMP makes it easy to make a Selection in the exact shape if the mask.

Select Just the Mask

Right click (two-finger tap on Mac) the Layer Mask icon and choose Mask to Selection. There will be a moving dotted line all the way around the thing (or things) that you colorized. That dotted line is called Marching Ants. Now you can make any edits you want. Your tools will only be effective inside the Selection.

*If you made a Path to help you “stay inside the lines” you already have a Selection to help you with this part. Just go to Select > From Path to reselect the Path you made earlier if it’s not already selected.

Paint with Dark Gray to Desaturate

Dark gray will let just a little bit of the black and white layer show over the color layer. This makes the color look less saturated. Since I made a Selection around the barn my paint brush doesn’t colorize the parts I want to keep totally black and white.

Make the Foreground color dark gray by double clicking it’s icon. This will bring up a menu called Change Foreground Color with a bunch of options. You can either pick a color from the big square on the left side of the menu or, since we’re just working with black white and gray, you can move the V (for value) slider way over to the left. (All the way to the left is black.) Click OK.  Now I can just brush over the wood to make it a little less colorful.

When you’re done go to Select > None to get rid of the Marching Ants. You could make changes to any part of the Layer Mask again. But you’re done, so, um, don’t.

Save!!!!

I really am going to write a whole post about Saving. For now let’s just say…

If you are really really done and you want to save a small file, go to Layer > Flatten. This will merge all of your layers into one. Then go to File > Save As and save as a .jpeg .

If you area really done and you don’t mind saving a big file, go to Layer > Flatten to merge your layers into one. Then go to File > Save As and save as a .tiff

If you think you may want to edit this some more later Don’t Flatten!! Keep those Layers!! Save in GIMP’s native format (.xcm).  Just got o File > Save As and choose .xcm from the File Format drop down menu. This saves all of your layers and keeps them editable so you can come back later and tinker some more.

If you’re interested in Paths (AKA the Pen Tool) that post is right here! I’m even working on the same photo in that tutorial to show you a good way to make selections that help you “stay in the lines”.

Related posts:

1. Remove Blemishes with a Free Photo Editor
2. A Look at GIMP Layer Blending Modes