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How to Add and Edit a Layer Mask in GIMP 3.0
In GIMP 3.0, add a mask from the Layers dialog, choose its starting transparency, and click the mask thumbnail to edit its pixels instead of the image.
To add a layer mask in GIMP 3.0, you can open the "Add Layer Masks..." command to set the initial transparency of your mask GIMP Add Mask Documentation. Once the mask is added, you can switch to editing its pixels by clicking the new mask thumbnail GIMP Edit Mask Documentation.
Accessing the Add Layer Masks command
You can access this command from the main menu by navigating through Layer → Mask → Add Layer Masks... GIMP Add Mask Documentation. If you are using the Layers Dialog, you can access it through the Add Layer Masks... command in the context menu, or by clicking on the Add Layer Masks... icon button located at the bottom of the dialog GIMP Add Mask Documentation. When you click the Add button, a thumbnail of the layer mask appears to the right of the layer thumbnail in the Layers Dialog GIMP Add Mask Documentation.
Choosing the initial transparency
The “Add Layer Masks...” dialog provides several choices under "Initialize Layer Mask to" for determining the mask's initial contents GIMP Add Mask Documentation. Choosing the White (full opacity) option will make all of the layer fully opaque, meaning you will not notice any difference in the layer's appearance until you paint on the layer mask GIMP Add Mask Documentation. Choosing the Black (full transparency) option will make all of the layer fully transparent, which is represented in the image by a checkered pattern GIMP Add Mask Documentation. With a black mask, you will need to paint on it to make any part of the layer visible GIMP Add Mask Documentation.
Switching to edit the layer mask
The Edit Layer Mask command acts as a toggle that switches you from editing the layer to editing the layer mask, if one is present, and vice versa GIMP Edit Mask Documentation. From the Layers Dialog, you can click on the layer mask thumbnail to make it active, or you can access it through the Edit Layer Mask command in the context menu GIMP Edit Mask Documentation. When the layer mask is in editing mode, a white border is shown around the mask in the Layers Dialog GIMP Edit Mask Documentation.
Displaying the mask while editing
Editing a layer mask is separate from showing the mask GIMP Edit Mask Documentation. You can make the layer mask visible, which makes it easier to see what you are changing on the mask GIMP Edit Mask Documentation.