To subtract from an existing layer mask in Photoshop, use Option + Command (Mac) / Alt + Control (Win) while creating the new mask or selection. This will remove the selected area from the existing mask.
Here's a more detailed breakdown:
Understanding Layer Masks
Layer masks are fundamental in Photoshop for non-destructively controlling the visibility of a layer. A layer mask is essentially a grayscale image that sits alongside your layer. White areas on the mask reveal the layer, black areas hide the layer, and shades of gray partially reveal it.
Subtracting From a Layer Mask
There are several ways to subtract from a layer mask:
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Using Selection Tools:
- Select the Area to Subtract: Use any selection tool (e.g., Lasso, Rectangular Marquee, Magic Wand) to create a selection of the area you want to remove from the mask.
- Subtract from the Mask:
- Press Option + Command (Mac) / Alt + Control (Win) simultaneously.
- Then, click the "Add Layer Mask" button at the bottom of the Layers panel. This creates a new mask where the selected area is subtracted (made black) from the existing mask. Alternatively, if a mask already exists, make sure the mask thumbnail in the Layers panel is selected, and fill your selection with black (Edit > Fill > Black) or use the Paint Bucket tool with black as the foreground color.
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Using the Brush Tool:
- Select the Layer Mask: Click on the layer mask thumbnail in the Layers panel to make sure it's active.
- Choose the Brush Tool (B): Select the Brush Tool from the toolbar.
- Set the Foreground Color to Black: Ensure your foreground color is set to black.
- Paint on the Mask: Paint directly onto the layer mask where you want to hide portions of the layer. Painting with black will subtract from the mask, making those areas of the layer transparent.
Example:
Let's say you have a layer with a photo of a person, and you've created a layer mask to remove the background. Now, you want to subtract a specific shape, like a circle, from the person in the image (revealing the background underneath).
- Select the Elliptical Marquee Tool.
- Hold Option + Command (Mac) / Alt + Control (Win).
- Draw a circle over the area of the person you want to subtract.
- Click the "Add Layer Mask" button in the Layers panel (if you haven't got a mask yet) or fill selection with black if you already have an existing mask. The circle will now be subtracted from the layer mask, revealing the background in that circular area.
Key Points to Remember:
- Make sure the layer mask thumbnail is selected in the Layers panel when editing the mask.
- Black hides, white reveals, and shades of gray partially reveal the layer based on the mask.
- You can always edit the mask later to refine your results.
Subtracting from layer masks allows for precise and non-destructive editing, giving you greater control over your image compositing in Photoshop.
