To enable the draw mask (Quick Mask) in GIMP, you need to activate Quick Mask mode. Here's how:
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Make a selection around the area you don't want to be masked. The easiest way to do this initially is often with the Rectangle Select Tool. Consider drawing a rectangle around the edge of the area you want to edit. Think of the selection as defining what won't be affected by your drawing.
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Click the Quick Mask icon. This icon is located in the lower-left corner of the image window and is typically represented by a rectangle with a red border (though the color is customizable). Clicking this toggles the Quick Mask on. The selected area will become transparent, while the unselected area will be overlaid with a colored mask (usually red by default).
Now, you can paint directly on the image using GIMP's paint tools. Painting with white removes the mask, making the underlying pixels editable when you exit Quick Mask. Painting with black adds to the mask, protecting the underlying pixels from changes when you exit Quick Mask. Grayscale values create semi-transparent masks.
In Summary:
- Select: First create a selection representing the part of the image you don't want masked.
- Activate Quick Mask: Click the Quick Mask icon.
- Paint: Use white to reveal, black to conceal (add to the mask), and grayscale for partial masking.
- Exit: Click the Quick Mask icon again to exit the mode and apply your mask. Your painted mask will become a selection.
