To create your mask, use the brush to outline the areas that require change, and then fill them in. When you release the mouse, the pin will appear on the document preview where you first placed the brush cursor down. When you mouse over the pin, your mask will be displayed with a red “rubylith” preview (similar to Photoshop’s Quick Mask). If you have a few areas that you masked by mistake, click the Erase option in the panel, set the size and feather for the Erase brush, and paint over the areas you do not want to include in your edits. The rubylith preview in those areas will disappear as you brush, letting you know they’ve been excluded from the mask.
The Auto Mask feature is quite helpful; I recommend you leave this checked as it makes the process of brushing along boundary areas (especially those marked by obvious color or contrast changes) much more accurate. Essentially, you can quickly brush around a contour and Lightroom will guess where you are trying to “draw the line”, intelligently limiting the mask to the areas you are targeting, based on the local color and contrast near the brush stroke (Figure 3).
Figure 3 Using the Auto Mask feature makes it simple to stay within the bounds of your mask area (right).