Today’s Question: I learned that you can use an intersect option when masking in Lightroom Classic for targeted adjustments, but I can’t seem to wrap my head around what this intersect option actually does. Can you explain?
Tim’s Quick Answer: The intersect option for masks in Lightroom Classic represents the ability to subtract from an existing mask with the inverse of another mask.
More Detail: In Lightroom Classic you can apply targeted adjustments with the masking features. The basic process involves creating a mask that defines the area of the image you want to adjust, and then applying adjustments that only affect that area. As a simple example you could create a mask representing the sky, and then apply adjustments that only affect the sky.
You can also create compound masks that are built up from several components. For example, you could start with a mask based on the sky, and then subtract from the sky mask using a linear gradient. In this example you could have an adjustment that only affects the sky but affects the sky with a gradient transition.
The standard options for creating a compound mask are add and subtract. You can start with a basic mask, and then add additional areas of the image to that mask, so that all areas represented by the mask will be affected by the targeted adjustment. You could also start with a basic mask and subtract areas from it, as outlined above.
You can also hold the Alt key on Windows or the Option key on Macintosh to cause the Add and Subtract buttons to change to Intersect. The intersect option represents the ability to specify that you want to subtract all areas from the existing mask except for the areas defined by the intersection mask. In other words, if you start with one mask and then use the intersect option for another mask, the final mask will represent only the areas where the two masks overlap with each other.
Considering the example above where the sky is being affected by targeted adjustments based on a linear gradient, the easiest approach would be to start with a mask for the sky and then subtract from that sky mask using a linear gradient. Let’s assume, however, that you didn’t anticipate this and so you initially created the mask with a linear gradient.
In this example you would want to subtract the non-sky areas from the initial mask. That equates to subtracting the inverse of the sky, and subtracting the inverse is exactly what the intersect option provides. So, after creating a linear gradient mask, you can hold the Alt/Option key and click the Intersect button, choosing “Select Sky” from the popup. This will cause all non-sky areas to be removed from the mask initially created based on a linear gradient.
I demonstrated compound masks in great detail in the lesson on “Advanced Targeted Adjustments”, which is Chapter 3, Lesson 6 in my new “Mastering Lightroom Classic” course. This comprehensive course is included in the GreyLearning Ultimate Bundle (http://timgrey.me/atg99bundle) at no additional cost, but is also available as a standalone course here: