Today’s Question: Can you explain what exactly is meant by the option to intersect a mask [or selection]? I understand the options to add or subtract, but can’t wrap my head around intersect.
Tim’s Quick Answer: The intersect option involves modifying a mask or selection to only include the overlap between the existing mask or selection and a new mask or selection. I recommend thinking of the intersect option as providing a way to say “subtract everything but this”.
More Detail: Admittedly, the intersect option for masks and selections can be a little confusing. Adding to a selection or mask is rather straightforward, as is subtracting from a selection or mask. The intersect option isn’t quite as clear.
Let’s assume you have a mask or selection of three hot air balloons in the sky, but you only want to modify the middle balloon. One approach you could take would be to subtract the balloon on the left and then subtract the balloon on the right, leaving only the balloon in the middle being included in the selection or mask.
A faster option, however, would be to subtract everything except the middle balloon in one step. That’s what the intersect option makes possible. Using the intersect option, you select only the middle balloon, and that middle balloon becomes the only balloon that is selected or included in the mask.
What’s happening in the background is that the two selections or masks are being compared, and the final selection or mask will represent only the overlap (or intersection) between both of them. However, I think it is easier to think of it as an option to “subract everything but this”. In this example that would mean you are selecting the middle balloon, effectively saying “remove all balloons from the selection or mask except this one”.
Another way you could look at this is that the intersect option is the same as subtracting the opposite of what you’ve selected. In the example above, that means intersect is the same as creating a selection of the middle hot air balloon, inverting that selection or mask, and using that to subtract from the selection of all three hot air balloons.
