Selection Merge Confusion


Today’s Question: I created a layer mask [in Photoshop] to separate my subject from the background so that I could apply more image noise reduction to the background than what the subject will get. I am also using the same mask to blur the background but not the subject. If I combine the two layer masks into a single selection, the entire image is not selected. How can that be?

Tim’s Quick Answer: The short answer here is that adding two selections together doesn’t produce a result where the luminance values in the shades of gray of a selection are exactly added together. In effect, adding luminance values of 50% plus 50% does not yield 100%, but rather 75%.

More Detail: In most cases this behavior is not a real problem, because the specific impact of a particular shade of gray on a layer mask isn’t critical. The result achieved with white on a layer mask (the full effect is applied) is important. And the result achieved with black on a layer mask (none of the effect is applied) is important. Shades of gray in between tend to be less important.

Still, this behavior can be problematic when combining different layer masks or selections together. For example, let’s assume a simple selection that is 50% selected. If you invert the selection that area would still be 50% selected. But if you merge the two selections together, you don’t end up with 100%.

In other words, merging two selections together, where the second selection represents the inverse (the exact opposite) of the first selection will not produce a result where all pixels in the image are selected.

I’m sure there is some complicated mathematical explanation for why this situation makes sense, rather than what we might consider a more “logical” approach to combining selections and layer masks. But the bottom line is that if you want to select the entire image, you should use the “Select All” command found on the Edit menu, rather than combining existing selections or layer masks.