Adobe Creative Cloud

June 13, 2009 /Region of Interest /

mask expansion and rounded corners

Someone just asked on a forum why expanding a rectangular mask resulted in a mask with rounded corners. I made a little visual aid to answer the question, so I thought that I should post it here.

Increasing mask expansion is not the same thing scaling a mask. Scaling preserves shape. Expansion extends the influence of the mask by growing outward from each point along the mask path by a certain number of pixels.

For each point on the original mask path, imagine a circle radiating outward by the number of pixels by which you’re expanding.
mask_expansion2.png
In the image above, the white rectangle is the original mask path. The red circles indicate the expansion radius. Note how these circles define a sharp inner rectangle, which is what you get with a negative expansion; and they define a rounded outer rectangle, which is what you get with a positive expansion.

Region of Interest