This mask is parametered by a 2D segment that ranges from 
                    
                      pixels fully unselected
                     to 
                    
                      pixels fully selected
                    .
        
                            Note that similar and more evolved masks can be obtained by combining a 
                    Layer mask
                    Turn any 2D generator into a mask
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                     with a 
                     Linear Gradient
                    This generator creates a linear gradient (by default from black to white), from top to bottom
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Linear Gradient
                    This generator creates a linear gradient (by default from black to white), from top to bottom
                    Read More
                    .
        
                            The corresponding mask in 3D space is: 
                    Linear
                    Using a line in space as mask
                    Read More
                    .