by Julieanne Kost
In the new Adaptive Wide Angle Filter in Photoshop CS6, shift-dragging will constrain the line to be either a vertical or horizontal constraint (depending on the direction that you drag).
If you have already drawn an arbitrary line and want to convert the line to a vertical or horizontal constraint, Control -click (Mac) / Right Mouse -click (Win) on the selected line and select Horizontal or Vertical from the list. Note that the color of the line changes from Teal (when the line is Arbitrary) to Yellow (horizontal) or magenta (vertical). You can use the Adaptive Wide Angle Filter Preferences to change the colors of the constraints (choose Preferences from the flyout menu to the right of the Correction option.)