Sharpen > Extend Tangent Lines calculates the exact position of a sharp edge by extending a tangent line from each of the two flat sides that adjoin a curve line.
The two black lines generated by this function are at a distance from the orange curve line where the surface becomes flat, and from which tangent lines can be drawn to identify the ideal location of a sharp edge.
Factor field - allows you to adjust the extension of the tangent lines until you are satisfied that they accurately define the end of the fillet. The larger the factor, the further from the edge the tangent lines are extended, and vice versa. Once you select a factor, the operation calculates the tangent lines.
Extend button - draws the black lines at the distance specified by Factor.
Reset button - erases the black lines.
Done button (applicable when this function is called individually, not as part of the Sharpening Wizard) - saves the black tangent lines to the object in the Model Manager. If the black tangent lines require additional tweaking, use Curves > Edit Vertices again. If the black tangent lines are optimal, use Sharpen > Sharpen Polygons to reconstruct the polygon mesh to fill the space defined by tangent lines, thus creating a final sharpened object.
Cancel button - terminates the dialog without saving tangent lines.
Next >> button (appears only when this function is called through the Polygons > Sharpening Wizard) - advances the sharpening wizard to the next step (Curves > Edit Vertices) so that tangent lines can be tweaked if necessary.
To extend tangent lines:
Click Sharpen > Extend Tangent Lines. The Extend Tangent Lines dialog opens.
Select the orange curve line that lies on an edge you want sharpen. If none are selected, every orange curve line will be processed.
Click Extend. Black tangent lines appear on each side of every selected curve line. The black lines represent the positions on each side of the fillet from which intersecting tangent lines will form a sharp edge.
Adjust the Factor to ensure that black lines lie on the flat region of the object, not on the rounded fillet.
Click Clear to remove the tangent lines and start over, or OK to accept the positions of the black lines.