Regardless of how many curves are drawn, there are work and crop* area in tools like Vertical Straighten and Horizontal straighten. If Vertical Straighten is chosen, these two areas are shown in this example:
If Horizontal Straighten is chosen, the work area is between the first bottom and last top curve:
*If only one curve is set, the whole image is the work area.
Work area in Advanced Vertical Straighten and Advanced Horizontal Straighten are defined between Border curves. As mentioned, Advanced Vertical straighten has a Left Border curve and a Right Border curve. Advanced Horizontal straighten has a Bottom Border curve and an Upper Border curve.
Everything in Advanced Straighten tools is going between Border curves. The area outside Border curves is unaffected.
The Guide curve in Advanced Straighten represents deformation that should be straighten, but the straighten method defines the Leading curve.
The user sets the Leading curve the way that he(she) wants to straighten make. See images below:
Guide curves (yellow colour) are set where deformations are located. Curved lines defined by Guide curves will be straightened in a way defined by Leading curves (cyan colour).
As You can see, the curved sidewalk from the previous image is straighten and now looks more natural. The complete job of straightening this image is explained step by step in the section "Advanced Vertical and Advanced Horizontal Straighten example".