1. A computer-readable memory medium that has program instructions stored thereon, wherein the program instructions are executable by a computer system to implement a method comprising: displaying a first image in a first window using a display system, wherein the first image corresponds to a first view of a graphical object set that includes one or more graphical objects in a three-dimensional (3D) space, wherein each of the one or more graphical objects represents a corresponding sub-surface geological structure; displaying a second window using the display system; generating a polygon in the 3D space, wherein said generating includes: receiving user input defining a closed planar polyline in the second window, wherein the closed planar polyline includes three or more 2D vertices in the second window; and mapping each of the three or more 2D vertices to a respective 3D point in the 3D space, thereby obtaining three or more 3D points that define 3D vertices for the polygon; rendering the polygon with respect to the first view to generate an update for the first image; displaying the updated first image in the first window, wherein the updated first image depicts at least a portion of the polygon and at least a portion of a first of the graphical objects of the graphical object set.