1. A computer-accessible memory medium storing program instructions, wherein the program instructions are executable by a computer system to: display a first window using a display system, wherein the first window represents a given portion of the earth's surface according to a given cartographic reference system; receive user input specifying a first plurality of points in the first window, wherein the first plurality of points define a first polyline, wherein the first plurality of points includes two end points and at least one knee point; receive user input specifying a first vertical range in a vertical dimension that extends perpendicular to the given portion of the earth's surface; receive user input identifying a first image that is stored in a memory; generate a first set of geometry data based on the first polyline and the first vertical range, wherein the first set of geometry data represents a first folded rectangle in three-dimensional space, wherein the first folded rectangle has one fold for each knee point in the first polyline; add the first set of geometry data to a 3D virtual world; add a copy of the first image to the 3D virtual world, wherein said adding the first image copy to the 3D virtual world includes specifying that the first image copy is to be applied as texture to the first set of geometry data; render the 3D virtual world to obtain a rendered image, wherein said rendering includes rendering the first set of geometry data using the first image copy as texture; and display the rendered image in a second window using the display system, wherein the displayed rendered image visually represents the first image as being painted onto the first folded rectangle in the three-dimensional space.