Computer system interface surface with reference points

A method of enabling user interaction with computer software running in a computer system via:

• an interface surface having printed thereon:
• visible information relating to the computer software, and a plurality of tags, each tag containing at least one perspective feature and coded data indicative of an identity of the interface surface and a location on the surface,
• said method comprising the steps of:
• (a) placing a pen, having a nib and an optical sensor,
• in an operative position relative to the interface surface and sensing at least one tag;
• (b) calculating a two-dimensional perspective transform of the tag using the sensed at least one perspective feature;
• (c) generating indicating data using:
• a known spatial relationship between the pen's physical axis and the pen optical axis, said axes being different from each other;
• the sensed coded data, and
• the two-dimensional perspective transform,
• said indicating data identifying the identity of the interface surface and a position of the pen nib relative to the interface surface;
• (d) sending the indicating data to a computer system,
• thereby enabling the computer system
• to identify at least one interactive element relating to the computer software, and
• operate the computer software in accordance with instructions associated with the at least one interactive element.