Method of detecting edge under non-uniform lighting background

An interactive projection system comprising:

• at least one light source that illuminates a surface;
• an optical detector that captures at least one image of said surface;
• a microprocessor coupled to said optical detector;
• and
• a non-transitory computer-readable storage medium coupled to said microprocessor, said non-transitory computer-readable storage medium encoded with computer-readable instructions for causing said microprocessor to execute:
• capturing a first image and a second image using said optical detector, wherein said first image is an image of said surface and said second image is an image having an object on top of said surface;
• determining a first gradient direction profile of said first image;
• determining a second gradient direction profile of said second image;
• computing a differential profile based on said first gradient direction profile and said second gradient direction profile;
• and
• determining said edge of said object based on said differential profile,
• wherein said differential profile registers gradient magnitudes of said second gradient direction profile and angular differences between said first gradient direction profile and said second gradient direction profile,
• wherein said differential profile includes absolute values of a product of said gradient magnitudes of said second gradient direction profile and sine values of said angular differences between said first gradient direction profile and said second gradient direction profile.