3D VISION


detected using regular cameras. It is much more sensitive to depth than structured light scanning, which is based on triangulation between camera and projector, and disregards specular reflection as noise. Te disadvantage of


deflectometry is that the reflection visible in an object – especially a convex object – is typically small. So, with standard methods, only a tiny patch of the surface can be mapped at one time, with the process repeated and all the patches stitched together to capture the entire 3D shape. A deflectometry sensor mounted on a robot would take hundreds of different images for each small neighbourhood when inspecting paintwork on a car body, for instance.


‘[Standard deflectometry] is over-engineered; we don’t need to display arbitrary images in order to encode the scene’


‘Te contribution of this


technology is to reinvent the scene encoder,’ he said. ‘An electronic display is very convenient hardware for generating a flat scene, but far less practical if you need to comprehensively surround an object like a car. It is also over- engineered, as we don’t need to display arbitrary images in order to encode the scene.’ Instead of a display, du


Preez’s method relies on a static pattern printed on the inside of an arch. Moving the arch over the car encodes a scene that is uniquely decodable. Te pattern consists of a set of latitude codewords that when translated with longitudinal motion encode each scene point’s latitude and longitude. To differentiate it from other varieties of deflectometry, du Preez calls this approach Latitude Code Scanning Deflectometry.


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10.09.21 10:55 DECEMBER 2021/JANUARY 2022 IMAGING AND MACHINE VISION EUROPE 7 Te pattern’s design is


optimised using a technique analogous to channel coding in telecommunications. Te codewords are packed together inside a codespace as efficiently as possible. ‘But where discrete communications codes are optimised by sphere packing, what we do could


more accurately be described as spaghetti packing, since we are encoding a continuous variable,’ du Preez explained. At one stage, du Preez said


the easiest way to design the pattern was to work with a physical analogy. He fixed up a jig shaped like a pyramid to represent the codespace, and


experimented with different ways of fitting a length of flexible pipe into the volume. ‘An efficient encoding ultimately allows for shorter codewords, reducing the depth and mass of the required arch and the machine as a whole,’ he explained. Du Preez is a software


g


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