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PhD Thesis - Automated Recognition of 3D CAD Model Objects in ...

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Literature Review 12<br />

object recognition problems are a sub-set <strong>of</strong> pattern match<strong>in</strong>g problems [13].<br />

The literature on model-based <strong>3D</strong> object recognition is extensive. Solutions<br />

are designed based on the constra<strong>in</strong>ts characteriz<strong>in</strong>g the problem <strong>in</strong> its specific<br />

context. In the specific problem <strong>in</strong>vestigated here, it can be assumed at this po<strong>in</strong>t<br />

that: (1) search objects may have any arbitrary shape; (2) they can be viewed<br />

from any location, mean<strong>in</strong>g that their pose <strong>in</strong> the sensed data is aprioriunknown;<br />

(3) the relative pose <strong>of</strong> two objects <strong>in</strong> the sensed data is also aprioriunknown;<br />

and (4) they can be partially or fully occluded.<br />

Object recognition systems rely on the choice <strong>of</strong> data representations <strong>in</strong>to which<br />

the sensed data and the search object models can be obta<strong>in</strong>ed (possibly after conversion)<br />

and from which both data can be described us<strong>in</strong>g similar features (or<br />

descriptors) [13]. The choice <strong>of</strong> the data representation determ<strong>in</strong>es the recognition<br />

strategy and thus has a significant impact on the efficiency and robustness <strong>of</strong><br />

the recognition system. An adequate representation is unambiguous, unique, not<br />

sensitive, and convenient to use [13]. However, the performance required by the<br />

application generally lead to the choice <strong>of</strong> representations that compromise some<br />

<strong>of</strong> these characteristics for the benefit <strong>of</strong> others. In the case <strong>of</strong> the problem <strong>in</strong>vestigated<br />

here, a data representation should be unambiguous and unique, because this<br />

would ensure that each object can only be represented <strong>in</strong> one dist<strong>in</strong>ctive way [27].<br />

The choice <strong>of</strong> a data representation must be accompanied by robust techniques for<br />

extract<strong>in</strong>g compatible features from both object models and <strong>in</strong>put range image.<br />

<strong>Model</strong>-based object recognition systems that can be found <strong>in</strong> the literature use<br />

data representations with different levels <strong>of</strong> complexity. <strong>3D</strong> data representations<br />

that have been used <strong>in</strong> the literature <strong>in</strong>clude parametric forms, algebraic implicit<br />

surfaces, superquadrics, generalized cyl<strong>in</strong>ders and polygonal meshes [13]. Polygonal<br />

meshes are very popular for at least three reasons: (1) meshes can faithfully approximate<br />

objects with complex shapes (e.g. free-forms) to any desired accuracy (given<br />

sufficient storage space); (2) <strong>3D</strong> po<strong>in</strong>ts, such as range po<strong>in</strong>ts, can easily be triangulated<br />

<strong>in</strong>to meshes; and (3) a variety <strong>of</strong> techniques exists for generat<strong>in</strong>g polygonal<br />

mesh approximations from other <strong>3D</strong> data respresentations such as implicit surfaces<br />

[89] or parametric surfaces [73]. Triangles are the most commonly used polygons<br />

<strong>in</strong> polygonal meshes.<br />

In the case <strong>of</strong> the problem <strong>in</strong>vestigated here, search objects are construction<br />

project <strong>3D</strong> objects. The specificity <strong>of</strong> construction project <strong>3D</strong> objects is that they<br />

are <strong>of</strong>ten designed <strong>in</strong> <strong>3D</strong> us<strong>in</strong>g Computer-Aided Design (<strong>CAD</strong>) model<strong>in</strong>g s<strong>of</strong>tware,<br />

and the object <strong>3D</strong> models are generally parametric forms. <strong>3D</strong> design is now particularly<br />

standard <strong>in</strong> <strong>in</strong>dustrial projects which are the specific type <strong>of</strong> projects<br />

identified with<strong>in</strong> the scope <strong>of</strong> this research. Parametric forms could thus be used<br />

as the search object data representation for the object recognition problem <strong>in</strong>ves-

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