Tilted Haar-like Features
Lienhart and Maydt introduced the concept of a tilted (45°) Haar-like feature. This was used to increase the dimensionality of the set of features in an attempt to improve the detection of objects in images. This was successful, as some of these features are able to describe the object in a better way. For example, a 2-rectangle tilted Haar-like feature can indicate the existence of an edge at 45°.
Messom and Barczak extended the idea to a generic rotated Haar-like feature. Although the idea sounds mathematically sound, practical problems prevented the use of Haar-like features at any angle. In order to be fast, detection algorithms use low resolution images, causing rounding errors. For this reason, rotated Haar-like features are not commonly used.
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