Viola-jones

Version 2 (Anthony Rowe, 05/19/2007 09:32 am)

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== Violoa Jones ==
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The Viola-Jones sample project that comes with the CMUcam3, is an example of a lightweight
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face detector. The algorithm is based on the wellknown
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paper “Robust Real-Time Face Detection” by P. Viola and M. Jones from 2004 [attachment:viola-ijcv04.pdf 1]. 
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The paper introduces a novel technique to detect faces in real-time and with very high
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detection rate. It is essentially a feature-based approach in which a classifier is trained for
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Haar-like rectangular features [http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/proceedings/iccv/1998/8295/00/8295toc.xml&DOI=10.1109/ICCV.1998.710772 6] selected by AdaBoost. The test image is scanned at
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different scales and positions using a rectangular window, and the regions which pass the
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classifier are declared as faces. One of the major contributions of this paper is the
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extremely rapid computation of these features using the concept of Integral Image, which
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enables the detection in real-time. Additionally, instead of learning a single classifier and
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computing all the features for all the scanning windows in the image, a number of
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classifiers are learnt which are put together in a series to form a cascade. The classifiers
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in the beginning of the cascade are simpler and consist of smaller numbers of features.
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However, as one proceeds in the cascade, the classifiers become more complex. A region
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is reported as detection only if it passes all the classifier stages in the cascade. If it is
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rejected at any stage, it is discarded and not processed further. This way, the easier
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patches in the image which the “cascade of classifiers” is sure of not being a face, are
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rejected very early while the difficult regions are operated on by more complex
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classifiers. This greatly speeds up the detection process without compromising on the
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accuracy and provides high detection rate. This overall system provides performance
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comparable to the existing best face detector systems (Rowley et al., 1998 [attachment:CVPR00.pdf 2];
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Schneiderman and Kanade, 2000 [attachment:nips00.pdf 3]; Roth at al., 2000 [attachment:rowely-ieee.pdf 4]) but with orders of magnitudes
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faster than any of these systems. On a conventional desktop, it can detect faces at 15
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frames per second.