Interest operators extract salient image features, which are distinctive in their neighbourhood and are reproduced in corresponding images in a similar way [4]; at the same time, interest operators supply one or more characteristics, which can be used in the image matching. Region detector operators, instead, search for a set of pixels which are invariant to a class of transformations (radiometric and geometric distortions). The term ��region�� differs from classical segmentation since the region boundaries do not have to correspond to changes in image appearance such as colour or texture [5].These operators have been developed for when the normal stereo image acquisition condition is not required. Region operators detect features that do not vary with different geometrical transformations (scale, affine transformation, etc.
). A descriptor, which describes the extracted feature using a 2D vector that contains gradient pixel intensity information, is associated to each region. This information may be used to classify the extracted regions or to perform the matching process.Although region detector/descriptors are computationally slower than those of interest points, the experimental results show that these detectors have a wider application range. Interest in these detectors in the photogrammetric field is quickly increasing due to the introduction of new image acquisition techniques, which do not comply with the normal stereoscopic case. Images acquired through Mobile Mapping Technology [6] are usually extracted from video-sequences with low-resolution quality.
Consequently, the orientation process is hampered by illumination problems, the limited dynamic range of the video-cameras, sensor noise, narrow baselines and projective distortions. Oblique photogrammetric images, Dacomitinib which are commonly used for the generation of 3D city modelling [7], offer images that are affected by high projective distortions which must be carefully processed. Finally, image sequences acquired using low-cost UAV platforms [8] do not assure the normal taking geometry.Interest point extractors and matchers, which are traditionally used in photogrammetry (Forstner operator [9], Harris operator [10], Cross-Correlation etc.), are usually inefficient for these applications as they are unable to give reliable results under difficult geometrical and radiometrical conditions (convergent taking geometry, strong affine transformations, lack of texture etc.)The SIFT operator is one of the most frequently used in the region detector field. It was first conceived by Lowe [11] and it is currently employed in different application fields.