1. A method for object detection as a function of a motion direction attribute, the method comprising: clustering training data set object images corresponding to object motion blobs into each of a plurality of motionlet sets as a function of similarity of their associated motion direction attributes, each of the motionlet sets comprising object image associated with similar motion direction attributes that are distinguished from the motion direction attributes of the object image blobs in others of the motionlet sets; resizing the clustered motionlet pluralities of object images from their respective original aspect ratios into a same aspect ratio, wherein the motionlet object images may have different original respective aspect ratios; learning motionlet detectors for each of the motionlet sets from features extracted from the resized training blobs and from sets of negative images of non-object image patches of the same aspect ratio obtained from background images; applying a deformable sliding window to detect an object blob in an input video obtained by background modeling by varying at least one of a size, a shape and an aspect ratio of the sliding window to conform to a shape of the detected input video object blob; extracting a motion direction of an underlying image patch of the detected input video object blob; selecting at least one of the motionlet detectors that has a motion direction similar to the motion direction extracted from an underlying image patch of the input video object blob; applying the selected at least one motionlet detector to the detected input video object blob; determining that an object has been detected within the detected input video object blob and extracting semantic attributes of the underlying image patch of the input video object blob if a one of the selected and applied at least one motionlet detectors fires; and storing the extracted semantic attributes of the underlying image patch of the input video object blob in a database for searching for the detected object as a function of its extracted semantic attributes.