1. A method for extraction of descriptors from video content, said descriptors being semantic descriptors adapted for a visual search of multiple video contents, the method comprising the following steps: receiving an incoming video stream at a machine that includes a processor, wherein the processor is configured to perform: a Key Frame Extracting step that includes applying a local descriptors-based approach to select pictures of an incoming video as key frames that are representative of a temporal region of the video which is visually homogeneous for the visual search of the multiple video contents; a Content Analysis step that includes analyzing content of said key frames and classifying image patches of said key frames as semantically interesting or not for said extraction of descriptors adapted for the visual search of the multiple video contents, said image patches having a key semantic meaning in queries for the visual search of the multiple video contents; a Descriptors Extracting step that includes extracting compact descriptors from said selected key frames, and defining a set of surrounding images also on the basis of input received from said Content Analysis step; a Temporal Coding step that includes multiplexing information about the time points at which said key frames have been extracted in said Key Frame Extracting step with said compact descriptors extracted in said Descriptors Extracting step, to obtain semantic descriptors characterizing said video content on the basis of contained objects for the visual search of the multiple video contents, said semantic descriptors comprising a multiplex of information about time points at which the key frames have been extracted and about said compact descriptors of the key frames, wherein the semantic descriptors establish relationships amongst the key frames along a video timeline of the incoming video stream; and an outputting step that includes outputting a stream of semantic descriptors for the incoming video stream that characterize the incoming video stream based on the contained objects, wherein the semantic descriptors account for spatial matching of individual pictures and of how these pictures inter-relate along the video timeline.