1 . A speed measurement method using a plurality of cameras, comprising the steps of: S1: calibrating positions of respective cameras in a camera group, which are arranged on a mobile platform at fixed relative positions, to obtain positional relationships among the cameras and a positional relationship between each camera and the mobile platform; S2: obtaining a sequence of synchronized images from the respective cameras in the camera group, and recording time stamps of the sequence of synchronized images; and S3: performing a registration on the images captured by the respective cameras in the camera group, wherein if a registration result of two of the images is greater than a first threshold, calculating a position transformation matrix for the two images, then calculating a position transformation matrix for the mobile platform over an interval between times when the two images were captured based on: i) a positional relationship between the cameras that captured the two image; ii) the positional relationships between the cameras that captured the two images and the mobile platform; and iii) the position transformation matrix for the two images, and finally calculating a speed information of the mobile platform based on: the position transformation matrix of the mobile platform, and the interval between the times when the two images were captured.
2 . The speed measurement method of claim 1 , wherein S3 comprises: S31: acquiring a position information of the mobile platform at a current time; calculating a first position of a first one of the cameras at the current time based on: the position information of the mobile platform at the current time; and the positional relationship between the first camera and the mobile platform, S32: based on the first position, successively searching second positions of the other camera(s) where images were captured; if a distance between a second position and the first position is smaller than a second threshold, determining that the search is successful, taking the images captured at the first position and the second position as a search image and a candidate image, respectively; recording the time stamps of the search and candidate images; and proceeding to S33, S33: registering the search and candidate images; if a registration result is greater than the first threshold, determining that the registration is successful and calculating a position transformation matrix for the search and candidate images; proceeding to S34, and S34: calculating a position transformation matrix for the first camera based on: the positional relationship of the cameras that captured the search and candidate images; and the position transformation matrix for the search and candidate images, calculating a position transformation matrix for the mobile platform based on: the position transformation matrix for the first camera; and the positional relationship of the first camera and the mobile platform, and calculating the speed information of the mobile platform based on: the position transformation matrix for the mobile platform; and an interval between times when the two images were captured.
3 . The speed measurement method of claim 2 , wherein determining the position information of the mobile platform at the current time comprises: registering the search image to an image of a previous frame of the first camera to derive a position transformation matrix for the first camera; and calculating the position information of the mobile platform based on the position transformation matrix of the first camera, and the positional relationship of the first camera to the mobile platform.
4 . The speed measurement method of claim 3 , wherein during calculating the position information of the mobile platform based on the position transformation matrix of the first camera derived by registering the search image of the first camera to the image of the previous frame thereof, and the positional relationship of the first camera to the mobile platform, the position information of the mobile platform is calculated by a weighted average, wherein weights are assigned to the respective cameras according to registration results thereof.
5 . The speed measurement method of claim 1 wherein the registration is accomplished using a two-dimensional image feature matching method comprising: extracting feature points from the two images and performing a rapid matching of the feature points to obtain the position transformation matrix for the two images.
6 . The speed measurement method of claim 1 , wherein the registration is accomplished using a two-dimensional image frequency-domain information matching method comprising: calculating a rotation and translational factor and a scaling factor for the two images using a Fourier-Mellin transform algorithm to obtain the position transformation matrix for the two images.
7 . The speed measurement method of claim 1 , wherein the registration is accomplished using a three-dimensional matching method, and wherein the three-dimensional matching method comprises calculating positions and orientations with three degrees of freedom in a space through coordinates of three-dimensional points corresponding to the two images to obtain the position transformation matrix for the two images.
8 . The speed measurement method of claim 2 , wherein the position information of the mobile platform at the current time is directly acquired from a position sensor.
9 . The speed measurement method of claim 2 , wherein if the search or the registration fails in step S32 or S33, the method proceeds to S35, and wherein S35 comprises: estimating the speed information of the mobile platform based on the position information of the mobile platform at the current time, position information thereof at a previous time and a difference between the current and previous times.
10 . A speed measurement apparatus using a plurality of cameras, comprising: a mobile platform, configured to move with respect to an object to be imaged; a camera group, comprising at least two cameras arranged on the mobile platform at fixed relative positions; and a processor, configured to implement the speed measurement method as defined in claim 1 .
11 . The speed measurement apparatus of claim 10 , wherein the camera group has a light source compatible with the cameras, wherein the light source comprises: a near-infrared light source, in case of the cameras being implemented as compatible cameras with lenses allowing transmission of a light emitted from the near-infrared light source; or a far-infrared light source, in case of the cameras being implemented as compatible far-infrared cameras; or an ultraviolet light source, in case of the cameras being implemented as compatible ultraviolet cameras; or a light source producing polarized light, in case of the cameras being implemented as compatible cameras with lenses allowing a transmission of the polarized light from the light source.
12 . The speed measurement apparatus of claim 10 , wherein the cameras capture images in synchronization or with time stamps.
13 . The speed measurement apparatus of claim 10 , further comprising a calibration tool arranged on the mobile platform, wherein the calibration tool is used to calibrate: a relative positional relationship among the cameras; and positional relationships of the cameras and the mobile platform.
14 . The speed measurement apparatus of claim 13 , wherein the calibration tool is a chessboard, a laser range finder, a lidar, a TOF sensor or an encoder.
15 . The speed measurement apparatus of claim 10 , wherein the camera group comprises at least one pair of binocular cameras.
16 . The speed measurement apparatus of claim 10 , wherein the camera group comprises at least two TOF cameras arranged along a principal moving direction of the mobile platform.
17 . The speed measurement apparatus of claim 10 , wherein the camera group comprises at least one pair of line scan cameras arranged along a principal moving direction of the mobile platform.
18 . The speed measurement apparatus of claim 17 , wherein the camera group further comprises at least one two-dimensional area scan camera with a global shutter, wherein the processor is configured to compare images captured by the line scan cameras with an image captured by the two-dimensional area scan camera in order to correct the images captured by the line scan cameras.
19 . The speed measurement apparatus of claim 10 , wherein when a registration result of two images is smaller than a first threshold, the processor is configured to estimate a speed information of the mobile platform based on a position information of the mobile platform at a current time, a position information thereof at a previous time and a difference between the current and previous times.
20 . The speed measurement apparatus of claim 10 , further comprising at least one laser measuring device for measuring a distance between different objects to be imaged, wherein the laser measuring device is a line laser measuring device or a cross laser measuring device.
21 . (canceled)
