1. A method for analyzing a multispectral image, said multispectral image comprising a plurality of spectral images of the same scene but corresponding to different spectral ranges, each spectral image assigning an intensity value to each pixel located at an intersection of a row and a column of a matrix of the multispectral image, and an origin point being defined at a corner of the peripheral boundary of the matrix; wherein a detection image is constructed by assigning a display value to each pixel of a working area of the matrix, said display value being obtained from a revealing function that quantifies a content shift in the multispectral image between two areas that are determined by said pixel; the method comprising the steps of: /1/ calculating integral images of at least one of the following types: for each spectral image, a first-order integral image, by assigning to each calculation pixel an integral value equal to the sum of the intensity values of said spectral image for all pixels contained within a rectangle having two opposite vertices respectively located on the origin point and on said calculation pixel; for each pair of spectral images obtained from the multispectral image, a second-order integral image, by assigning to each calculation pixel another integral value equal to the sum, for all pixels contained within the rectangle having two opposite vertices respectively located on the origin point and on said calculation pixel, of products of the two intensity values relative to the same pixel but respectively assigned by each spectral image of the pair; /2/ defining a fixed window frame and a mask internal to the frame, said mask defining a target area and a background area within said frame; and /3/ for each pixel of the working area of the matrix, performing the following sub-steps: /3-1/ placing a window at a position in the matrix that is determined by the pixel, said window being delimited by the frame defined in step /2/; /3-2/ using the integral values read from the integral images calculated in step /1/ to determine a value of the revealing function that quantifies the content shift in the multispectral image between the target area and the background area within the window, in the position determined by said pixel; and /3-3/ assigning the value of the revealing function, calculated in sub-step /3-2/, to the pixel for which step /3/ is performed, wherein the display value is obtained from the value of the revealing function, for each pixel in the detection image, using one or a combination of the following methods: comparing the value of the revealing function to a threshold, and the display value is set to zero if said value of the revealing function is below the threshold, otherwise said display value is set to said value of the revealing function; or applying a linear scale conversion to the value of the revealing function, and the display value is set to the result of the conversion.