1. A three-dimensional face recognition method based on intermediate frequency information in a geometric image, the method comprising: step 1: making a pre-treatment to a test model and a pre-treatment of a library model, wherein the making of either pre-treatment involves: step 1.1 face cutting: locate a nose tip point from a plurality of points according to a shape index feature and geometric constraints of a facial point cloud, define a sphere with the nose tip point as a center of the sphere and a radius of 90 mm, discard outside points of the plurality of points that are outside of the sphere, wherein the facial point cloud comprising the points that are within the sphere; step 1.2 facial surface smoothing: carry out posture correction for the facial point cloud by principal component analysis (PCA), to obtain 3 orthogonal axial directions; take the nose tip point as an origin, choose a first eigenvector corresponding to a maximum eigenvalue as Y-axis, choose a second eigenvector corresponding to a minimum eigenvalue as Z-axis, establish a right-handed spatial three-dimensional coordinate system, wherein each point in the facial point cloud can be denoted by coordinates x, y, and z uniquely in the right-handed spatial three-dimensional coordinate system; and carry out triangularization for the facial point cloud in the right-handed spatial three-dimensional coordinate system, to obtain spatial triangular meshes; smooth the facial triangular meshes with a mesh-based smoothing algorithm for 10 iterations for noise removal, to obtain surface smoothed three-dimensional facial meshes; step 1.3 cutting of upper face discard the points below a y=−10 plane among the three-dimensional facial meshes, and keep an upper part of the three-dimensional face that is less affected by a facial expression; and step 1.4 diluting of facial point cloud: take samples from the facial point cloud evenly by a spatial distance at 1 mm interval, to obtain a diluted point cloud; carry out triangular meshing for the diluted point cloud, calculate and save side lengths γ l1 , γ l2 , γ l3 (l=1,2,. .. , η) of each spatial triangular patch among the three-dimensional facial meshes, where, η is a number of the spatial triangular patches among the three-dimensional facial meshes, and an average side length among all triangular patches is γ ; if any spatial triangular patch has a side in length greater than 4 γ , discard the spatial triangular patch but keep vertexes of the spatial triangular patch; step 2 mapping and extending of intermediate frequency information in a geometric image: step 2.1 mapping the coordinate information of facial point clouds of the test model and the library model to a plane respectively, to form geometric images of the test model and the library model respectively, wherein the method for obtaining the geometric images is as follows: step 2.1.1 mesh parameterization: map boundary points among the pre-treatment of the three-dimensional facial meshes to four sides of a square with a size of 512×512 pixels, and map non-boundary points among the three-dimensional facial meshes except the boundary points to an area within the square by mesh parameterization, to obtain planar meshes φ; take any vertex of the square as origin, take the directions in which two sides intersect at the origin as positive directions, and establish a counter-clockwise coordinate system MON, wherein any point in the plane can be denoted uniquely by coordinates m and n; on the four sides of the square, take b points evenly in a counter-clockwise direction, starting from the origin, wherein the coordinates of the samples are (m t 0 , n t 0 ) (t=1,2,. .. b), b is a number of boundary points among the three-dimensional facial meshes; and denote the vertexes of the three-dimensional facial meshes as f q (q=1,2,. .. , τ), where, τ is a number of vertexes, coordinates of corresponding points mapped from the vertexes to the area within the square are (m q , n q ), where, m q and n q are solutions of the following linear equation set: { Lm q = Ln q = 0 , ∀ f q ∉ B m q = m q 0 , n q = n q 0 , ∀ f q ∈ B , where, L is a Laplacian matrix of the three-dimensional facial meshes, and B is the boundary points among the three-dimensional facial meshes; and step 2.1.2 generating a geometric image: append the three-dimensional coordinates of the vertexes f q =(x q ,y q ,z q ) among the three-dimensional facial meshes to the corresponding points (m q ,n q ) as attributes of the points (m q ,n q ); determine the attributes of the points within the area within the square by linear interpolation, to obtain a two-dimensional image with three-dimensional coordinate attributes, which is referred to as a geometric image G; and step 2.2 Filter the geometric images G of the test model and the library model respectively, to obtain intermediate frequency information of the test model and the library model, wherein a method of filtering geometric image is as follows: step 2.2.1 carrying out multi-scale Haar wavelet filtering for a geometric image G; step 2.2.1.1 carrying out row transformation and column transformation for the geometric image G with a Haar transformation matrix sequentially, to obtain a low frequency coefficient set and horizontal, vertical, and diagonal high frequency coefficient sets; denote the low frequency coefficient set as LL 1 , and denote the horizontal, vertical, and diagonal high frequency coefficient sets as HL 1 , LH 1 , and HH 1 respectively; and step 2.2.1.2 carrying out Haar wavelet filtering for the low frequency coefficient set LL 1 again as that described in step 2.2.1.1, and output secondarily filtered low frequency coefficient set and horizontal, vertical, and diagonal high frequency coefficient sets, which are denoted as LL 2 , HL 2 , LH 2 , and HH 2 respectively; repeat the filtering for 5 cycles, with the low frequency coefficient set obtained in a previous cycle as an input in each filtering cycle, and output new low frequency coefficient set and horizontal, vertical, and diagonal high frequency coefficient sets; and step 2.2.2 extracting intermediate frequency information from a geometric image: extracting and saving a horizontal high frequency coefficient set HL 5 , vertical high frequency coefficient set LH 5 , and diagonal high frequency coefficient set HH 5 that are outputted in a final filtering cycle, take HL 5 , LH 5 , and HH 5 as attributes of the pixels, and construct three images in size of 16×16 pixels, which are referred to as horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image respectively; step 3: calculating a degree of similarity between the test model and the library model with wavelet domain structure similarity algorithm respectively, wherein a method of calculation is as follows: step 3.1 calculating an HL degree of similarity S HL between the horizontal intermediate frequency information image of the test model and the horizontal intermediate frequency information image of the library model, a LH degree of similarity S LH between the vertical intermediate frequency information image of the test model and the vertical intermediate frequency information image of the library model, and an HH degree of similarity S HH between the diagonal intermediate frequency information image of the test model and the diagonal intermediate frequency information image of the library model, sum up S HL , S LH , and S HH , and take the sum as the degree of similarity between the test model and the library model, wherein; the S HL , S LH , and S HH are obtained with the horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image to be matched and are calculated with wavelet domain structure similarity algorithm; the wavelet domain structure similarity algorithm is as follows: step 3.1.1 sorting the x of all pixels in the horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image in an order of corresponding pixel respectively, according to the three attributes (x, y, z) of each pixel in the horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image, and construct an x channel for the horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image respectively; construct and obtain y channels and z channels, similarly, for the horizontal intermediate frequency information image, vertical intermediate frequency information image, and diagonal intermediate frequency information image, and denote the channels as: C χ = [ c 1 , 1 c 1 , 2 … c 1 , 16 c 2 , 1 c 2 , 2 … c 2 , 16 ⋮ ⋮ ⋱ ⋮ c 16 , 1 c 16 , 2 … c 16 , 16 ] , where, χ is x, y, or z, C χ represents x channel, y channel, or z channel, c 1,1 is an element in row 1 and column 1 in C χ , c 1,2 is an element in row 1 and column 2 in C χ ,. .. , c 2,1 is an element in row 2 and column 1 in C χ ,. .. , and C 16,16 is an element in row 16 and column 16 in C χ ; the horizontal intermediate frequency information image, vertical intermediate frequency information image, or diagonal intermediate frequency information image is referred to as an intermediate frequency information image; calculate a degree of similarity s x of x channel, a degree of similarity s y of y channel, and a degree of similarity s z of z channel between two intermediate frequency information images to be matched, sum s x , s y , and s z , and take the sum as the degree of similarity S HL , S LH , or S HH between the two intermediate frequency information images to be matched, wherein the s x , s y , and s z are obtained with the following method: the x, y, or z channel of the intermediate frequency information image of the test model is represented by C p χ = [ c 1 , 1 p c 1 , 2 p … c 1 , 16 p c 2 , 1 p c 2 , 2 p … c 2 , 16 p ⋮ ⋮ ⋱ ⋮ c 16 , 1 p c 16 , 2 p … c 16 , 16 p ] , and a same channel of the corresponding intermediate frequency information image of the library model is represented by C g χ = [ c 1 , 1 g c 1 , 2 g … c 1 , 16 g c 2 , 1 g c 2 , 2 g … c 2 , 16 g ⋮ ⋮ ⋱ ⋮ c 16 , 1 g c 16 , 2 g … c 16 , 16 g ] , where, p indicates C p χ comes from the test model, g indicates C g χ comes from the library model; a number of rows and a number of columns of elements in C p χ and C g χ are denoted as α and β respectively, a 3×3 pixel neighboring region in C p χ is represented by C p χ ( α , β ) = [ c α - 1 , β - 1 p c α - 1 , β p c α - 1 , β + 1 p c α , β + 1 p c α , β p c α , β + 1 p c α + 1 , β + 1 p c α + 1 , β p c α + 1 , β + 1 p ] , the element c α,β p is a central element in the 3×3 pixel neighboring region in C p χ , a 3×3 pixel neighboring region in C g χ is represented by C g χ ( α , β ) = [ c α - 1 , β - 1 g c α - 1 , β g c α - 1 , β + 1 g c α , β + 1 g c α , β g c α , β + 1 g c α + 1 , β + 1 g c α + 1 , β g c α + 1 , β + 1 g ] , , the element c α,β g is a central element in the 3×3 pixel neighboring region in C g χ , and the structure similarity {tilde over (s)} χ (α,β) between c α,β p and c α,β g is: s ~ χ ( α , β ) = ( 2 ∑ e 1 = α - 1 α + 1 ∑ e 2 = β - 1 β + 1  c e 1 , e 2 p   c e 1 , e 2 g  + 0.1 ) ( 2  ∑ e 1 = α - 1 α + 1 ∑ e 2 = β - 1 β + 1 c e 1 , e 2 p ( c e 1 , e 2 g ) *  + 0.1 ) ( ∑ e 1 = α - 1 α + 1 ∑ e 2 = β - 1 β + 1  c e 1 , e 2 p  2 + ∑ e 1 = α - 1 α + 1 ∑ e 2 = β - 1 β + 1  c e 1 , e 2 p  2 + 0.1 ) ( 2 ∑ e 1 = α - 1 α + 1 ∑ e 2 = β - 1 β + 1  c e 1 , e 2 p ( c e 1 , e 2 g ) *  + 0.1 ) where, e1 and e2 represent a row suffix and a column suffix of the elements in C p χ (α,β) and C g χ (α,β) and (c e1,e2 g )* is a conjugate value of c c1,c2 g ; Let α=2,3,. .. , 15, β=2,3,. .. ,15, take the average of {tilde over (s)} χ (α,β) as the structure similarity between C p χ and C g χ : s χ = 1 196 ∑ α = 2 15 ∑ β = 2 15 s ~ χ ( α , β ) ; and step 4: identity recognizing of three-dimensional face: repeat steps 1 to 3, to obtain the degree of similarity between the test model and each library model, compare the degrees of similarity between the test model and the library models, and judge that the library model with a maximum degree of similarity is a same identity as the test model.