Background
The airborne remote sensing image has very important application value in the military field. Due to the safety of the acquisition mode and the richness of the acquired image data, the method plays a vital role in the aspects of camouflage effect detection and evaluation, firepower investigation and accurate fighting, battlefield environment perception and the like. However, due to the influence of various factors such as the vibration of the airborne platform and the airflow disturbance, preprocessing is also needed before the airborne remote sensing images are fused for analysis, wherein the realization of high-precision registration of the images is an important prerequisite for image analysis.
The purpose of image registration is to spatially align images of the same region, which are derived from different time instants, different angles, different environments, different imaging mechanisms, and the like. The registration of multi-source images is always a difficult problem in the field of remote sensing image processing, and mainly shows two aspects: firstly, the imaging mechanisms of different sensors are different, and although the characteristics of the ground object can be reflected from different aspects, the characteristics of the same ground object expressed in different source images are greatly different; secondly, due to the influence of various factors such as flight attitude, position and the like, the remote sensing images of different sources have differences in shooting angles, distortion degrees and the like.
The current registration method of remote sensing images can be roughly divided into three categories: a registration method based on grey correlation, a registration algorithm based on a transform domain and a registration method based on characteristics. The image registration method based on the gray-scale related information mainly utilizes the gray-scale information of the image to establish similarity measurement, and realizes registration by seeking a transformation model when the similarity measurement is maximum or minimum, and common algorithms comprise normalized cross correlation, sequential similarity detection and the like. The algorithm is generally high in robustness and precision, but large in calculation amount. The image registration algorithm based on the transform domain firstly carries out the transformation from space domain data to frequency domain data, and then determines matching parameters by measuring the similarity between two images, wherein the commonly used method comprises Fourier transform and Gabor transform. The method is simple and has high precision, but the requirement on the image is high, estimation errors are easy to generate, and image registration fails. The feature-based registration method is the most widely studied algorithm, and mainly comprises feature extraction, feature matching and image interpolation. The algorithm has good robustness in the aspects of processing image rotation, scale change, brightness change and noise, and the matching method based on the image characteristics has great advantages under the conditions of low image quality and obvious deformation. However, the method has the defects of large calculation amount, more extracted feature points, easy mismatching and the like in different degrees in the using process.
Images shot by different types of sensors represent different characteristic information of the ground object, for example, a visible light image and a full-color image reflect space reflection information of the ground object, and an infrared image reflects heat radiation characteristics of the ground object. Therefore, the fusion of multi-source remote sensing image data is realized, the advantages of multi-source images can be fully exerted, and more accurate, comprehensive and rich information is provided. The realization of high-precision multi-source image registration is a key step and an important premise of data fusion, but most of the existing image registration algorithms register specific single-type remote sensing images, the method is not strong in universality, and generally cannot be directly applied to the registration of multi-source images.
Disclosure of Invention
Aiming at the problems in the background art, the invention provides a high-precision and quick registration method for an airborne remote sensing image by combining geographic coordinates and feature point matching, which comprises the following steps:
s1, selecting a characteristic region according to the influence characteristics of the image to be registered, wherein the characteristic region comprises at least one pair of corresponding characteristic points;
s2, acquiring the position of the feature point in the feature area;
s3, connecting the corresponding feature point pairs in the reference image and the image to be registered;
and S4, registering the reference image and the image to be registered according to the position relation of the characteristic point pairs.
Preferably, step S4 further includes: and establishing a mapping relation between the image to be registered and the reference image according to the positions of the matched feature points on the image to be registered and the reference image.
Preferably, the method further comprises: and S5, registering the image to be registered by using the mapping relation between the image to be registered and the reference image.
Preferably, step S5 further includes: and calculating the positions corresponding to the feature points of the image to be registered, and performing registration on the image to be registered by utilizing the mapping relation between the image to be registered and the reference image.
Preferably, step S5 further includes: and converting the coordinates of the feature points in the image to be registered into a reference image coordinate system according to the mapping relation to obtain the positions of the feature points in the reference image.
Preferably, the method further comprises: s6, calculating the matching error and the total registration error of each pair of feature points, if the total error is less than a given threshold, finishing the image registration process, otherwise, returning to the step S3 to continue the feature point matching.
Preferably, in step S1, the feature region includes 4-6 sets of feature point pairs.
Preferably, in step S2, feature point position information in the feature region of the reference image and the image to be registered is calculated by the Harris corner detector.
Preferably, step S3 further includes: and respectively searching and marking the characteristic point pairs in the two images according to the calculation result in the S2, and completing the connection of the characteristic point pairs.
Preferably, in step S1, a region including an edge point, a corner point, or a line intersection of the object is taken as the feature region.
The invention has the beneficial effects that: the method improves the operation efficiency and the accuracy of characteristic point selection by pertinently adopting the characteristic region to extract the characteristic points, and simplifies the calculation process by utilizing the characteristics of high processing speed and powerful functions of ArcGIS software, thereby quickly obtaining the high-accuracy remote sensing image registration image. Moreover, the method is suitable for registering various remote sensing image combinations, and multi-source images such as visible light, infrared and synthetic aperture radars and the like can be registered by the method. The method is simple to operate, reliable, good in applicability, strong in practicability, high in precision of the obtained experimental result and wide in application prospect.
Detailed Description
Embodiments of the present invention will be described below with reference to the drawings, and technical features in the following examples and embodiments may be combined with each other without conflict.
Fig. 1-2 are flow charts of the registration method of the present invention. Referring to fig. 1, in step S1, a feature region is selected according to the influence characteristics of the image to be registered, wherein the feature region includes at least one pair of corresponding feature points.
Specifically, the reference image and the image to be registered are observed, the image distribution characteristics are analyzed, the corresponding characteristic area is selected, and the coordinate and the size of the selected characteristic area are recorded. In one embodiment of the present invention, the normal image is used as a reference image, and the visible image and the infrared image are respectively used as images to be registered. The ortho image is an airborne image corrected with UTM (Universal transmercator coordinate system) coordinates, has no distortion and high precision, contains an area of the image to be registered, and is suitable for being used as a reference image.
The purpose of selecting the feature regions is to calculate the feature point information more efficiently, so the feature regions should be selected based on the more obvious feature points in the reference image and the image to be registered, and each set of feature regions should include at least one pair of corresponding feature points for selection. The principle of selecting the characteristic point pairs is as follows: object edge points, corner points, line intersections, etc. are selected. The feature area should be as small as possible to avoid the portion of the image that affects the feature point calculation, and large enough to include the portion where the feature point may exist, ensuring the accuracy and validity of the calculated feature point position.
In one embodiment, two images, i.e., a reference image and an image to be registered, are opened, the shape and distribution characteristics of the two images are observed, and an area including a crossing point or an angular point of a road or a building, which is relatively obvious, is selected as a feature area, so as to further extract feature points. As shown in fig. 3 (a) and (b), fig. 3 is a schematic diagram of selecting feature regions, including feature point distribution, where fig. 3 (a) is a reference image and fig. 3 (b) is an image to be registered, and feature regions with the same number in the two images correspond to each other one by one, and a feature region 1 is selected. And (3) acquiring coordinate information of the upper left corner of the selected feature region, wherein the coordinate information is (x, y) and (x ', y') in the reference image and the image to be registered respectively.
Referring again to fig. 1, in step S2, the positions of the feature points in the feature region are acquired.
Finding the exact coordinates of the feature points in the feature area requires the use of a feature detector. According to the property of the characteristic points, Harris angular points can be selected for characteristic point extraction. Due to the feature region selection operation in step S1, the accuracy of the feature detector in selecting the corresponding feature points in the image can be effectively enhanced. The characteristic point extraction process is carried out based on Harris angular point detection, and the specific implementation comprises the following steps.
S21, calculating the gradient I of the image I (X, Y) in the X and Y directionsx、Iy。
S22, calculating the product of the gradients of the two directions of the image.
Ixy=Ix·Iy
At S23, the resulting three gradient products are gaussian-weighted (assuming σ as 1) using a gaussian function, and elements A, B and C of the matrix M are generated.
S24, calculating a Harris response value r for each pixel, and setting r less than a certain threshold t to zero, α taking 0.05.
r={r:det M-α(traceM)2<t}
S25, performing non-maximum suppression in a neighborhood of a certain size (e.g., 3 × 3 or 5 × 5), where the local maximum point is a corner point in the image.
The position information of the feature points in the feature areas of the reference image and the image to be registered is calculated by the Harris corner detector, and is (u, v) and (u ', v'), respectively, so that the corresponding relation of the feature point pairs can be obtained. Assuming that the coordinates of the selected feature areas at the upper left corners in the reference image and the image to be registered are (x, y) and (x ', y'), respectively, the positions of the feature points in the corresponding original images obtained by simple addition calculation are:
(x0,y0)=(x+u,y+v)
(x1,y1)=(x′+u′,y′+v′)
referring again to fig. 1, in step S3, pairs of characteristic points are connected.
And respectively searching and marking the characteristic point pairs in the two images according to the calculation result in the S2, thus completing the connection work of a group of characteristic point pairs.
And selecting n groups of corresponding characteristic points for matching (the matching is to determine the corresponding relation of the characteristic point pairs, and for a group of characteristic point pairs, the characteristic point pairs are located at the same position in the registered image), wherein n cannot be too small, otherwise, the precision cannot be ensured, and the precision cannot be too large, otherwise, the calculation efficiency is influenced. Generally, n is 4-6, so that the accuracy of the result can be ensured, the calculated amount can be reduced, and the registration efficiency is improved.
And respectively connecting other 3 groups of characteristic point pairs according to the method to finally obtain the corresponding relation of 4 groups of characteristic point pairs, as shown in fig. 3.
Referring to fig. 1 again, in S4, a mapping relationship between the image to be registered and the reference image is established according to the coordinates of the matching feature points.
Establishing a mapping relation between a reference image and an image to be registered by the following formula:
I0(x0,y0)=f(I1(x1,y1))
wherein, I1Pointing to the image, I0Representing a reference image. By resolving the corresponding relation of the characteristic point pairs, the mapping relation between the reference image and the image to be registered can be established and resolved.
Referring to fig. 1 again, in step S5, the mapping relationship between the image to be registered and the reference image is used to perform registration of the image to be registered, and the geographic coordinates corresponding to the feature points of the image to be registered are calculated.
And converting the image to be registered into the coordinate space of the reference image by utilizing the mapping relation between the image to be registered and the reference image, thereby completing the registration. And simultaneously, converting the coordinates of the feature points in the image to be registered into a coordinate system of the reference image according to the mapping relation to obtain the geographic coordinate values of the feature points in the reference image.
Referring to fig. 1 again, in step S6, the matching error and the total registration error of each pair of feature points are calculated, if the total error is smaller than a given threshold, the image registration process is completed, otherwise, the process returns to step S3 to continue the feature point matching.
The feature point matching error refers to a deviation between the feature point geographical coordinate value calculated by the mapping relationship and the real (or assumed) feature point geographical coordinate value. The coordinates of the ith corresponding feature point in the reference image and the image to be registered can be obtained in step S2, which are (x)i0,yi0) And (x)i1,yi1) The geographic coordinate value of the mapped feature point obtained in step S5 is (x)i2,yi2) Then the set of feature point matching errors is
And the same calculation method can be used for completing the calculation of the matching errors of all the feature points. Calculating the total error:
if the total error is smaller than the given threshold E, i.e. Δ < E, the image registration process is completed, otherwise, the process returns to step S3 to continue the feature point matching.
The method is realized through ArcGIS software, the registration result is shown in figure 4 for the reference image and the image to be registered in figure 3, and the image edge can be seen to be well connected with the ground objects such as roads, buildings and the like corresponding to the base map, so that the subsequent image operation requirement is met. In addition, the linktable of the georreferencen module of the ArcGIS software can also realize high-precision automatic correction on the matching of the feature points, and fig. 5 shows that the linktable of the georreferencen module of the ArcGIS software includes the matching error and the total registration error of each group of feature points. Therefore, the matching error of each group of feature points is less than 2, the total registration error value is less than 2, and the registration accuracy of the visible image is good. In addition, the method can also be used for registering the visible light image and the infrared image, and the registration effect is as shown in fig. 6, so that the effect is very good.
The above-described embodiments are merely preferred embodiments of the present invention, and general changes and substitutions by those skilled in the art within the technical scope of the present invention are included in the protection scope of the present invention.