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Poster Session Q
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The registration and mosaic of digital images remotely sensed
- Registration between On-Line Image and Off-Line Map/Image
The registration between on-line image and off-line map/image is the problem encountered frequently in application of remote sensing information. In an ordinary way, first the off-line map/image is digitized into an on-line image, then registration is conducted according to the registration way of on-line images, This process is tedious and restricted by hardware. A new registration way in which only the image to be corrected is needed to input to the system
Tem and the reference map/image is used by off-line, User can select a control point on the system monitor and find the corresponding position on the off-line map/image. The coordinates of the position are typed in the system at terminal and converted to the coordinates as consistent with the image to be corrected. So that a control point pair of on-line image and off-line map/image is determinate. The user repeated selecting control point on both on-line and off-line ones until the control are as such as enough. Based on these control points the on-line image is registered to the off-line map/image geometrically.
- Generating CNPT File by Typing the Coordinates of Images into the System to Achive Image Registration
Sometimes image registration is not completed by using the methods of control point selecting as mentioned above. In this situation the coordinates of control point need to converted and calculated in the light of specific conditions. Before spatial transformation of image new coordinates of control point are typed in at terminal by user and a CNPT (control point) file is generated. The image registration is brought about based on the control point pairs in the file.
Mosaic of digital images
Then two or more overlapping images are combined for mosaic; there would be no exat mosaic geometrically without accurate registration. Nevertheless, it must be pointed out that if one wants to acquire a high quality mosaic not only an accurate registration but also an appropriate matching of gray level or hue for the images to be mosaicked is needed. It is important to make good choice for the candidates of images to be mosaicked. Moreover, the preprocessing, which is designed to improve, geometric and radiometric qualities of the images have to be conducted before putting them together. The specific techniques of image mosaic adopted as follows.
- Adjustment of Gray Level.
For some reasons, the difference of gray levels or hues between selected images is in existence frequently. The gray level matching between the images must be conducted 4. The method is taking one image as a reference and transforming the other to match the reference one in histogram. The mathematic formula used in histogram matching is:
Fig. 1 A sketch map of mosaic and juncture point selection.
Where IN is the gray level transformed. IA and IB are gray levels of the reference image and one to be adjusted respectively, sa and sb are variances corresponding to IA and IB,  and  are means of IA and IB respectively.
- Selection of Juncture Point of Mosaic
A slide window is adopted to search the position where the difference of gray levels is the minimum in a neighborhood in overlapping area of two adjacent images line by line in order to eliminate the seam, which is often apparent in mosaic. The algorithm utilized is of the following form:
Where IA(N+i) and IB (n+i) are gray levels of the overlapping area of two adjacent images respectively. Let the window size W be much less than the width of the overlapping area. The value of n which causes D(n) to get the minimum is found out by using the window and as a juncture point of a line. To do this repeatly, untill all juncture points are searched for all of the lines. Sometimes the seams are apparent along the direction perpendicular to the mosaic direction if the images are mosaicked directly by the juncture points. (Due to two juncture point of two adjacent lines may be far from each other). To avoid this problem, except the first line, the candidate juncture point will be selected from the following three points. One is the point in the same column. The other two points are situated on the left and the right of this point. Then the juncture point is selected among the three points with the slide window. Fig. 1 is a sketch map of mosaic and juncture point selection.
For a color image mosaic the juncture line of the first band of the composite image is as a juncture line for all three bands in order to avoid artificial hues due to each band using different juncture line.
- Gray Level Smoothing of Neighborhood of Juncture Point
Sometime there is still the difference of gray levels of both sides of the juncture point. The smoothing processing technique is adopted to eliminate the difference. The algorithm used is as follows:
Where INi is gray level smoothed, IAi and IBi are gray levels of the juncture point. The smoothing processing. K is a weight. W is much less than the width of the overlapping area and its size is specified as a parameter.
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