Tutorial files are available from the Exelis VIS website or on the ENVI Resource DVD in the image_reg directory.

Transcription

1 Image Registration Tutorial In this tutorial, you will use the Image Registration workflow in different scenarios to geometrically align two overlapping images with different viewing geometry and different terrain distortions into the same coordinate system so that corresponding pixels represent the same objects. Files Used in this Tutorial Tutorial files are available from the Exelis VIS website or on the ENVI Resource DVD in the image_reg directory. File ikonos_4.0m.dat ikonos_4.0m.hdr quickbird_2.4m.dat quickbird_2.4m.hdr Description IKONOS true-color image of Boulder, CO, at 4 meter spatial resolution Header file for above QuickBird true-color image of Boulder, CO, at 2.4 meter spatial resolution Header file for above Performing Automatic Registration In the following steps, you will perform image registration by selecting base and warp images, setting parameters to generate tie points, reviewing and editing those tie points, and generating an aligned output image. Select Base and Warp Images and Set Parameters 1. Start ENVI. 2. From the Toolbox, select Geometric Correction > Registration > Image Registration Workflow. The File Selection panel appears. 3. In the File Selection panel, click Browse next to the Base Image File field, navigate to the \image_reg directory and select the quickbird_2.4m.dat file, then click OK. Page 1 of 16

2 4. In the File Selection panel, click Browse next to the Warp Image File field, navigate to the \image_reg directory and select the ikonos_4.0m.dat file, then click OK. 5. Click Next to proceed to the Tie Points Generation panel. The two images open in the Layer Manager and the base image (quickbird_2.4m.dat) displays. 6. In the Layer Manager, click the check box next to ikonos_4.0m.dat so that it displays in the Image window view, then select the layer in the Layer Manger. 7. Move the Transparency slider back and forth to see how the images are offset. In the image below, setting the transparency to 50% reveals blurriness in the middle, lower, and left areas because the two images are not aligned with one another. Page 2 of 16

3 8. The Tie Points Generation panel is where you specify the parameters for automatic tie point generation. You could create seed tie points in this panel, though they are not needed for this tutorial. Seed tie points are recommended only if the images have poor or no georeferencing. If only one image has poor georeferencing and the other image does not, use the image with accurate georeferencing as the base image. If the warp image is pixel based, arbitrary, or a pseudo projection, you need to add or import at least three seed tie points before you continue. For this step, keep the default parameter settings on the Main tab and Advanced tab. You will not need to use the Seed Tie Points tab because this data has good georeferencing. Page 3 of 16

4 9. Click Next to automatically generate tie points and go to the Review and Warp panel. Review Generated Tie Points When automatic tie point generation is complete, the Review and Warp panel appears and the view is populated with tie points in the overlapping area of the base and warp images. Tie points in the base image are magenta. Page 4 of 16

5 Click the Switch to Warp button in the Review and Warp panel to see the corresponding tie points in green. In the Review and Warp panel, click Show Table to open the Tie Points Attribute Table. This table lists details for all of the generated tie points, such as the location of the tie points on the base and warp images, and the SCORE and ERROR. The total root mean square (RMS) error displays at the bottom of the table. Page 5 of 16

6 The SCORE is from area-based auto tie point generation. A patch around the tie point location is used as a matching window. Depending on the matching method you choose, the normalized cross-correlation or normalized Mutual Information between the window in the base image and the window in the warp image is computed as the matching score. The ERROR measurement represents the distance of each tie point in the warp image to its predicted location, in pixel units. If the base and warp image do not have RPC map information and an associated DEM, a first-order polynomial transform is used to compute the predicted location from tie points on the base image to points on the warp image. If the base image or warp image have RPC map information and you specified a DEM file under the Main tab, ENVI performs a quick orthorectification on the base and warp image, resulting in a more accurate image registration. Higher errors may indicate bad tie points or an inappropriate geometric model used. It does not necessarily reflect the positional errors in the tie points. The best way to check the accuracy of the tie points is to visually examine their placement in the Image window. The ERROR measurements are updated if you add or delete tie points. On Windows platforms, you can use keyboard keys to navigate the table rows. When you select a record in the table, the Image window centers over and highlights the tie point. The Up and Down arrow keys select the previous or next record in the table. The Home and End keys select the first or last record in the table. The left arrow key switches to the base image view; the right arrow key switches to the warp image view. The Delete key deletes the selected record. Use the keyboard keys to check each tie point and switch between base and warp views. If the features at the tie point location do not match on the two images, press the Delete key to delete the selected tie point. 1. In the Tie Points Attribute Table, click on the ERROR column to select it, then right-click and select Sort by selected column reverse. The tie points with the highest error appear at the top of the table. You will visually examine the tie points with the highest error. Page 6 of 16

7 2. Select the row of the first tie point listed in the table. The view moves to that point and highlights it. If needed, zoom in to the view to see the image detail. For example, zoom in to 400%. 3. Toggle between the base and warp images by clicking Switch To Warp/Switch To Base in the Review and Warp panel; as you do this, check to Page 7 of 16

8 see if the placement accuracy of the tie point is good. The tie points above appear to have good accuracy. Though it is not necessary for this tutorial, you can also sort the tie points by matching score. In the Tie Points Attribute Table, click on the SCORE column to select it, then right-click and select Sort by selected column forward. The tie points with the lowest score appear at the top of the table. 4. Check the accuracy of other tie points by selecting them by row in the Tie Points Attribute Table and toggling between the base and warp images. If you find tie points that do not have good accuracy, you can delete them so they will not be included in the warping process and affect the accuracy of the image registration. The following is an example of what tie points with poor accuracy might look like: Page 8 of 16

9 To delete a bad tie point, make sure it is selected in the attribute table, then click Delete Selected at the bottom of the attribute table. 5. When you have finished checking the tie points with the highest error numbers or the lowest scores and deleting them as necessary, select the Warping tab in the Review and Warp panel. 6. Keep the default settings, including the Warping Method of Polynomial. 7. Enable the Preview check box to open a Preview Window showing what the output will look like using these settings. Page 9 of 16

10 Most areas of the warped image produce a good result. If you pan the image to the mountainous areas in the left side of the image, some areas of the preview produce a good result, while other areas still appear to be offset. This image distortion is due to terrain relief; therefore, fewer tie points were automatically generated in this area. Disable the Preview check box. 8. Manually add tie points on the road that is offset, to see if the registration Page 10 of 16

11 improves in the preview. To add the points, return to the Tie Points tab in the Review and Warp panel and click Start Editing. Note: If you used a different toolbar tool in between adding tie points (such as panning to a new location in the image), click the Vectors drop-down on the toolbar and select Vector Create to reenable adding tie points. Or, use the middle mouse button to pan and zoom the image so you do not have to reselect the tool. 9. Click in the Image window to mark a tie point, then right-click and select Accept as Individual Points. The Image window view switches to the second image; mark the corresponding tie point in the second image, then right-click and select Accept as Individual Points. The number of tie points is updated and the Image window view returns to the first image. 10. Add additional tie points as necessary. When finished, click Stop Editing in the Review and Warp panel. Page 11 of 16

12 11. Enable the Preview check box again and see if the registration preview has improved. Page 12 of 16

13 You can see that adding tie points to the road did not correct the misalignment. Because Polynomial is the selected Warping Method, warping fits a global polynomial transform and the two images do not align exactly at the tie point locations. 12. In the Warping tab of the Review and Warp panel, change the Warping Method to Triangulation and examine the result in the Preview Window. Page 13 of 16

14 Triangulation warping fits triangles to the irregularly gridded data points and interpolates values to the output grid. Applying the Triangulation method aligns the two images exactly at the tie point locations. 13. If needed, add more tie points to the base and warp images in places where image misalignment occurs, particularly in the mountainous areas. Page 14 of 16

15 14. Click Next to generate the geometrically aligned output image and go to the Export panel. 15. Provide an output directory and filenames for the Export Warped Image and Export Tie Points to ASCII outputs. 16. Click Finish to generate the output image and complete the workflow. ENVI loads the warped image into the Layer Manager. 17. Exit ENVI. Copyright Notice: ENVI is a registered trademark of Exelis Inc., a subsidiary of Harris Corporation. Page 15 of 16

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