Automatically Coregister Images

Image coregistration is the process of geometrically aligning two or more images so that corresponding pixels representing the same objects may be integrated or fused. Typically, the geometric relationship between a warp image and a base image can be obtained through a number of tie points and modeled using transformation capabilities, such as polynomial functions, Delaunay triangulation, or affine transform. The image coregistration tool allows for automatic tie point generation, thus making fully automatic image registration possible.

You can access the automatic tie point collection in ENVI when you are working with:

Image-to-image ground control points. This is described in "Autoregistration Using Ground Control Points" in the next section.

Image-to-image mapping. This is described in Autoregistration Using Image-to-Image Mapping.

Image matching is always performed between one band from the base image and one band from the warp image.

The automatic image coregistration tool uses area-based matching and feature-based matching algorithms to obtain tie points, which are as follows:

Area-based image matching compares the grayscale values of patches of two or more images and tries to find conjugate image locations based on similarity in those grayscale value patterns. The results of area-based matching largely depend upon the quality of the approximate relationship between the base image and the warp image. This is determined through map, RPC, or pseudo map information, or by using three or more tie points. If both map information and three or more tie points exist, the three or more tie points condition is used. In the case where the pseudo map information is not sufficiently accurate, it is recommended that you select three or more tie points to obtain good matching results.

Feature-based image matching extracts distinct features from images then identifies those features that correspond to one another. This is done by comparing feature attributes and location.

Autoregistration Using Ground Control Points

With two images open in Image displays, from the ENVI main menu bar, select Map

Registration

Select GCPs:Image to Image. The Image to Image Registration dialog appears.

In the Image to Image Registration dialog:

Select the Base Image to use.

Select the Warp Image to use.

Click OK. The Ground Control Points Selection dialog appears.

From the Ground Control Points Selection dialog menu bar, select Options

Automatically Generate Tie Points.

If either the base image or the warp image displays in color, the Base Image Band Matching Choice and/or Warp Image Band Matching Choice dialogs appear for you to select the band to use for matching. Select the base image band and/or warp image band and click OK. If both images display in grayscale, this step is skipped.

In the Automatic Tie Points Parameters dialog, set the parameters for automatic matching. You can set parameters for area-based matching (described in Area-Based Matching Parameters) or feature-based matching (described in Feature-Based Matching Parameters).

Area-based matching is disabled if both the base image and warp image do not have map, RPC, or pseudo map information, or if three or more tie points have not already been selected. If area-based matching is disabled and you want to use this method, click Cancel, return to the Ground Control Points Selection dialog and manually select three or more tie points, then select Options Automatically Generate Tie Points.

When all parameters are set, click OK. A status dialog displays as ENVI automatically generates tie points. When processing is complete, the Ground Control Points Selection dialog displays.

In the Ground Control Points Selection dialog, click Show List to view the GCPs. The Image to Image GCP List dialog appears. See Using the Image to Image GCP List for information on viewing and editing the GCP list.

Autoregistration Using Image-to-Image Mapping

From the ENVI main menu bar select Map

Registration

Automatic Registration:Image to Image. The Select Input Band from Base Image dialog appears.

Select the band to use then click OK. The Select Input Warp File dialog appears.

Select the warp file to use. If desired, select spatial subsetting by clicking the Spatial Subset. For details on spatial subsetting, see Selecting a Spatial Subset. Click OK.

If the warp file has multiple bands, the Warp Image Band Matching Choice dialog appears for you to select the bands to use for matching. Select the warp image band and click OK. If the warp file has only one band, this step is skipped.

The ENVI Question dialog displays with the following prompt: Would you like to select an optional existing tie points file to aid in the autoregistration? Do one of the following:

Click Yes to select a tie points file. This opens the Select Existing Tie Points File dialog. Navigate to and select the desired file, then click Open. After you click Open, the Automatic Registration Parameters dialog appears.

Click No to skip selecting a tie points file. This opens the Automatic Registration Parameters dialog.

Click Cancel to close the automatic image coregistration tool.

In the Automatic Registration Parameters dialog, set the parameters for automatic matching. You can set parameters for area-based matching (described in Area-Based Matching Parameters) or feature-based matching (described in Feature-Based Matching Parameters).

Area-based matching is disabled if both the base image and warp image do not have map, RPC, or pseudo map information, or if three or more tie points have not already been selected. If area-based matching is disabled and you want to use this method, click Cancel, go to the Ground Control Points Selection dialog (Map Registration Select GCPs:Image to Image from the ENVI main menu bar) and manually select three or more tie points, then select Options Automatically Generate Tie Points from the Ground Control Points Selection dialog.

When all parameters are set, click OK. A status dialog displays as ENVI automatically generates tie points.

The final steps depend upon the Examine tie points before warping parameter setting:

If the Examine tie points before warping value was Yes, the Image to Image GCP List dialog displays. See Using the Image to Image GCP List for information on viewing and editing the GCP list.

If the Examine tie points before warping value was No, the warped image is added to the Available Bands List.