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32.
R2V를 이용하여 Image Warping / Registration / Image Geo-Referencing
등의 작업은 어떻게 합니까?
R2V를
이용한 Image Warping and Geo-referencing
Very often,
we need to register a raster image to a base map or a different
image with overlapping regions or we need to geo-reference a raster
image to a real world coordinate system.
Although
both image warping and geo-referencing change pixel locations, they
are implemented using different methods. Image warping is done in
the image coordinate system and control points are specified using
pixel coordinates in integer, not real world coordinates.
As a result,
image warping physically creates a new image with pixel locations
properly adjusted according to the given control points. To maintain
the maximum accuracy in image warping processes, R2V uses the Delaunay
Triangulation method to create a mathematical transformation for
each triangle region and geometrically corrects each triangle region
separately.
On the
other hand, raster image geo-referencing does not change the image
itself in any way. Instead it attaches additional information
about how to place the image under a real world coordinate system.
For example, in ArcView, this is done through a World File which
contains 6 parameters to provide a linear transform to display the
image at the right location. In MapInfo, a TAB file which contains
a group of control points is used for displaying the image. If an
image is not geometrically distorted in any way (not rotated, not
skewed, not in a mountain area, etc) as compared to the base map,
than a one step geo-referencing may work. However, when the raster
image does not match the base map geometrically, image warping should
be used as the first step to remove the geometric distortion and
then apply the geo-referencing process.
The following
are step by step instructions on how to do image warping against
another image or a vector map in an image coordinate system, warp
an image against a geo-coded vector map, and geo-referencing an
image.
(1) Register/Warp An Image Against Another Image or
A Vector Map In Image Coordinate System
Here are the steps for image warping:
Step 1. Open both the image to be warped and the target image
using the File/Open Image command. Tile the two display windows
side by side. Here we call the image to be warped the source image
and the other image is the target image. If you use a vector map
instead of an image, use New Workspace and Import Vector to show
the data.
Step 2. Activate the source image and start the control point
editor using Edit/Control Points or Vector/Select Control Points.
A control point is a point you can identify in both the source and
the target image. Move the mouse within the source and target image
to find such a point, normally road intersections or some other
landmarks. Write down the coordinate values (X, Y) displayed
at the bottom of the window for the point in the target image. The
coordinate values should be in image coordinates (integer values),
not in geo-coordinates such as Lat/Long for image warping. Go back
to the source image, click the left mouse button at the point to
define this control point. The "From" fields are the coordinates
in the source image. Enter the coordinate values you have
for the target image in the "To" fields. Click "OK"
to confirm.
Step 3. Repeat Step 2 to define more control points for the source
image. You need at least 4 control points, and normally you should
have 15 or more to have a more accurate warping result. Click the
right mouse button and select Done to exit from control point editor.
Step 4. Once control points are collected, use the Edit/Delaunay
Triangulate command to view the triangulation result. Long narrow
triangles should be avoided by adding more control points.
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