An image feature is a raster image, that is, an image made up of points like in a newspaper picture. For example, a scanned paper map or satellite ground photo can be image features.
Raster graphics in Windows always have rectangular form. Each raster point can be specified by its raster coordinates, that is, by numbers of the line and the column in a rectangle. There are monochrome raster graphics and color ones with different number of colors. Files of standard Windows format storing raster graphics have BMP extension.
An image feature in ObjectLand is always built on the basis of raster graphics which is stored in some BMP file or in TAB file of MapInfo system. For building an image feature, apart from indicating the file, it is necessary to provide two more things:
Perform raster binding to the map for which it is required to indicate the coordinates of any three control points not lying on one straight line on the map. Usually such control points are either rectangle corners or noticeable marks which coordinates are known for sure (for example, geodetic signs). The system executes linear (affine) raster coordinate transformation into coordinates of the map points.
Specifying more than three control points is allowed, however in this case, as a rule, it is impossible to provide exact binding of all these points to the specified coordinates (affine transformation by four or more arbitrary points can't be determined). In this case the system makes such affine transformation which gives the minimum of sum of squares of control point coordinates errors (least-squares transformation).
Indicate the feature clipping boundary, that is, such a closed polyline outside which feature points are not displayed. Due to it, an image feature can have any multangular, not only rectangular form in ObjectLand. The presence of inner boundaries is allowed, which makes an image feature similar to area feature, but it has “personal fill style” in the form of an arbitrary raster graphics.
At figure 9-5 two variants of binding one and the same raster are shown, and at figure 9-6 – the influence of the clipping boundary on image feature display.
Please pay attention that raster bounded to the map does not necessarily keep rectangular form when displayed. Since control point coordinates may be chosen at random, rectangular raster display may be an arbitrary parallelogram.
Figure 9-5. Binding of an image feature to the map
Figure 9-6. Clipping boundary of an image feature
For monochrome image features ObjectLand presents additional capabilities. The user can choose two colors for background and foreground (for example, black image against white background or red one against green background), he can also make the background transparent, not covering the visibility of lower map layers. At figure 9-7 an effect of background transparency in image feature display is shown.
Figure 9-7. Image feature with transparent background
There is a possibility to invert monochrome images when they are displayed, that is, display the foreground by the background color and vice versa.
Each feature type of the map layer can contain features of one spatial category only (for example, the type “Building” contains only area features), however, there may be any number of feature types having the same spatial category. For example, “Building” and “Parcel” – are area types, and “Pillar” and “Road sign” are point types.
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