The ``Uncorrected Image Overview'' and the ``Georeferenced Image Overview'' windows work in exactly the same way. However, when vector tie downs are being performed, vectors will also appear in the georeferenced image windows. When ``User Entered Coordinates'' or map tiedowns are being performed, no georeferenced windows will be opened at all.
Several possible enhancements are provided. The default is ``Linear'' which is a simple contrast stretch. The darkest pixel in the source image is forced to black, the lightest to white, and all the values in between are stretched proportionally.
The ``Root'' enhancement is similar to ``Linear'' except that the values between the minimum and maximum are stretched in such a way that they fit on a square root curve, instead of a straight line as with ``Linear''.
The ``Equal'' enhancement modifies the brightness of pixels with the intention of having a similar number of pixels at each level of brightness. This is an equalization of the output image histogram.
The linear, root, and equalization enhancements are computed based on the area of the image that is currently displayed on the screen. If you zoom in on a dark subarea of the image, and are using a linear enhancement, the window will adapt by attempting to lighten up the dark area. This dynamic, or ``Tracking'' enhancement can be disconcerting. To force the window to continue using the current enhancement without tracking, select ``Hold'' on the option menu. This effectively freezes the current LUT until another option is selected.
The ``None'' option displays the real satellite data, without any enhancement.
Understanding the details of the different enhancements is not as important as experimenting to find the one that brings out the interesting features on the image with which you are working.
The desired zoom level can be selected on the option menu, or the up and down arrows can be used to increase or decrease the zoom level. When the zoom option menu is in ``off'' mode, the window is in overview mode.
Note that no matter how far the user zooms in on the overview window, it still only contains decimated data.
When you select an uncorrected or georeferenced image, an option is provided to select one or three image planes. Three image planes are usually displayed as an RGB image with the first image plane used as the red component, the second as the green component, and the third as the blue component. If only one image channel is selected for tiedown, the RGB option menu is disabled (shaded out).
The RGB option menu allows the user to study one image plane at a time by selecting ``B&W 1'', ``B&W 2'', or ``B&W 3''. The ``RGB'' option can be selected to return to an RGB display of the image.
The fifth option (``PCT'') can be used to display the first image plane using it's default PCT table. This is the default mode when loading an image with a default PCT.
The RGB Display Control option menu is provided because certain types of image features are often clearer when a single image plane is viewed. It is also possible on 8-bit graphic systems to display much finer graduations of grey than is possible for colour images.