Epipolar Stereo Pair - Creation window

Functionality

In the Epipolar Stereo Pair - Creation window, you can create a stereo pair from two images with overlap.

In short, you will need to:

mark on each photograph the position of two, three, or four fiducial marks (FM),
mark the position of the calculated principal point (PP) of the left photograph, on the right photograph (transferred principal point or TP),
mark the position of the calculated principal point (PP) of the right photograph, on the left photograph (transferred principal point or TP),
in case of scale differences between the two input maps, mark the position of one or two scaling points (SP) in both photographs.

Legend:

1,2,3 = Fiducial Marks

PP = Principal Point

TP = Transferred Principal Point

When you close the Epipolar Stereo Pair - Creation window with the Exit and Show button , both input photographs will be resampled to output maps, i.e. the stereo pair is calculated. Rotation differences and possible scale differences between the photographs will be removed.

The output stereo pair can then be viewed in stereo, either with a stereoscope or as an anaglyph.

The window has a menu bar, a context-sensitive menu, a toolbar, dialog panes at the bottom of the window, and a status bar.

To open the Epipolar Stereo Pair - Creation window:

For the creation of an epipolar stereo pair, you need two scanned images (for instance two aerial photographs with fiducial marks) with at least 50% overlap.

When you wish to view the stereo pair later on with a stereoscope, you can use input raster maps with the Color domain, the Image domain, or another value domain.
When you wish to view the stereo pair as an anaglyph later on, you can only use raster maps with the Image domain, or with another value domain.

Tips:

As scanned images may have a very large file size, it is advised to create Pyramid Layers for both input raster maps. This will speed up display in the Epipolar Stereo Pair - Creation window.
If one or both of the images was scanned in the wrong direction, you can use the Mirror Rotate operation before starting to create a stereo pair. In the input pair, you should make sure that the ground track of the flight line is more or less from left to right, and thus more or less at a right angle to the columns of the images.
To convert a map with the Color domain to the Image domain, you can use the Color Separation operation; choose option Intensity. See also How to change the domain of a map.

You can start the Epipolar Stereo Pair operation:

by double-clicking the Epipolar Stereo Pair item in the Operation-list,
by opening the Image Processing item in the Operation-tree, and double-clicking the Epipolar Stereo Pair item,
by choosing Epipolar Stereo Pair from the Operations, Image Processing menu in the Main window.

The Create Stereo Pair dialog box will appear in which you can specify the names of the left and right input maps, and the name of the output stereo pair. The Epipolar Stereo Pair - Creation window will also be opened.

The Epipolar Stereo Pair window consists of four parts:

upper left pane: displays the left-hand photograph,
upper right pane: displays the right-hand photograph,
lower left pane: shows the row/col positions of marked points in the left-hand photograph,
lower right pane: shows the row/col positions of marked points in the right-hand photograph.

Functionality

To add a fiducial mark:

Activate the photograph/image on which you wish to work by clicking the Activate Left Pane button or the Activate Right Pane button.
Zoom in on the area where you expect to find a fiducial mark.
Indicate that you wish to mark the position of a fiducial mark:

choose Select Fiducial Marks from the Edit menu or from the context-sensitive menu, or
press the Fiducial Mark button in the toolbar , or
select a fiducial mark check box in the lower dialog pane.

Check that the cursor displays the number of the fiducial mark that you wish to enter, and that the row/col boxes of the fiducial mark that you are about to enter appear highlighted (by default yellow).

Position the cursor as exact as possible and click the location of the fiducial mark in the photograph.
Repeat for the other fiducial marks.

You are supposed to enter two, three or four fiducial marks for each photograph. For one photograph, use either the fiducial marks located in the corners of the photograph or the fiducial marks that are in the middle along an edge of the photograph.

To remove a fiducial mark:

Clear a fiducial mark check box in the lower dialog pane.

To re-enter a wrong fiducial mark:

first clear the check box of the wrong fiducial mark in the lower dialog pane,
then reactivate the check box of the fiducial mark that you wish to re-enter, and
click the position of the fiducial mark again in the photograph.

Automatic positioning of principal point:

When for a photograph, enough fiducial marks are entered, then the position of the principal point will automatically appear in the photograph.

The principal point of a photograph, is also called the pivot; it is the point around which the photograph will be rotated.

The principal point is only an approximation of the best possible rotation point or pivot. Theoretically, the nadir point (i.e. the terrain point perpendicular under the camera) is the point around which the photos should be rotated to make the photos epipolar to each other. However, without ground control points, the nadir point is hard to find, and in case of vertical photographs, the nadir point coincides with the principal point.

Manual positioning of principal point (optional):

If you don't have fiducial marks, you can also indicate the position of the principal point yourself:

Do not enter fiducial marks, or clear the fiducial mark check boxes.
Activate the photograph/image on which you wish to work by clicking the Activate Left Pane button or the Activate Right Pane button.
Zoom in on the area where you expect to find the principal point.
Indicate that you wish to mark the position of the principal point:

choose Select Principal Point from the Edit menu or from the context-sensitive menu, or
press the Principal Point button in the toolbar, or
position the cursor in the Principal Point row box or column box in the lower dialog pane.

Check that the cursor displays PP, and that the row/col boxes of the principal point appear highlighted (by default yellow).

Position the cursor as exact as possible and click the location of the principal point in the photograph, or type row/column numbers in the lower dialog pane.

To transfer the principal point of the left photograph to the right photograph (and vice versa):

Zoom in on the area of the principal point in the left photograph.
Zoom in on the area where you expect to find this point in the right photograph.
Activate the photograph/image in which you wish to work (the right), by clicking the Activate Left Pane button or the Activate Right Pane button .
Indicate that you wish to mark the position of a transferred principal point:

choose Select Transferred Principal Point from the Edit menu or from the context-sensitive menu, or
press the Transferred Principal Point button in the toolbar, or
position the mouse pointer in the Transferred Principal Point row and column boxes in the lower dialog pane.

Check that the cursor displays TP, and that the row/col boxes of the transferred principal point appear highlighted in the lower dialog pane (by default yellow).

Position the cursor as exact as possible and click the location of the transferred principal point in the (right) photograph.

Now do the same vice versa, and transfer the principal point of the right photograph to the left photograph.

The imaginary line between the principal point and the transferred principal point in a photograph is called the photo base; in normal circumstances, it coincides with the flight line projected on the ground (ground track).

Notes:

In the left-hand photograph, the transferred principal point must be located to the right of the principal point of that photo, i.e. the column number of transferred principal point should be greater than the column number of the principal point.
In the right-hand photograph, the transferred principal point must be located to the left of the principal point of that photo, i.e. the column number of transferred principal point should be smaller than the column number of the principal point.
As long as this is not fulfilled, you cannot start the calculation of the stereo pair's output maps.

To specify scaling points (optional):

When there are scale differences between the left and right photograph, you should specify one or two scaling points in both photographs. Like the principal point, the scaling point that you mark in one photograph has also to be marked in the other.

A scaling point should be located at the top or the bottom of photograph, at a distance of at least half the photo base away from the photo base/flight line, otherwise calculation of the stereo pair's output maps is not possible.

Mark the position of a scaling point in one photograph.
Mark this scaling point also in the other photograph.
You can use the Scaling Point button in the toolbar, or the Select Scaling Points option on the Edit menu or the context-sensitive menu.

As soon as one or two scaling points are placed, a 'scaling distance' is displayed as 'Total distance from flight line'. This distance is expressed in photo pixels (rowcols). It is (the sum of) the distance(s) between the scaling point(s) and the base line. During resampling, the resolution of the photograph with the smallest scaling distance will be increased to the resolution of the other photograph.

To exit the Epipolar Stereo Pair - Creation window and show the resulting stereo pair (Exit and Show):

To save all entered points, and to calculate the resulting stereo pair which can be viewed in stereo:

click the Exit and Show button in the toolbar, or
choose Exit and Show from the File menu.

The Epipolar Stereo Pair - Creation window will be closed.

The stereo pair and its two output raster maps will be calculated.

By default the stereo pair will be displayed in a Stereo Pair - Stereoscope window. The default Open action (i.e. open in Stereoscope window or open as Anaglyph) can be specified in the Preferences.

Other ways to exit the Epipolar Stereo Pair - Creation window:

In some cases you may wish to leave the Epipolar Stereo Pair - Creation window in another way than described above.

Exit: the changes you made in the Epipolar Stereo Pair - Creation window will be stored, but the stereo pair will not be calculated yet.

To exit:

click the Exit button in the toolbar,
choose Exit from the File menu, or
click the Close button of the window.

The Epipolar Stereo Pair - Creation window will be closed. The stereo pair will be calculated when necessary, e.g. when opened to be displayed.

Quit: none of the changes you made in this session of the Epipolar Stereo Pair - Creation window will be stored.

To quit:

choose Quit from the File menu.

The Epipolar Stereo Pair - Creation window will be closed.