Landmarks and curves

This page enables you to place landmarks on images, or curves on surfaces, associated with a prerecorded data set. You can then make measurements of distances and angles between these landmarks, or use the landmarks and curves to help define similar features between surfaces. You can also include fiducial landmarks located using a tracked pointer in these measurements.

The landmarks task can be selected either by using the tabs at the top of the task pages, or by selecting from the task menu, or by selecting the landmarks tool in the toolbar.

The text used to display landmark and curve names in the 2D and 3D windows, and the size of the dots, can both be altered by using the 'graphics font' option in the GUI options configuration menu.

Landmarks and curves are all stored in the Stradwin ".sw" file. So the file will need to be saved in order to retain them for next time. In addition, surface-based landmarks and curves are also included as comments when exporting the associated surface as a ".ply" or ".wrl" file. This feature can be used to indicate, to external software, specific locations of interest in the surface, for instance to assist in registration of surfaces.

Landmarks

Landmarks can be created by using a pointer attached to a position sensor to locate points in 3D space as described on the pointer task page, and pressing the 'Grab pointer location' button. Alternatively, you can just click on locations in the review (top left), reslice or 3D windows after selecting the landmark tool.

When defining landmarks in any of the reslice visualisations (but not in the review or 3D windows), the default behaviour is to move the landmark slightly so that it is located at the nearest point in 3D on the nearest frame. This frame is then automatically shown in the review window. In a normal data set this movement will not be perceptible, unless you define a landmark outside of the recorded data. This behaviour can be disabled by un-checking the 'Snap mouse clicks to frames' checkbox. Note that if disabled, landmarks can be defined anywhere in space, and hence may not show up on any of the recorded frames, though they will always be displayed in the 3D window.

Surface-based landmarks are displayed slightly differently in the 2D review and reslice windows. So long as the landmark is within the equivalent of five image pixels from the plane location, the nearest in-plane location will be marked with a dot. This is surrounded by a dashed circle, where the radius of this circle shows how far away the point is from this plane. Hence a surface-based landmark with no dashed circle is coincident with the plane, whereas one with a larger circle is further away.

All the landmarks you have defined are listed, in alphabetical order, in the box at the top of the landmarks task page. In this list, landmark names based on a surface will be preceded by the number of the surface object on which they were defined, followed by a colon. Up to three landmarks can be selected using the left mouse button. If you select more than three, the least recently selected landmark will be deselected to make way for the new one.

As you select a landmark, its name is highlighted and it will be allocated to one of the slots A, B or C. You can then change the landmark's name by editing the text in the appropriate slot and pressing return (or moving to another control). A selected landmark can be deselected by clicking on its name again. The distances and angles between the selected landmarks are displayed underneath the names.

Next to each of the selected landmarks A, B and C, there is a 'Delete' button. Note that this deletes the landmark completely. If you just wish to deselect the landmark, you should click on the landmark name in the box at the top of the task page. Landmarks can also be deleted using the erase tool.

Finally, each selected landmark also has a 'Find' button. This button can be used to locate orthogonal views which intersect with the landmark, as well as the nearest frame.

Curves

Curves are created by clicking with the draw tool on a surface in the 3D window: this surface must have already been updated from the underlying contours, or imported from an appropriate file. Curves are defined one point at a time, and unlike contours, curves can either be open or closed. To finish drawing the curve, either click again on the last point (in which case the curve will be left open) or click again on the first point (in which case the curve will be closed, i.e. joined to this first point).

Curves points can be moved, inserted and deleted in just the same way as contours: see the draw task page for more information on this.

However, having defined a curve, the points will remain even if the surface is deleted. This allows the curve information to be re-used even after the surface has been edited, for instance to change the resolution with which it is created. Like landmarks, curves are also stored in the Stradwin ".sw" file.

Curves are also displayed in the 2D review and reslice windows, in a similar way to surface-based landmarks. Any curve, or portion of a curve, which is within five image pixels of the plane, will be displayed as a solid line. A dot is drawn on this curve at the point that it actually intersects the plane. Dashed lines are drawn around each curve point, and alongside each line segment. The distance of these dashed lines away from the solid lines shows how far away the curve actually is from this plane.

The selection list shows to which object each curve belongs, as well as the curve name. This name must be unique within each object but can be repeated across objects. Unlike landmarks, only one curve can be selected at a time from the list. Having selected a curve, the controls beneath the list then apply only to this selected curve.

The curve name can be edited, and just as with landmarks there are also 'delete' and 'find' buttons. It is also possible to change whether the curve is closed or open, and which object the curve is associated with.

The final option is to 'split' the surface using this curve. This is only possible if the curve is closed, and the associated surface exists. In this case the surface triangulation will be altered so that the triangles exactly align with the curve (traced out over the surface), and the patch within the curve will be internally separated out from the rest of the surface, though at this point it will still belong to the same object.

This splitting can also be achieved by using the 'erase' tool in the 3D window, and the same tool can be used to create a new object using this patch: see the draw task page for more information on this.