Three-Dimensional Cursor

To overcome the limitations of the two-dimensional active cursor we turn to a true three-dimensional cursor. Here a three-dimensional co-ordinate is defined by the user (usually with the aid of a three-dimensional pointing device such as a Spaceball) and is projected onto the screen. Where a stereoscopic display is available the cursor can be seen to move closer and further away.

The two main difficulties in implementing an effective three-dimensional cursor are: (i) how to draw it onto the screen, and (ii) how to enable the user to judge depth accurately. A number of techniques have been experimented with, each with its particular advantages and disadvantages. In the following sections it may be assumed that for a stereoscopic display the operations described may be done separately for each eye-view.

Two-Dimensional Overlay Cursor

Here the three-dimensional cursor position is projected onto the screen. A small crosshair is drawn through this projected point. This cursor is always visible, never disappearing even if it is pushed behind a feature. This cursor is very simple to implement and requires the least computation. Unfortunately, depth perception is often limited, even when displaying in stereo. Feedback from users indicates that it was too difficult to measure the depth of the cursor. Therefore, while stereoscopic display is an important tool, it should not be solely relied upon to provide depth cues.

Three-Dimensional Overlay Cursor

Here the cursor position defines three intersecting lines, each line parallel to one of the principle axes of the volume (see Figure 1). These three lines extend from one face of the volume to the opposite face. This may be thought of as a large, three-dimensional crosshair. Depth perception is improved with this cursor. In addition, if a wireframe is drawn around the volume the user gets an improved sense of where the cursor lies in the volume. As with the previous cursor, this cursor does not disappear when moved behind a feature.

  
Figure 1: NGC4631 with Three-Dimensional Overlay Cursor

Two-Dimensional Shaded Cursor

This cursor is similar to the two-dimensional overlay cursor except that an estimate is made whether the cursor lies in front of or behind an obscuring feature. This estimate is based on the same technique used in the two-dimensional active cursor. If the cursor is in front of the feature, it is drawn, otherwise it is not drawn. While an improvement, this cursor still does not leave the user with much of a sense where it lies in the volume.

Three-Dimensional Shaded Cursor

This cursor defines the same three lines as in the three-dimensional overlay cursor, except that, instead of drawing the projected lines over the rendered volume, they are effectively merged into the data. This merging process differs according to the shading algorithm used to render the volume.

For a maximum voxel algorithm each point along the three lines is drawn or not drawn according to whether the point is considered to be in front of or behind an obscuring feature, using the same depth estimation algorithm used by the two-dimensional active cursor.

For an opacity-based shader each point is properly blended with the ray of voxel values passing through that point. If the point is in front of a feature, it is visible; if it is moved behind an opaque feature it will gradually fade to invisibility. See Figure 2.

  
Figure 2: NGC4631 with Three-Dimensional Shaded Cursor

Of these three-dimensional cursors disussed so far, this one is clearly the best in terms of visual effect, depth perception and placement inside the volume, and received modest praise from users. It is of course the most computationally expensive, but even this cost is relatively moderate.

© Copyright Astronomical Society of Australia 1997