Two-Dimensional Active Cursor

A simple but quite useful cursor for three-dimensional position determination is a modification of the familiar two-dimensional cursor. Here the user moves the conventional cursor over the projected volume-rendered data to a feature of interest. The position of the two-dimensional cursor is then projected back into the volume and an estimate is made of the depth down the ray the feature the user sees. The algorithm, used to compute the estimate, changes, depending on the shading algorithm used to render the volume.

For a simple ``maximum voxel'' algorithm, where the maximum voxel value along each projected ray is displayed, the depth of the feature is approximated to the depth of the voxel at which the intensity is a maximum.

For more complex ``hot gas'' shaders (described in Gooch 1995c) which use a radiative transfer algorithm, the depth is computed using the following:
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where depth is the estimated depth, is the opacity of voxel i, and is the depth of the voxel. This gives meaningful results for compact, nearly opaque features and for extended, less opaque features. It does not perform so well where there are two compact, nearly opaque features, one in front of the other. I have added a threshold function which is effective if the nearer feature has sufficient cumulative opacity. The threshold function ignores data that lie behind quite opaque features.

Once the approximate depth of the feature is computed a simple geometric transformation may be applied to convert this and the two-dimensional screen co-ordinate to a three-dimensional co-ordinate. This ``active cursor'' allows the user simply to point to a feature and obtain the full position information.

As indicated above, the estimate of the depth is not ideal, so to remove ambiguity a further extension has been made which interactively displays the voxel data and opacity data (where appropriate) along the projected ray. The estimated depth is also shown on this display, which allows the user to determine if the estimate is reasonable.

Another limitation of this cursor is that it cannot work with a stereoscopic display. This cursor requires that every point on the screen is projected from a unique ray through the volume. Clearly, this is not the case with stereo, where each point on the screen is projected from two unique rays through the volume. Hence, the user cannot use stereo to assist in perceiving depth if using the active cursor.

б© Copyright Astronomical Society of Australia 1997