Ashlar-Vellum’s patented, 23-year-old “Drafting Assistant” is the central component of Ashlar’s “Vellum interface”.^[5]

The Drafting Assistant tracks the position of the designer’s cursor and looks for nearby geometry. It then automatically displays information alongside the cursor regarding nearby geometric features to which the designer can snap. The designer can create new geometry at those snap points, or create construction lines to serve as guides. The Drafting Assistant is sensitive to the following geometric attributes:^[6]

• Centers
• Endpoints
• Intersections
• Midpoints
• Perpendicularities
• Quadrants
• Tangents
• Vertexes

Drafting Assistant remembers the last snaps with a weighted algorithm to intuit the designer’s intentions; thus, it is easy to snap to intersections in empty 3‑D space.

In the animation at right, the designer first snaps to the X-, Y-, and Z-axis coordinates at the midpoint of the top edge and then snaps to the same spot on the leading edge, which has different X- and Z-axis coordinates. He moves his cursor to a point in 3D space where there are no geometric attributes to snap to. Although there may be 3D surfaces underneath the cursor, Drafting Assistant intuits the designer’s intent and offers an intersection point comprising the Y- and Z-axis coordinates of the first edge and the X-axis coordinate of the nearest edge. At this location, the designer adds a circle freehand and then specifies a diameter of 200 millimeters by typing it into the box at bottom right. Last, the designer uses the “Remove profile from solid” tool to cut through the block. Here again, Drafting Assistant enables prompt definition of the depth of the cut by snapping to the back quadrant of the intersecting hole.

The Drafting Assistant also provides a “Message line” at the top. This displays instructions appropriate for the selected tool, prompts the designer with what he should do next with any given tool, and reminds the designer of optional modes for those tools.

Cobalt’s parametrics and history tracking work permits the designer to later edit the diameter and/or location of either circle—both of which have dependencies (holes in the block)—and the model updates accordingly.