Introduction

Picking is the means of identifying which parts of a segment network fall within a specified space, or pick aperture. A common use of picking is to take a location returned from an input device to define the location of the pick aperture, the size of the pick aperture being some reasonable size, and pass all segments represented on the display device through the picking operation to determine which segment(s) and corresponding element(s) fall near the location defined by the input device.

The picking process in the context of Display List is very similar to the picking process in the context of "raw" Starbase. Most of the steps are identical; only a few are different. One possible approach is described below:

Both the position and size of the pick aperture are defined using the Starbase calls set_pick_window and set_pick_depth (see the Starbase documentation for more information on these calls). It should also be noted that picking is not directly coupled to any particular input device. If desired, the application can define a pick aperture based on input device values, thereby relating picking to input device values.

What Can Be Picked?

A primitive that can be picked is a primitive that is:

• Visible. The primitive must not be invisible by being in the set of invisible objects in the context of the invisibility filter.

• Pickable. The primitive must not be considered by the unpickability filter.

• Unclipped. The primitive must not be outside the clip limits.

• Unobscured. In the case of 3-Dimensional picking with hidden surface removal, using a device with a Z-buffer, if two primitives are inside the pick aperture and the second primitive covers the first primitive because of hidden surface removal, then the second primitive will produce a hit and the first one will not because it is obscured.

• Detectable. With element control, a primitive with its visibility flag set to DISP_IGNORE and its pickability flag not set to PICK_IGNORE is invisible, but can be picked.

• In a detectable segment; that is, segment control flags of the segment containing the element do not include PICK_IGNORE.

• Not a Device Coordinate (DC) - primitive (e.g., dcdraw, dcpolyline, dcpolygon, etc.).

• Not a partial- primitive (e.g., partial_polygon3d, partial_arc, partial_ellipse, etc.).

	NOTE: For the devices HP 98705 and HP 98766, with hidden surface removal activated, the primitive picked is determined by the following conditions: First, the primitive closest to the center of the pick aperture. Second, the primitive closest in Z.

	NOTE: When a Hidden Line/Hidden Surface Removal (HLSHR) pick is executed in an application, the Z-buffer is used. A pick can normally be done to an obscured window without a problem. However, if a pick is performed with Hidden Line/Hidden Surface Removal enabled, the Z-buffer for obscured regions of a window becomes undefined requiring that the Z-buffer be cleared before the application executes any drawing primitive. For predictable results, do not use HLHSR when picking to obscured windows.

The set_disp_traversal_control and set_pick_traversal_control calls also influence which primitives are picked. If pick traversal encounters either a pick-control element or a display-control element, and the control element is TRAVERSAL_ABORT or TRAVERSAL_RETURN, the segment structure from that point on will not be traversed. Any primitives in that part of the structure can not be picked simply because they are not processed.