Control of Viewing Transformations

It is possible to produce closed loop control by the user of the viewing transformations without passing through the application program. This is achieved via use of locator prompt/echo types and the use joystick or puck as follows:

• Locator Prompt/Echo types -11, -12, and -13 produce rotations matrices corresponding to rotation about two axes which are applied in VRC before the view mapping matrix. The axes are y+x (-11), x+z (-12), and y+z (-13).
• Locator Prompt/Echo -14 enlarges or reduces ('scales' or 'zooms') the view window in VRC symetrically by up to 10 or 1/10 times.
• Locator Prompt/Echo -15 shifts (pans) the view window in VRC by up to $\pm 1.0$ .

To start closed loop interaction the application sets the mode of the locator device via GSLCM and then issues a request locator (GRQLC3), or sets the device into event mode. Dynamic modification is then enabled by pressing the joystick button or puck key and moving the joystick or puck, depending on which physical device was selected. Interaction is terminated by releasing the key. Only one viewing transformation can be modified at a time, and the view index is specified by calling GINLC3. The BREAK action <ESC> works normally, and resets the viewing transformation to the value before closed loop control began. As closed loop control is performed in the driver, the application program must call GQVWR to inquire what is the current state of the viewing transformation.

Closed loop control implies the following restrictions:

• Locator devices used in this mode may only be enabled in Request and Event mode. No entries are placed in the event queue.
• Whilst a viewing transformation is being modified it should not be set via GSVWR, nor should output be produced using it.
• Whilst a viewing transformation is being modified the workstation window and viewport should not be modified, nor should GUWK be called.
• A viewing transformation cannot be modified by more than one locator device at a time.