Display Form

Figure 4.9: The Display form

The Display form controls many of the characteristics of the graphics display window. The characteristics which may be modified include:

• AA - Turns antialiasing on or off. Antialiasing helps smooth out the jagged appearance of displayed geometry resulting from the inherently discrete pixels on the display device. The antialiasing feature is currently only available on platforms which support full-screen antialiasing, sometimes known as ``multisample antialiasing''. Several Silicon Graphics systems fully support this feature. On platforms lacking the multisample capability, there may be alternate ways to perform full-screen antialiasing by selecting an option in the display driver setup. Windows machines most commonly place these controls in the display driver configuration panel.

• Depthcue - Turns depth cueing on or off. Depth cueing causes the more distant objects to blend into the background color. This can help increase apparent the 3D effect for both stereo and non-stereo display modes. The depth cueing factors are currently fixed, with the depth cueing starting at the default front clipping plane ending at the default rear clipping plane. In order to get the desired visual effect, one may need to scale and translate the molecule for best effect. Scaling the molecule up (making it larger) will increase the amount of depth cueing effect that is visible on the molecule. Scaling the molecule size down decreases the depth cueing effect. Translating the molecule away from the camera will cause it to take on more of the background color, translating it toward the camera will cause it to take on less of the background color.

• Axes - A set of XYZ axes may be displayed at any one of five places on the screen (each of the corners or the center) or turned off. This is controlled by the Axes chooser.

• Stage - The Stage browser controls the stage, which is a checkerboard plane that can be located in any one of six places or turned off.

• Detail - The degree of detail in the displayed image. Nothing: show nothing; Points: show everything as points; Wireframe: show only wireframe representation for spheres, cylinders, etc.; Flat: show a flat image (no 3D effects); Full: show complete detail. The performance increases as the level of detail decreases.

• Alt. Detail - Same as Detail, but for the display during mouse operations on the displayed molecules, such as rotation, translation and scaling. Since VMD redraws the screen often while the molecule is rotated, it is sometimes beneficial to set lower level of detail for the time of rotation for better performance.

• Perspective - The view of the scene can be Perspective or Orthographic. In the perspective view (the default), objects which are far away are smaller than those near by. In the orthographic view, both objects appear at the same scale. Note that several of the supported external rendering programs do not support orthographic rendering. As such, it may be necessary to ``fake it'' by translating the scene far away from the camera, and apply a zoom factor. This has the effect of significantly reducing the perspective, while not truly an orthographic view.

• Stereo, Eye Sep, and Focal L - These controls set the stereo mode and parameters; stereo is discussed fully in chapter §. The Stereo chooser changes the stereo mode, while the Eye Sep and Focal L controls change the eye separation distance and the focal length, respectively.

• Lights - The graphics display window can use up to four separate light sources to add a realistic effect to displayed graphical objects. The Lights On browser turns these light sources on or off. If the number is highlighted, the light is on, and clicking on it turns the light off. See section § for more discussion regarding lights.

• Clipping Planes (Near Clip and Far Clip) - Clipping occurs when only the part of an image within a certain region of space is drawn. Only those parts of graphical objects which are between the near and far clipping planes are drawn. This feature is useful for viewing a slice of the molecule. The clipping planes also affect the depth cueing, if it is turned on. Objects at the near clipping plane are distinct while objects at the far clipping plane are indistinguishable from the background color.

  The positions of the clipping planes are changed with the Near Clip and Far Clip controls. It is not possible for the near clip to be farther away than the far clip. When using stereo, it may be useful to set the near clip plane much lower than the default value, or even set to zero. This makes the geometry ``pop out of the screen'' a bit more, and can be used for greater dramatic effect.

• Screen Height (H) and Distance (D) - The screen distance parameter determines the distance, in `world' coordinates, from the origin to the display screen. If this is zero, the origin of the coordinate system in which molecules (and all other graphical objects) are drawn coincides with the center of the display. If it is less than 0, the origin is located between the viewer and the screen, while if it is greater than 0, the screen is located closer to the viewer than the origin. A negative value puts any stereo image in front of the screen, aiding the three-dimensional effect; a positive value results in a stereo image that is behind the screen, a less dramatic effect (but easier to see, for some people) stereo effect.

  The screen height, along with the screen distance, defines the geometry and position of the display screen relative to the viewer. The screen height is the vertical size of the display screen, in `world' coordinates. Each molecule is initially scaled and translated to fit within a 2 x 2 x 2 box centered at the origin; so the screen height helps determine how large the molecule appears initially to the viewer. This parameter is used mainly to configure the VMD display to the dimensions and position of a large-screen display, such as a projector, that may be used as a stereo display.

  Figure 4.10: Relationship between screen height (SCRHEIGHT), screen distance to origin (SCRDIST), and the viewer

  

  Figure 4.10 describes the relationship between the screen height, the screen distance, and the world coordinate space.