3D view definition is based on intuitive concepts such as the direction in which the observer is looking, his position, the zoom factor etc. A view may be defined with projections in parallel or in perspective.
In 3D, the center of the view xc, yc, zc corresponds to the observer's focal point in WC space.
In a perspective view, primitives which are parallel in WC are not projected parallel on the display surface. The closer the observer is to the point he is observing (center of the view), the further from parallel these primitives appear. This type of view is closer to human perception.
Furthermore, the visual cone or width of the field of vision has been set as close as possible to that of the human eye. The field of view may not be calibrated. It has been determined according to the following rule : an observer located at a distance d=3 from the observed point C sees the whole of a cube with sides of 1 centered on C in WC if zoom factor = 1
A 3D perspective view is defined by the following parameters :
the observed point (xc,yc,zc) : expressed in WC as in 2D
the position of the observer (in cartesian or spherical coordinates)
the zoom factor
the display surface viewport
N.B. : the image is magnified as the observer moves closer to the observed point. Increasing the zoom factor also makes the image larger. This does not, however make the zoom factor redundant, as it is not capable of altering the perspective, which does change in function of the variation of distance from the observer. This phenomenon is comparable to framing a view in photography : increasing the focal length of the lens or moving the camera closer to the subject both magnify the subject on the photograph, but the 2 images are by no means the same.
observer defined in spherical coordinates in relation to a marker centered on the observed point
This type of marking makes it easy for the observer to move around the observed point. The vertical axis is either X, Y or Z according to the function call :
TV3SPX (WKID, VIEWID, xc,yc,zc, phi,teta,d, zoom, viewport)
TV3SPY (WKID, VIEWID, xc,yc,zc, phi,teta,d, zoom, viewport)
TV3SPZ (WKID, VIEWID, xc,yc,zc, phi,teta,d, zoom, viewport)
observer defined in cartesian coordinates in relation to a marker centered on the observed point
The vertical axis is either X, Y or Z according to the function call :
TV3CAX (WKID, VIEWID, xc,yc,zc, xo,yo,zo, zoom, viewport)
TV3CAY (WKID, VIEWID, xc,yc,zc, xo,yo,zo, zoom, viewport)
TV3CAZ (WKID, VIEWID, xc,yc,zc, xo,yo,zo, zoom, viewport)
Primitives which are parallel in WC are projected in parallel on the display surface. The observer's position is no longer a factor (it is supposed to be at infinity), but the viewing direction is important.
The following rule has been chosen: a cube with sides of 1 (in WC) observed perpendicularly to one of the sides, is entirely visible on the viewport if zoom factor = 1
A 3D view in parallel is defined by the following parameters :
the observed point (xc,yc,zc) : in WC as in 2D
the viewing direction (defined by two angles : phi and theta)
the zoom factor
the display surface viewport
This type of marking makes it easy for the (fictitious) observer to move around the observed point. The vertical axis is either X, Y or Z according to the function call :
TV3SLX (WKID, VIEWID, xc,yc,zc, phi,teta, zoom, viewport)
TV3SLY (WKID, VIEWID, xc,yc,zc, phi,teta, zoom, viewport)
TV3SLZ (WKID, VIEWID, xc,yc,zc, phi,teta, zoom, viewport)