Cameras

TMatrix3DState provides a 3D transformation matrix for the camera. TLinkableCamera provides protocol for linking the camera to a TMatrix3DState and TClipState. See Chapter 19 for more information on TMatrix3DState and TLinkableCamera.

Projection from 3-D to 2-D

Perspective projection: A center of projection is required, which is the point at which the projectors converge. The size of the projected image depends on the relative distances between the camera, the projection plane, and the 3D scene.

• Moving the center of projection changes the drawing size of a graphic.
• Moving the projection plane alone changes the size of the graphic, but not the level of perspective.
• Placing the projection plane on the graphic in space, projects the 3D graphic onto the projection plane in its original size.

Concrete camera classes

The concrete camera constructors take parameters that let you position and orient the camera, and define the viewing distances and clipping planes of the scene. Figure 216 shows the perspective camera, and Figure 217 shows the parallel camera. The elements in these figures are described with the parallel and perspective camera constructors below.

Perspective camera constructors

  TPerspectiveCamera( const TGPoint3D& cameraPosition,
                      const TGPoint3D& referencePoint,
                      const TGPoint3D& upVector,
                      GCoordinate viewDistance,
                      GCoordinate nearPlaneDistance,
                      GCoordinate farPlaneDistance,
                      const TGPoint projectionPoint = TGPoint::kOrigin,
                      const TGRect& cropWindow = TGRect::kInfiniteRect );

  TPerspectiveCamera( const TGPoint3D& cameraPosition,
                      const TGPoint3D& referencePoint,
                      GDegrees twistAngle,
                      GCoordinate viewDistance,
                      GCoordinate nearPlaneDistance,
                      GCoordinate farPlaneDistance,
                      const TGPoint projectionPoint = TGPoint::kOrigin,
                      const TGRect& cropWindow = TGRect::kInfiniteRect );

  TParallelCamera( const TGPoint3D& cameraPosition,
                      const TGPoint3D& referencePoint,
                      const TGPoint3D& upVector,
                      GCoordinate viewDistance,
                      GCoordinate nearViewPlaneDistance,
                      GCoordinate farViewPlaneDistance,
                      const TGPoint projectionPoint = TGPoint::kOrigin,
                      const TGRect& cropWindow = TGRect::kInfiniteRect );

  TParallelCamera( const TGPoint3D& cameraPosition,
                      const TGPoint3D& referencePoint,
                      GDegrees twistAngle,
                      GCoordinate viewDistance,
                      GCoordinate nearViewPlaneDistance,
                      GCoordinate farViewPlaneDistance,
                      const TGPoint projectionPoint = TGPoint::kOrigin,
                      const TGRect& cropWindow = TGRect::kInfiniteRect );

referencePoint: A TGPoint3D that represents a point in the scene at which the camera points. The reference point is the center of interest.

Orientation and roll

upVector: A TGPoint3D that sets the orientation and roll of the camera. The upVector parameter is defined relative to the y-axis in the world-coordinate system.

twistAngle: A TGPoint3D that sets the orientation and roll of the camera.

Figure 218 shows that if the camera is sitting on the y-axis looking straight up or down, a twist angle of zero is not uniquely defined. In this case, the system chooses an orientation where the up vector is ( 1, 0, 0 ) so the upper edge of the projection plane faces the negative x-axis.

Viewing distances and clipping planes

farPlaneDistance (perspective cameras only): A GCoordinate value that sets the distance between the camera and the far clipping plane.

farViewPlaneDistance (parallel cameras only): A GCoordinate value that sets the distance between the camera and the far clipping plane.

projectionPoint: A TGPoint that defines the 2D origin. The default is TGPoint::kOrigin, which corresponds to the center of the projection plane.

cropWindow: A TGRect that represents the area of interest. It is analogous to the view finder on a camera. Graphics within the view finder are part of the scene; areas outside the view finder are not part of the scene. The default is TGRect::kInfiniteRect, which corresponds to no cropping.

• You can define the cropping window in the projection plane to make partial screen updates faster.
• You can scroll the cropping window by setting up the projection point so that it is the intersection of the camera's direction and the projection plane.

Default camera settings

	cameraPosition	TGPoint3D( 0.,0., 72*18. )
	referencePoint	TGPoint3D( 0., 0., 0. )
	upVector	TGPoint3D( 0., 1., 0. )
	viewDistance	72. * 18.
	nearPlaneDistance	0.1
	farPlaneDistance	2000.0
	projectionPoint	TGPoint( 0., 0. )
	cropWindow	TGRect( TGPoint( kInfinity, kInfinity, TGPoint( kInfinity, kInfinity ) )

These default settings center the TGBox3D constructed in the code fragment below as a 2inch cube on the screen when viewed 18 inches away.

TGBox3D( TGPoint3D( 72., 72., 72. ), TGPoint3D( 72., 72., 72. ) );