scene.rst

The Scene

The scene is the central registry for graphical objects and manages rendering parameters. Among other parameters, it is used to setup the lighting, fog, background color and the position and viewing direction of the user. The scene is a singleton, meaning that there is only one scene available. The instance can be accessed via :func:`gfx.Scene`. Because the scene is so important and commonly used, the scene is also available as the scene variable in the interactive python shell as well as from scripts.

Manipulating the Camera

The users position and orientation in the scene is specified by the camera. The camera is represented by a center, a rotation and an offset from the center. These 3 attributes can be set independently. Manipulation of the camera is done by assigning a new :class:`~ost.mol.Transform` to the :attr:`Scene.transform` attribute.

Orbiting around a Point

The following code example will let the camera orbit around the center of the camera.

# tweakable parameters:
#  - axis of rotation
#  - angular step size. Making it bigger speeds up the rotation
#  - number of frames
axis=geom.Vec3(0, 1, 0)
step_size=0.01
num_frames=100

angle=0.0
for i in range(num_frames):
  angle+=step_size
  if angle>math.pi*2:
    angle-=math.pi*2
  camera=scene.transform
  rot=geom.AxisRotation(axis, angle)
  camera.SetRot(rot)
  camera.SetTrans(geom.Vec3(25.0, 0.0, -50))
  scene.transform=camera
  scene.Export('frame-%03d.png' % i, 500, 500)

It is interesting to note that the offset from center (trans) is given in rotated coordinates. Transforming along z shifts the camera along the viewing direction. Setting x and y to non-zero causes the center of the camera not to be projected onto the center of the screen any longer. For example, setting the value trans to geom.Vec3(50, 0, 0) gives a viewing direction perpendicular to the vector from the camera position to the center.