Concepts of 3D Graphics
This topic explains basic concepts related to 3D rendering. Ignoring
difference in naming these concepts are used in most 3D graphical
applications, not only DG Kernel software.
The main elements are:
Global Frame,
Viewing Volume,
Eye Frame and
Viewing Plane
The Global Frame is
used for modeling. It describes the physical 3D space, which is being modeled. For example, if a sphere is added at point x, y, x these
coordinates are related to the global axes.
Viewing Volume is the part of 3D space, which is displayed in
the 3D View. It is a rectangular box (tapered in case of perspective projection), located and oriented arbitrary in
3D space. Image on the computer screen is created by projecting objects
inside the box to its front side, mapping to the window on the screen
and converting it to pixels. Any object or parts of them which are
outside the Viewing Volume will appear fully or partially clipped.
Viewing transformations of 3D view, namely Zoom Pan and Rotate,
result in change of size, location and orientation of the Viewing
Volume. Zoom reduces width and height of the Viewing Volume so that a
smaller part of the scene mapped to the screen and the object will
appear larger. Holding shift key and moving mouse to the right will
cause the Viewing Volume to move to the left relative to its current
position. Rotate transformations with the mouse cause the Viewing Volume
to rotate in 3D around its center.
Experiment with Birds Eye view to see the above elements in action: Start any sample. Click
on the DGK window. Press v,b. Try rotate/pan/zoom in the main window of the sample and observe how
it affects the elements of the scene.
Infinite plane parallel to the front side of the Viewing Volume is
called Viewing Plane. Viewing plane determines direction of view. Viewing plane has a set of axes attached to it. It is called Eye
Frame. Viewing Plane is the pane z=0 of the Eye Frame. Origin of the
Eye Frame coincides with origin of the Global Frame on start of the
application, but it is modified by viewing rotations. Eye Frame is used for programmatic
manipulations of Viewing Volume. Viewing Volume is aligned to axes of
Eye Frame. Its dimensions are expressed relative to Eye Frame.
When visible, the Global Frame is rendered in 3D View with
conical pointers at the end of axes. There is no much reason to display the
Eye Frame. If visible, it would always appear on the screen as
horizontal X axis is pointing to the right and vertical Y axes pointing
up. Z axis is orthogonal to the plane of the screen
and directed towards the viewer.
Concept of the Local Frame
is secondary to the Global Frame. Local Frames are used for modeling
surface of the object, it belongs to, and for moving objects relatively to
the rest of the model. During viewing transformations relative position
of global and local frames remains constant.
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