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# 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 KernelCAD 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 3D space which is currently displayed in the 3D View. It is a rectangular box 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.

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.

Unless turned off 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.