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An OpenGL projection is the process of translating 3D objects into a 2D image for display on a screen. There are two types of projection modes: orthographic and perspective. The challenge is to adjust the data to be consistent with the projection mode, display volume, and virtual camera settings. The most commonly used mode is perspective projection, which scales and positions objects based on their distance from the viewer.
An OpenGL® projection is the process and image resulting from translating three-dimensional (3D) objects into an aligned two-dimensional (2D) image that can be displayed on a screen. The word “projection” in computer graphics programming refers to the fact that 3D objects within a scene must be mathematically moved, or projected, onto a virtual 2D surface so that it can be shown on a flat display on a device . While the entire sequence of events that take a 3D object and render it onto a 2D surface can be called a projection, the term is also more strictly applied to the specific set of geometric transformations in the graphics pipeline that occur as a switch between other sets of transformations. The two specific types of OpenGL® projection modes available to programmers are perspective and orthographic.
The challenge of creating an OpenGL® projection is to take the data that includes all the objects in a given scene and adjust it so that when displayed on a screen, the objects are consistent with the projection mode, display volume, and the settings of the virtual camera. Viewing volume is the visible area of a scene from the viewer’s position within the scene or from the eye’s position. Six numbers define the viewing volume, including right, left, top, and bottom distances from a virtual center of the viewing frame. Also, two values known as the near and far planes define the depth of the viewing volume. This volume defines which objects will be projected, while all other objects outside the volume will be ignored or clipped so that the non-visible parts are not evaluated.
The first OpenGL® projection mode that can be used is known as orthographic projection. In this mode, the coordinates of objects within a scene are not adjusted for their distance from the viewer, or perspective. This means that all objects are drawn in the same size, regardless of the distance from the viewer. This OpenGL® projection mode is often used in 3D modeling programs, engineering design programs, and applications that use 3D polygons to create 2D composite images and don’t require accurate perspective.
The most commonly used OpenGL® projection mode is known as perspective projection. In perspective projection mode, objects in a scene are scaled and positioned based on their distance from the viewer. This means that more distant objects appear smaller. It also means that objects in the scene are adjusted to slowly converge at a point on the observation horizon known as the vanishing point. Perspective projection closely models how the real world looks and is used to create realistic versus technical renderings.
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