In computer graphics, especially three-dimensional (3D) graphics, the term "image plane" is used to indicate the conceptual plane that represents the actual display screen through which a user views a virtual 3D scene. The plane is not usually an actual geometric object in a 3D scene, but instead is usually a collection of target coordinates or dimensions that are used during the rasterization process so the final output can be displayed as intended on the physical screen. The term also can loosely be used in other ways, including to indicate a geometric plane within a 3D scene that has an image texture attached to it, or to describe a single slice that is geometrically identified as a plane within a larger volumetric object, such as a single frame from a completed magnetic resonance imaging (MRI) scan.
An image plane within a 3D scene can have several purposes. The location of the image plane can be used to help determine which objects within a scene require processing and which can be ignored. This can be easily done because objects on one side of the image plane are technically behind the viewer, will not be displayed and, thus, can be ignored.
If a scene is rendered using classic ray tracing, then light is followed from the virtual eyes of the viewers into a scene and then from the surface of an object to the defined light source. The image plane gives the location of the viewer in the scene and is used to help calculate how the rays scatter and how they should be rendered. If the plane is defined only as a shape that extends infinitely in two of the three axial directions, then it also is the basis for the viewport, which is a rectangular area within the plane that matches the aspect ratio of the display screen and can be used for some per-pixel operations.
When used in the context of 3D modeling, an image plane can be a geometric primitive that has an image texture attached to it. These are commonly used to represent a sky, background or floor in a scene. In some modeling programs, the image plane is an object in the scene that represents the angle at which a scene will be rendered, sometimes also called the camera.
In volume rendering in which an object has some type of content within its boundaries, an image plane is a slice of that volume. This can be visualized with an MRI scan in which several planes are compressed to form a complete 3D object. Each of the slices can be isolated and viewed by itself as a planar image.
