The “picture plane” in computer graphics refers to the virtual viewing screen for a 3D scene. It can be used to determine which objects require processing and is important for ray tracing. It can also be a geometric primitive with an associated image texture or a slice of a larger volumetric object.
In computer graphics, particularly three-dimensional (3D) graphics, the term “picture plane” is used to mean the conceptual plane that represents the actual viewing screen through which a user views a virtual 3D scene. The plane is usually not a true geometric object in a 3D scene, but is usually a collection of target coordinates or dimensions that are used during the rasterization process so that the final output can appear as intended on the physical screen. The term can also be used loosely in other ways, including to mean a geometric plane within a 3D scene that has an associated image texture, or to describe a single slice identified geometrically 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 serve several purposes. The position of the image plane can be used to determine which objects within a scene require processing and which can be ignored. This can be done easily because objects on one side of the picture plane are technically behind the viewer, will not be displayed, and, therefore, can be ignored.
If a scene is rendered using classic ray tracing, the light is followed by the virtual eyes of viewers in a scene and then from the surface of an object to the defined light source. The image plane provides the position of the observer in the scene and is used to calculate the ray scattering and rendering mode. If the plane is defined as just a shape that extends to infinity in two of the three axial directions, then it is also the basis for the viewport, which is a rectangular area within the plane that matches the aspect ratio of the viewing 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 associated image texture. These are commonly used to represent a sky, background or floor in a scene. In some modeling programs, the picture plane is an object in the scene that represents the render angle of a scene, also sometimes called a camera.
In volume rendering where an object has some type of content within its boundaries, an image plane is a portion of that volume. This can be visualized with an MRI scan where several planes are compressed to form a complete 3D object. Each of the slices can be isolated and viewed on its own as a planar image.
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