The computer graphics pipeline, or simply graphics pipeline, also known as the rendering pipeline, is a series of procedures for transforming a three-dimensional (3D) scene into a two-dimensional (2D) representation on a screen. Once a 3D model is generated, the 3D computer graphics pipeline converts the model into a visually perceivable format on the computer display. Due to the dependence on specific software and hardware configurations and desired display attributes, a universally applicable graphics pipeline does not exist. Nevertheless, graphics application programming interfaces (APIs), such as Direct3D, OpenGL and Vulkan were developed to standardize common procedures and oversee the graphics pipeline of a given hardware accelerator. These APIs provide an abstraction layer over the underlying hardware, relieving programmers from the need to write code explicitly targeting various graphics hardware accelerators like AMD, Intel, Nvidia, and others. The model of the graphics pipeline is usually used in real-time rendering. Often, most of the pipeline steps are implemented in hardware, which allows for special optimizations. The term "pipeline" is used in a similar sense for the pipeline in processors: the individual steps of the pipeline often run in parallel when a given stage has enough data.
Concept The 3D pipeline usually refers to the most common form of computer 3-dimensional rendering called 3D polygon rendering, distinct from raytracing and raycasting. In raycasting, a ray originates at the point where the camera resides, and if that ray hits a surface, the color and lighting of the point on the surface where the ray hit is calculated. In 3D polygon rendering the reverse happens – the area that is given the camera is calculated and then rays are created from every part of every surface given the camera and traced back to the camera.
Structure A graphics pipeline can be divided into three main parts: Application, Geometry, and Rasterization.
Application The application step is executed by the software or hardware on the main processors (CPU, GPU). During the application step, changes are made to the scene as required, for example, by user interaction using input devices or during an animation. The new scene with the modified primitives (points, lines, triangles, etc.) is then passed on to the next step in the pipeline. Examples of application step are collision detection, animation, morphing, and acceleration techniques using spatial subdivision schemes such as quadtrees or octrees. These are also used to reduce the amount of main memory required at a given time. The "world" of a modern computer game is usually larger than available onboard memory.
Geometry The geometry pipeline and the vertex pipeline are responsible for most of the operations with polygons. These can be further divided into the five tasks. How these tasks are organized as actual parallel pipeline steps is implementation specific.
Model and camera transformations Lighting Projection (Z-buffering) Clipping Viewport
World Coordinate System The world coordinate system is the coordinate system in which the virtual world is created. The objects contained within the scene (houses, trees, cars) are often designed in their object coordinate system (also called model coordinate system or local coordinate system) for reasons of simpler modelling. To assign these objects to coordinates in the world coordinate system or global coordinate system of the entire scene, the object coordinates are transformed using translation, rotation, or scaling. This is done by multiplying the corresponding transformation matrices. In addition, several differently transformed copies can be formed from one object, for example, a forest from a tree. This is called instancing.
Camera Transformation
In addition to the objects, the scene also defines a virtual camera or viewer that indicates the position and direction of view relative to which the scene is rendered. The scene is transformed so that the camera is at the origin looking along the Z-axis. The resulting coordinate system is called the camera coordinate system and the transformation is called camera transformation or View Transformation.
Projection The 3D projection step transforms the view volume into a cube with the corner point coordinates (−1, −1, 0) and (1, 1, 1); Occasionally other target volumes are also used. This step is called projection, even though it transforms a volume into another volume, since the resulting Z coordinates are not stored in the image, but are only used in Z-buffering in the later rastering step. In a perspective illustration, a central projection is used. To limit the number of displayed objects, two additional clipping planes are used; The visual volume is therefore a truncated pyramid (frustum). The parallel or orthogonal projection is used, for example, for technical representations because it has the advantage that all parallels in the object space are also parallel in the image space, and the surfaces and volumes are the same size regardless of the distance from the viewer. Maps use, for example, an orthogonal projection (so-called orthophoto), but oblique images of a landscape cannot be used in this way – although they can technically be rendered, they seem so distorted that we cannot make any use of them.# For reasons of efficiency, the camera and projection matrix are usually combined into a transformation matrix so that the camera coordinate system is omitted. The resulting matrix is usually the same for a single image, while the world matrix looks different for each object. In practice, therefore, view and projection are pre-calculated so that only the world matrix has to be adapted during the display. However, more complex transformations such as vertex blending are possible. Freely programmable geometry shaders that modify the geometry can also be executed. In the actual rendering step, the world matrix * camera matrix * projection matrix is calculated and then finally applied to every single point. Thus, the points of all objects are transferred directly to the screen coordinate system (at least almost, the value range of the axes is still −1..1 for the visible range, see section "Window-Viewport-Transformation").
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