3D graphics eBook - Course Materials Repository

Graphics pipeline 52
Graphics pipeline
In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current
state of the art method of rasterization-based rendering as supported by commodity graphics hardware pipeline . The
graphics pipeline typically accepts some representation of a three-dimensional primitives as an input and results in a
2D raster image as output. OpenGL and Direct3D are two notable 3d graphic standards, both describing very similar
graphic pipeline.
Stages of the graphics pipeline
Generations of graphic pipeline
Graphic pipeline constantly evolve. This article describe graphic pipeline as can be found in OpenGL 3.2 and
Direct3D 9.
Transformation
This stage consumes data about polygons with vertices, edges and faces that constitute the whole scene. A matrix
controls the linear transformations (scaling, rotation, translation, etc.) and viewing transformations (world and view
space) that are to be applied on this data.
Per-vertex lighting
Geometry in the complete 3D scene is lit according to the defined locations of light sources, reflectance, and other
surface properties. Current hardware implementations of the graphics pipeline compute lighting only at the vertices
of the polygons being rendered. The lighting values between vertices are then interpolated during rasterization.
Per-fragment (i.e. per-pixel) lighting can be done on modern graphics hardware as a post-rasterization process by
means of a shader program.
Viewing transformation or normalizing transformation
Objects are transformed from 3-D world space coordinates into a 3-D coordinate system based on the position and
orientation of a virtual camera. This results in the original 3D scene as seen from the camera’s point of view, defined
in what is called eye space or camera space. The normalizing transformation is the mathematical inverse of the
viewing transformation, and maps from an arbitrary user-specified coordinate system (u, v, w) to a canonical
coordinate system (x, y, z).
Primitives generation
After the transformation, new primitives are generated from those primitives that were sent to the beginning of the
graphics pipeline.
Projection transformation
In the case of a Perspective projection, objects which are distant from the camera are made smaller (sheared). In an
orthographic projection, objects retain their original size regardless of distance from the camera.
In this stage of the graphics pipeline, geometry is transformed from the eye space of the rendering camera into a
special 3D coordinate space called "Homogeneous Clip Space", which is very convenient for clipping. Clip Space
tends to range from [-1, 1] in X,Y,Z, although this can vary by graphics API(Direct3D or OpenGL). The Projection
Transform is responsible for mapping the planes of the camera's viewing volume (or Frustum) to the planes of the
box which makes up Clip Space.