The Graphics Rendering Pipeline

• Rendering is the conversion of a scene into an image:

  \

• The scene composed of models in three space.
  Models, composed of primitives, supported by the rendering system.
• Models entered by hand or created by a program.
  For our purposes today, models already generated.
• The image drawn on monitor, printed on laser printer, or written to a raster in memory or a file.
  These different possibities require us to consider device independence.
• Classically, ``model'' to ``scene'' to ``image'' conversion broken into finer steps, called the graphics pipeline.
  Commonly implemented in graphics hardware to get interactive speeds.
• At a high level, the graphics pipeline usually looks like

  \

• Each stage refines the scene,
  converting primitives in modelling space to primitives in device space,
  where they are converted to pixels (rasterized).
• A number of coordinate systems are used:

  MCS:

  Modelling Coordinate System.

  WCS:

  World Coordinate System.

  VCS:

  Viewer Coordinate System.

  NDCS:

  Normalized Device Coordinate System.

  DCS or SCS:

  Device Coordinate System or equivalently the Screen Coordinate System.

  Keeping these straight is the key to understanding a rendering system.
• Transformation between two coordinate systems represented with matrix.
• Derived information may be added (lighting and shading) and primitives may be removed (hidden surface removal) or modified (clipping).

Readings: Hearn and Baker, Section 6-1 (but they give a more detailed version than used here). Red book, 6-1, 6-6 (intro). White book, 8-3, Blinn: 16.

We begin with a description of forward rendering, which is the kind of rendering usually supported in hardware and is the model OpenGL uses. In forward rendering, rendering primitives are transformed, usually in a conceptual pipeline, from the model to the device.

However, raytracing, which we will consider later in the course, is a form of backward rendering. In backward rendering, we start with a point in the image and work out what model primitives project to it.

Both approaches have advantages and disadvantages.