2. Displaying 3D Polygon Animation

API

API, an abbreviation of application program interface, is a set of routines, protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

Most operating environments, such as MS-Windows, provide an API so that programmers can write applications consistent with the operating environment. Although APIs are designed for programmers, they are ultimately good for users because they guarantee that all programs using a common API will have similar interfaces. This makes it easier for users to learn new programs.

Direct3D

An API for manipulating and displaying three-dimensional objects. Developed by Microsoft, Direct3D provides programmers with a way to develop 3-D programs that can utilize whatever graphics acceleration device is installed in the machine. Virtually all 3-D accelerator cards for PCs support Direct3D.

OpenGL

A 3-D graphics language developed by Silicon Graphics. There are two main implementations: Microsoft OpenGL, developed by Microsoft and Cosmo OpenGL, developed by Silicon Graphics. Microsoft OpenGL is built into Windows NT and is designed to improve performance on hardware that supports the OpenGL standard. Cosmo OpenGL, on the other hand, is a software-only implementation specifically designed for machines that do not have a graphics accelerator.

This is an overview of the graphics pipeline in OpenGL: 

Graphics Pipeline

In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refer to the way in which the 3D mathematical information contained within the objects and scenes are converted into images and video. The graphics pipeline typically accepts some representation of a three-dimensional primitive as input and results in a 2D raster image as output. OpenGL and Direct3D are two notable 3d graphic standards, both describing very similar graphic pipelines.

Stages of the graphics pipeline

Per-vertex lighting and shading

Geometry in the complete 3D scene is lit according to the defined locations of light sources, reflectance, and other surface properties. Some (mostly older) 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 or per-pixel lighting, as well as other effects, can be done on modern graphics hardware as a post-rasterization process by means of a shader program. Modern graphics hardware also supports per-vertex shading through the use of vertex shaders.

Clipping

Geometric primitives that now fall completely outside of the viewing frustum will not be visible and are discarded at this stage.

Projection Transformation

In the case of a Perspective projection, objects which are distant from the camera are made smaller. This is achieved by dividing the X and Y coordinates of each vertex of each primitive by its Z coordinate (which represents its distance from the camera). In an orthographic projection, objects retain their original size regardless of distance from the camera.

Viewport Transformation

The post-clip vertices are transformed once again to be in window space. In practice, this transform is very simple: applying a scale (multiplying by the width of the window) and a bias (adding to the offset from the screen origin). At this point, the vertices have coordinates which directly relate to pixels in a raster.

Scan Conversion or Rasterisation

Rasterisation is the process by which the 2D image space representation of the scene is converted into raster format and the correct resulting pixel values are determined. From now on, operations will be carried out on each single pixel. This stage is rather complex, involving multiple steps often referred as a group under the name of pixel pipeline.

Texturing, Fragment Shading

At this stage of the pipeline individual fragments (or pre-pixels) are assigned a color based on values interpolated from the vertices during rasterization, from a texture in memory, or from a shader program.

Display

The final colored pixels can then be displayed on a computer monitor or other display.