15. General information on OpenGl

15.1 Common information

As a software interface for graphics hardware, OpenGL’s main purpose is to render two- and three-dimensional objects into a frame buffer. These objects are described as sequences of vertices (which define geometric objects) or pixels (which define images). OpenGL performs several processing steps on this data to convert it to pixels to form the final desired image in the frame buffer.

The following topics present a global view of how OpenGL works:

Primitives and Commands discusses points, line segments, and polygons as the basic units of drawing; and the processing of commands.

OpenGL Graphic Control describes which graphic operations OpenGL controls and which it does not control.

Execution Model discusses the client/server model for interpreting OpenGL commands.

Basic OpenGL Operation gives a high-level description of how OpenGL processes data and produces a corresponding image in the frame buffer.

Primitives and Commands

OpenGL draws primitives : points, line segments, or polygonssubject to several selectable modes. You can control modes independently of one another. That is, setting one mode doesn’t affect whether other modes are set (although many modes may interact to determine what eventually ends up in the frame buffer). To specify primitives, set modes, and perform other OpenGL operations, you issue commands in the form of function calls.

Primitives are defined by a group of one or more vertices. A vertex defines a point, an endpoint of a line, or a corner of a polygon where two edges meet. Data (consisting of vertex coordinates, colors, normals, texture coordinates, and edge flags) is associated with a vertex, and each vertex and its associated data are processed independently, in order, and in the same way. The only exceptions to this rule are cases in which the group of vertices must be clipped so that a particular primitive fits within a specified region. In this case, vertex data may be modified and new vertices created. The type of clipping depends on which primitive the group of vertices represents.

Commands are always processed in the order in which they are received, although there may be an indeterminate delay before a command takes effect. This means that each primitive is drawn completely before any subsequent command takes effect. It also means that state-querying commands return data that is consistent with complete execution of all previously issued OpenGL commands.

OpenGl Graphic Control

OpenGL provides you with fairly direct control over the fundamental operations of two- and three-dimensional graphics. This includes specification of such parameters as transformation matrices, lighting equation coefficients, antialiasing methods, and pixel update operators. However, it doesn’t provide you with a means for describing or modeling complex geometric objects. Thus, the OpenGL commands you issue specify how a certain result should be produced (what procedure should be followed) rather than what exactly that result should look like. That is, OpenGL is fundamentally procedural rather than descriptive. To fully understand how to use OpenGL, it helps to know the order in which it carries out its operations.