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Best Practices for Working with Vertex Data

Understanding how vertex data flows through an OpenGL program is important to choosing strategies for handling the data. Vertex data can travel through OpenGL in two ways, as shown in Figure 8-2. The first way, from vertex data to per-vertex operations, is as part of an OpenGL command sequence that is issued by the application and executed immediately (immediate mode). The second is packaged as a named display list that can be preprocessed ahead of time and used later in the program.

Figure 8-3 provides a closer look at the vertex data path when using immediate mode. Without any optimizations, your vertex data can be copied at various points in the data path. OpenGL is required to capture the current vertex state when you use immediate mode. If your code uses functions that operate on vertex arrays, you can eliminate the command buffer copy shown in Figure 8-3. The OpenGL commands glDrawRangeElements, glDrawElements, and glDrawArrays render multiple geometric primitives from array data, using very few subroutine calls. It's best to use glDrawRangeElements, with glDrawElements the second choice, and glDrawArrays the third.

In addition to using functions that operate on vertex arrays, there are a number of other strategies that you can adopt to optimize the flow of vertex data in your application:

• Minimize data type conversions by supplying OpenGL data types for vertex data. Use GLfloat, GLshort, or GLubyte data types because most graphics processors handle these types natively. If you use some other type, then OpenGL may need to perform a costly data conversion.

• The most desirable want to handle vertex data is to use the GL_APPLE_vertex_array_range or GL_ARB_vertex_buffer_object extensions. (See “Using Extensions to Improve Performance.”) If you can't use these extensions, then make sure you use vertex arrays and display lists. Avoid using immediate mode. But if your code must use immediate mode, maximize the number of vertices per draw command or within a begin-end code block.

• Use vertex programs to perform computations on vertex data instead of using the CPU to perform the computations.

• If your code must use immediate mode, use CGL macros (for Cocoa or Carbon) or AGL macros (Carbon only). Macros use the function call dispatch table directly, which can dramatically reduce function call overhead. See “Use OpenGL Macros.”

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