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programming hints

OpenGL performance hints

Chapter 5 53

draw array set extensions

glDrawArraySethp is a Hewlett-Packard OpenGL 1.1 extension to

vertex arrays which provides a high-speed mechanism for rendering

multiple primitives. Use of glDrawArraySethp will be easy for

applications which currently store geometry in vertex arrays and use

multiple calls to glDrawArrays for rendering primitives from the arrays.

glDrawArraySethp is especially useful when multiple connected

primitives, such as GL_LINE_STRIP, GL_TRIANGLE_STRIP, etc., are

consecutively drawn from a vertex array. However all OpenGL primitive

types are supported.

Since only glDrawArray calls are made while rendering the vertex array

set, primitive attributes, such as material colors, must be established for

the entire array set or changed per vertex. If OpenGL library calls other

than glDrawArrays are required during rendering the set to properly

draw the array set, then glDrawArraySethp is not appropriate.

benefits of glDrawArraySethp: glDrawArrays and glDrawArraySethp

provide basically the same programmatic benefits, that is reduced

function calls and less user code. Note that glDrawArraySethp’s major

benefit is performance. glDrawArraySethp provides from 10%-55%

performance advantage over using glDrawArrays alone.

The amount of performance benefit depends upon several factors,

including the number of primitives in the set, the length of the primitives

in the set, and maximum rendering speed of the graphics device.

To achieve optimum glDrawArraySethp rendering performance, group

as many primitives in each set as possible.

using glDrawArraySethp: To use glDrawArraySethp, the current vertex

array must be set and enabled. This is done using glNormalPointer,

glVertexPointer, glEnableClientState, etc., or

glInterleavedArrays. After the vertex array is established and

enabled, glDrawArraySethp may be used.

The C declaration of glDrawArraySethp is:

void glDrawArraySethp(GLenum mode,

const GLint* list, GLsizei count);

where:

OGLImp.book Page 53 Tuesday, February 14, 2006 9:15 AM

HP j6700 Supplementary Guide

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