Introduction

OpenGL implements extensions as a way for hardware vendors to introduce new features into the GL without having to wait for the ARB to decide on a spec. This is useful in two ways: it speeds up the introduction of new features in the GL, and at the same time, doesn't force the ARB to redo the spec every year or more (like DirectX).

The most notable extension in the recent gaming industry is ARB_multitexture. While it wasn't always an ARB extension (GLQuake used SGIS_multitexture), it has been one of the best examples of how the extension scheme works. The only problem is making sure a hardware vendor introduces new extensions when they add a new feature to their chipset. Luckily OpenGL is popular enough and has strong industry support (John Carmack at id and lots of ex-SGI engineers at NVIDIA), so we're seeing many new cool extensions.

Check out my table of extensions, download the simple program and send me new information.

Links

Here are some links before I start showing some code:

• Offical list of extensions @ opengl.org
• ATI's Rage 128 SDK : 3d programming

Checking for extensions

This is from the sample:

char *pext;
const char *ext;
bool ext_pal_support;

[...]

ext_pal_support = FALSE;

ext = glGetString (GL_EXTENSIONS);
pext = ext;
while( !ext_pal_support && pext )
{
	int cnt = 0;
	pext2 = pext;
	while( *pext2 && *pext2++ != ' ' )
		cnt++;
	if( strncmp( pext,"GL_EXT_paletted_texture", cnt)==0 )
		ext_pal_support = TRUE;
	pext += cnt + 1;
}
					

07/7/99 - Here's a simpler way to do it (if you like that sort of thing) sent in by Michael Steinberg:

boolean suppExt(char extName[])
{
 if ( strstr( (const char*)glGetString( GL_EXTENSIONS ), extName ) == NULL) return false;
 else return true;  // "else" not necessary...
}
					

Using extensions

This is a little troublesome because there are several ways in which extensions are used. It can be simply a new constant value (S3TC), or it can require new functions (multitexture) or both. The best thing to do is look at the spec to get any new constants and function declarations.

This example is taken from my GLConsole for loading and using palettized images.

extern BOOL ext_pal_support;
static PFNGLCOLORTABLEEXTPROC glColorTable;
					

BOOL GLSetupPalSupport( void )
{
	if( !ext_pal_support )
		return FALSE;
	glColorTable = (PFNGLCOLORTABLEEXTPROC) wglGetProcAddress( "glColorTableEXT" );
	if( !glColorTable )
		return FALSE;
	return TRUE;
}
					

glColorTable ( GL_TEXTURE_2D,
	GL_RGBA,
	256,
	GL_RGBA,
	GL_UNSIGNED_BYTE,
	paldata );					
					

Conclusion

Aren't extensions great? Now if we can get vendors to post more information about their driver's extensions (especially the vendor specific ones). Kudos to ATI for putting up their list of extensions. Let me know if other vendors have done the same thing. I may have overlooked some.