OpenGL Multitexturing - glActiveTexture is NULL

Question

I have started a new project, which I want to use multitexturing in. I have done multixexturing before, and is supported by my version of OpenGL

In the header I have:

GLuint  m_TerrainTexture[3];//heightmap, texture map and detail map
GLuint  m_SkyboxTexture[5]; //left, front, right, back and top textures

PFNGLMULTITEXCOORD2FARBPROC glMultiTexCoord2fARB;
PFNGLACTIVETEXTUREARBPROC   glActiveTexture;

In the constructor I have:

glActiveTexture = (PFNGLACTIVETEXTUREARBPROC) wglGetProcAddress((LPCSTR)"glActiveTextureARB");
glMultiTexCoord2fARB = (PFNGLMULTITEXCOORD2FARBPROC) wglGetProcAddress((LPCSTR)"glMultiTexCoord2fARB");

if(!glActiveTexture || !glMultiTexCoord2fARB)
{
    MessageBox(NULL, "multitexturing failed", "OGL_D3D Error", MB_OK);
}

glActiveTexture( GL_TEXTURE0_ARB );
...

This shows the message box "multitexturing failed" and the contents of glActiveTexture is 0x00000000

when it gets to glActiveTexture( GL_TEXTURE0_ARB ); I get an access violation error

I am implementing the MVC diagram, so this is all in my terrain view class

Are you sure you have a current GL context when you call all that? — genpfault
I have #included <windows.h>, <gl\gl.h>, <gl\glu.h> and "glext.h" Is there something Ive missed out? — Tim Orton
Did you check the contents of glGetString(GL_EXTENSIONS)? It's the only reliable way to know what's supported and what's not. You should also check glGetString(GL_RENDERER) and glGetString(GL_VENDOR). It may very well be, that your program falls into software rasterizer mode. — datenwolf
They all return a bad pointer - expression cannot be evaluated. I dont know what software rasteriser mode is... — Tim Orton
@Tim: Windows falls back into software emulation if you request an OpenGL context configuration that's not supported by (the) hardware driver. For example enabling accum buffer or BITMAP rendering. However glGetString should not return invalid pointers, unless you don't have a valid render context active. — datenwolf

datenwolf datenwolf · Accepted Answer · 2011-04-29T06:54:27

You quoted your code to load the extensions like following:

PFNGLMULTITEXCOORD2FARBPROC glMultiTexCoord2fARB;
PFNGLACTIVETEXTUREARBPROC   glActiveTexture;

glActiveTexture = (PFNGLACTIVETEXTUREARBPROC) wglGetProcAddress((LPCSTR)"glActiveTextureARB");
glMultiTexCoord2fARB = (PFNGLMULTITEXCOORD2FARBPROC) wglGetProcAddress((LPCSTR)"glMultiTexCoord2fARB");

This is very problematic, since it possibly redefines already existing symbols. The (dynamic) linker will eventually trip over this. For example it might happen that the assignment to the pointer variable glActiveTexture goes into some place, but whenever a function of the same name is called it calls something linked in from somewhere else.

In C you usually use a combination of preprocessor macros and custom prefix to avoid this problem, without having to adjust large portions of code.

PFNGLMULTITEXCOORD2FARBPROC myglMultiTexCoord2fARB;
#define glMultiTexCoord2fARB myglMultiTexCoord2fARB

PFNGLACTIVETEXTUREARBPROC   myglActiveTexture;
#define glActiveTexture myglActiveTexture

glActiveTexture = (PFNGLACTIVETEXTUREARBPROC) wglGetProcAddress((LPCSTR)"glActiveTextureARB");
glMultiTexCoord2fARB = (PFNGLMULTITEXCOORD2FARBPROC) wglGetProcAddress((LPCSTR)"glMultiTexCoord2fARB");

I really don't know of any other reason why things should fail if you have a valid render context active and the extensions supported.

OpenGL Multitexturing - glActiveTexture is NULL

4 Answers