borland c & glut      source      loading objects      texture coordinates      mail      links

urce source source source source source source source source source source

[program ten]  I'm fairly new to hardware multi-texturing.  My old 3d card didn't support it so I never had the chance to play with it.  This page takes you through setting up the extensions, vertex arrays and the texture units to render our light mapped wooded quad from [program 7] in one multi-textured pass.

Note that this program requires your card to have the GL_ARB_multitexture extension, otherwise it won't work.  

urce source source source source source source source source source source

We define macros to name our textures as per usual.

#define MAX_TEXTURES 

#define WOOD_IMAGE   0
#define LIGHTMAP     1

Function pointer variables are declared for the the multi-texture extension functions we are going to be using.  If you look in program10.h, there are many more functions that we don't need.  These are for sending texture coordinates to the texture units one at a time (or in display lists).  We're not doing that, so we don't need them. 
Line 3 is a global variable so we know whether compiled vertex arrays are supported or not. 

In fact, these lines aren't really needed, you should download a file called 'glext.h' from your video card vendor's website (Nvidia and ATI both have these on their developer websites).  Doing this ensures you'll always have an up to date set of headers for extensions that YOUR video card supports.

[1]    PFNGLCLIENTACTIVETEXTUREARBPROC glClientActiveTextureARB = NULL;
[2]    PFNGLACTIVETEXTUREARBPROC       glActiveTextureARB       = NULL;

We declare our vertex, texture coordinate and index arrays as before.  Both texture units will be using the same coordinates.  Remember that there are 4 texture coordinates, one for each vertex of the quad.  For an explanation of texture mapping you can go to [texture coordinates]

...
[1]    GLfloat tex_coords[] = { 0.0, 1.0,
                                1.0, 1.0,
                                1.0, 0.0,
                                0.0, 0.0 };
...

There isn't any lighting again in our initialisation function.  Lines 3 calls a function to grab the extensions, if they are supported - it also prints out some driver statistics.  Lines 8 through 12 load our textures and bind them to the texture names we've generated in line 6.

[1]    void init ( void )
[2]    {
[3]        get_extensions ( );
[4]
[5]        glPixelStorei ( GL_UNPACK_ALIGNMENT, 1 );
[6]        glGenTextures ( 2, tex_id );
[7]
[8]        glBindTexture ( GL_TEXTURE_2D, tex_id[WOOD_IMAGE] ); 
[9]        tgaLoadImage ( "board.tga", &temp, TGA_FREE ); 
[10]
[11]       glBindTexture ( GL_TEXTURE_2D, tex_id[LIGHTMAP] ); 
[12]       tgaLoadImage ( "lightmap.tga", &temp, TGA_FREE | TGA_LUMINANCE ); 

Line 13 enables the depth testing, 14 enables vertex arrays on the client side.  Line 16 is an extension function that switches OpenGL client state to effect the first texture unit - GL_TEXTURE0_ARB.  Line 17 enables the texture coordinate arrays on this texturing unit.  Lines 19 and 20 do the same for the second texture unit.

[13]       glEnable ( GL_DEPTH_TEST );
[14]       glEnableClientState ( GL_VERTEX_ARRAY );
[15]
[16]       glClientActiveTextureARB ( GL_TEXTURE0_ARB ); 
[17]       glEnableClientState ( GL_TEXTURE_COORD_ARRAY );
[18]
[19]       glClientActiveTextureARB ( GL_TEXTURE1_ARB ); 
[20]       glEnableClientState ( GL_TEXTURE_COORD_ARRAY );

Line 22 selects the first texture unit and 23 enables texturing on this unit.  Again, lines 25 and 26 do the same on the second texture unit.  Currently most hardware has only two units.  The PowerVR3 chipset (when it ever gets released) will support 8-level multi-texturing - that'll be nice.

[22]       glActiveTextureARB ( GL_TEXTURE0_ARB ); 
[23]       glEnable ( GL_TEXTURE_2D ); 
[24]
[25]       glActiveTextureARB ( GL_TEXTURE1_ARB ); 
[26]       glEnable ( GL_TEXTURE_2D ); 
[27]   }

The function to set the object is quite similar to the one in program7.  Line 3 passes OpenGL the address of our vertices array.  When glDrawElements is called later in the function it will look here for the vertex data.
Line 5 is that extension function again.  As before it switches OpenGL client state to effect the first texture unit - GL_TEXTURE0_ARB.  Line 6 passes the address of the text coordinates we are going to be using.   Lines 8 and 9 do the same for the second texture unit.

[1]    void set_and_draw ( void )
[2]    {
[3]        glVertexPointer ( 3, GL_FLOAT, 0, &vertices );
[4]
[5]        glClientActiveTextureARB ( GL_TEXTURE0_ARB ); 
[6]        glTexCoordPointer ( 2, GL_FLOAT, 0, &tex_coords ); 
[7]
[8]        glClientActiveTextureARB ( GL_TEXTURE1_ARB ); 
[9]        glTexCoordPointer ( 2, GL_FLOAT, 0, &tex_coords );

Line 11 selects the first texture unit and 12 binds our wood texture.  Line 13 sets the texture environment to GL_DECAL.  In other words the texture will 'draw over' anything under it.  Lines 15 and 16 select the second texture unit and bind the lightmap to it.  Note the the texture environment is set to GL_MODULATE here, this gives us the desired lighting effect.

Multi-texturing hardware is usually capable of many different blending operations.  Lines 11 - 17 could be replaced by something much more complex.  GL_ARB_texture_env_combine supplies a new texture environment mode.  This has many more blending operations and is not within the scope of this simple program (plus I've only had a few weeks to play with it).  I'll post a program that uses the texture combiners at a later date.

[11]       glActiveTextureARB ( GL_TEXTURE0_ARB ); 
[12]       glBindTexture      ( GL_TEXTURE_2D, tex_id[ WOOD_IMAGE ] ); 
[13]       glTexEnvi          ( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL );
[14] 
[15]       glActiveTextureARB ( GL_TEXTURE1_ARB ); 
[16]       glBindTexture      ( GL_TEXTURE_2D, tex_id[ LIGHTMAP ] ); 
[17]       glTexEnvi          ( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE );

The final line renders the quad.

[18]       glDrawElements ( GL_QUADS, 4, GL_UNSIGNED_INT, &indices ); 
[19]   }

[1]    void display ( void )
[2]    {
[3]        glClear ( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
[4]
[5]        glLoadIdentity ( );
[6]        glTranslatef   ( 0.0, 0.0, -3.0 );
[7]        glRotatef      ( angle, 1.0, 1.0, 0.0 );
[8]
[9]        set_and_draw   ( );
[10]
[11]       glutSwapBuffers ( ); 
[12]   } 

Website and content, Paul Groves