How to apply multiple textures to a VBO of a cube?

Question

Dear reader,

I'm new to VBO's and read some articles on creating and using VBO's. I am working on a application where I want to create a dozen of rotating cubes. I'm connected to a server which pushes text messages to my application. When I receive 6 text messages, I render these messages using Pango and Cairo. Then I want to create textures from these 6 rendered messages and apply those to a new Cube.

Though I normally just use a 2D texture and apply these to a GL_QUAD, I'm not sure how I can apply 6 textures to a cube which is created using VBOs?

I've found an example on how to create a cube using VBO. Those are created like:

The VBO

//    v6----- v5
//   /|      /|
//  v1------v0|
//  | |     | |
//  | |v7---|-|v4
//  |/      |/
//  v2------v3

GLfloat vertices[] = {1,1,1,  -1,1,1,  -1,-1,1,  1,-1,1,        // v0-v1-v2-v3
                          1,1,1,  1,-1,1,  1,-1,-1,  1,1,-1,        // v0-v3-v4-v5
                          1,1,1,  1,1,-1,  -1,1,-1,  -1,1,1,        // v0-v5-v6-v1
                          -1,1,1,  -1,1,-1,  -1,-1,-1,  -1,-1,1,    // v1-v6-v7-v2
                          -1,-1,-1,  1,-1,-1,  1,-1,1,  -1,-1,1,    // v7-v4-v3-v2
                          1,-1,-1,  -1,-1,-1,  -1,1,-1,  1,1,-1};   // v4-v7-v6-v5

    // normal array
    GLfloat normals[] = {0,0,1,  0,0,1,  0,0,1,  0,0,1,             // v0-v1-v2-v3
                         1,0,0,  1,0,0,  1,0,0, 1,0,0,              // v0-v3-v4-v5
                         0,1,0,  0,1,0,  0,1,0, 0,1,0,              // v0-v5-v6-v1
                         -1,0,0,  -1,0,0, -1,0,0,  -1,0,0,          // v1-v6-v7-v2
                         0,-1,0,  0,-1,0,  0,-1,0,  0,-1,0,         // v7-v4-v3-v2
                         0,0,-1,  0,0,-1,  0,0,-1,  0,0,-1};        // v4-v7-v6-v5

    // color array
    GLfloat colors[] = {1,1,1,  1,1,0,  1,0,0,  1,0,1,              // v0-v1-v2-v3
                        1,1,1,  1,0,1,  0,0,1,  0,1,1,              // v0-v3-v4-v5
                        1,1,1,  0,1,1,  0,1,0,  1,1,0,              // v0-v5-v6-v1
                        1,1,0,  0,1,0,  0,0,0,  1,0,0,              // v1-v6-v7-v2
                        0,0,0,  0,0,1,  1,0,1,  1,0,0,              // v7-v4-v3-v2
                        0,0,1,  0,0,0,  0,1,0,  0,1,1};             // v4-v7-v6-v5

    glGenBuffersARB(1, &vbo_id);
    glBindBufferARB(GL_ARRAY_BUFFER_ARB, vbo_id);
    glBufferDataARB(
        GL_ARRAY_BUFFER_ARB
        ,sizeof(vertices)+sizeof(normals)+sizeof(colors)
        ,0
        ,GL_STATIC_DRAW_ARB
    );


    glBufferSubDataARB(GL_ARRAY_BUFFER_ARB, 0, sizeof(vertices), vertices);
    glBufferSubDataARB(GL_ARRAY_BUFFER_ARB, sizeof(vertices), sizeof(normals),normals);
    glBufferSubDataARB(GL_ARRAY_BUFFER_ARB, sizeof(vertices)+sizeof(normals), sizeof(colors), colors);  // copy colours after normals
    s_vertices = sizeof(vertices);
    s_colors = sizeof(colors);
    s_normals = sizeof(colors);

    cube = new Cube(vbo_id, ofxVec3f(0.0, 0.0, 0.0),0.1, s_vertices, s_colors, s_normals);

The Cube class which uses the VBO

#include "Cube.h"
Cube::Cube(
    GLuint nVBO
    ,ofxVec3f oPosition
    ,float fSize
    ,int nVertices
    ,int nNormals
    ,int nColors
)
:vbo_id(nVBO)
,position(oPosition)
,size(fSize)
,s_vertices(nVertices)
,s_normals(nNormals)
,s_colors(nColors)  
{
r = 0;
}

void Cube::update() {
}


void Cube::draw() {
    glPushMatrix();

        glTranslatef(position.x,position.y,position.z);
        glRotatef((r++),0,1,0);
        glScalef(size, size, size);

        // bind VBOs with IDs and set the buffer offsets of the bound VBOs
        // When buffer object is bound with its ID, all pointers in gl*Pointer()
        // are treated as offset instead of real pointer.
        glBindBufferARB(GL_ARRAY_BUFFER_ARB, vbo_id);

        // enable vertex arrays
        glEnableClientState(GL_NORMAL_ARRAY);
        glEnableClientState(GL_COLOR_ARRAY);
        glEnableClientState(GL_VERTEX_ARRAY);


        // before draw, specify vertex and index arrays with their offsets
        glNormalPointer(GL_FLOAT, 0, (void*)s_vertices);
        glColorPointer(3, GL_FLOAT, 0, (void*)(s_vertices+s_normals));
        glVertexPointer(3, GL_FLOAT, 0, 0);

        glDrawArrays(GL_QUADS, 0, 24);

        glDisableClientState(GL_VERTEX_ARRAY);  // disable vertex arrays
        glDisableClientState(GL_COLOR_ARRAY);
        glDisableClientState(GL_NORMAL_ARRAY);

        // it is good idea to release VBOs with ID 0 after use.
        // Once bound with 0, all pointers in gl*Pointer() behave as real
        // pointer, so, normal vertex array operations are re-activated
        glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);
        glPopMatrix();

}

Answer 1

Though I normally just use a 2D texture and apply these to a GL_QUAD, I'm not sure how I can apply 6 textures to a cube which is created using VBOs?

The simplest, most straightforward approach is to stitch them together into a single large texture, and adjust your texture coordinates to match.

Alternatively, since the cube is so simple, a display list instead of VBO might suffice.