iPhone OpenGL : Texturing a cube issue

Question

I been really enjoying playing with OpenGl and have followed some examples etc

My issue is texturing a cube. Most side texture fine (front, back, left) but right is a massed up mess.

My cube

// cube
static const GLfloat cubeVertices[] = { 
    -5.0f, 15.0f, 5.0f,
    5.0f, 15.0f, 5.0f, 
    5.0f,0.0f, 5.0f,
    -5.0f,0.0f, 5.0f,
    -5.0f, 15.0f,-5.0f, 
    5.0f, 15.0f,-5.0f,
    5.0f,0.0f,-5.0f,
    -5.0f,0.0f,-5.0f
};

static const GLubyte cubeNumberOfIndices = 36;

const GLubyte cubeVertexFaces[] = { 
    0, 1, 5, // Half of top face 
    0, 5, 4, // Other half of top face
    4, 6, 5, // Half of front face 
    4, 6, 7, // Other half of front face
    0, 1, 2, // Half of back face 
    0, 3, 2, // Other half of back face
    1, 2, 5, // Half of right face 
    2, 5, 6, // Other half of right face
    0, 3, 4, // Half of left face 
    7, 4, 3, // Other half of left face
    3, 6, 2, // Half of bottom face 
    6, 7, 3, // Other half of bottom face 
};

My Texture map

const GLshort squareTextureCoords2[] = {
    0, 5, // top left
    0, 0, // bottom left
    15, 0, //bottom right
    15, 5  //top right
};

My cube code

//tell GL about our texture
    glBindTexture(GL_TEXTURE_2D, 1);
    glTexCoordPointer(2, GL_SHORT, 0, squareTextureCoords2);

    glVertexPointer(3, GL_FLOAT, 0, cubeVertices);

    for(int i = 0; i < cubeNumberOfIndices; i += 3) {
        //glColor4ub(cubeFaceColors[colorIndex], cubeFaceColors[colorIndex+1], cubeFaceColors[colorIndex+2], cubeFaceColors[colorIndex+3]);
        int face = (i / 3.0);
        if (face % 2 != 0.0) {
        }
        glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, &cubeVertexFaces[i]);
}

So as I said, everything renders but one side of the cube (cant see top and bottom so not worried) the texture goes all weird

Thanks

UPDATE **

I now tried these Coords and the texture does not appear on each side it textures it from front to back and so the sides are like lines from the other edge of the texture. I got to be close to cracking this

const GLfloat squareTextureCoords3[] = {
    // Front face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Top face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Rear face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Bottom face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Left face
    5, 5,       // top left
    0, 5,       // bottom left
    0, 0,       // bottom right
    5, 0,       // top right

    // Right face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right
};

Tommy Tommy · Accepted Answer · 2010-11-11T00:49:12

First of all, to describe eight vertices you need eight texture coordinates. You're supplying a buffer containing only four, so behaviour is technically undefined (and will probably depend on what, if anything, your compiler put in memory after the array).

It's also not really valid to pass a constant as the texture name to glBindTexture. You should pass in whatever glGenTextures gave you back earlier.

Even if those problems were fixed, surely when you think about it the texture coordinates on the edge faces won't be very useful if you're sticking rigidly to having just eight vertices? That means you have to find texture coordinates for each vertex that make sense for every connected face. Try sketching it out on paper - I'm not sure it's possible. To avoid that you need to supply duplicate vertex positions with different texture coordinates.

EDIT: example geometry for a cube where each face has a completely unique set of vertices (typed directly in here, untested, to make the point):

// this is unchanged from your original
static const GLfloat squareTextureCoords3[] = {
    // Front face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Top face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Rear face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Bottom face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right

    // Left face
    5, 5,       // top left
    0, 5,       // bottom left
    0, 0,       // bottom right
    5, 0,       // top right

    // Right face
    0, 5,       // top left
    0, 0,       // bottom left
    5, 0,       // bottom right
    5, 5,       // top right
};

// this is changed, to match the tex coord list
static const GLfloat cubeVertices[] = { 
    // front face
    -5.0f, 15.0f, 5.0f,
    5.0f, 15.0f, 5.0f, 
    5.0f, 0.0f, 5.0f,
    -5.0f, 0.0f, 5.0f,

    // top face
    -5.0f, 15.0f, 5.0f,
    5.0f, 15.0f, 5.0f, 
    5.0f, 15.0f,-5.0f,
    -5.0f, 15.0f,-5.0f,         

    // rear face
    -5.0f, 15.0f,-5.0f, 
    5.0f, 15.0f,-5.0f,
    5.0f,0.0f,-5.0f,
    -5.0f,0.0f,-5.0f

    // bottom face
    -5.0f, 0.0f, 5.0f,
    5.0f, 0.0f, 5.0f, 
    5.0f, 0.0f,-5.0f,
    -5.0f, 0.0f,-5.0f,

    // left face
    -5.0f, 0.0f, 5.0f,
    -5.0f, 0.0f,-5.0f,
    -5.0f, 15.0f,-5.0f,         
    -5.0f, 15.0f, 5.0f,

    // right face
    5.0f, 0.0f, 5.0f,
    5.0f, 0.0f,-5.0f,
    5.0f, 15.0f,-5.0f,         
    5.0f, 15.0f, 5.0f,
};

// this is changed also, to match the new arrays above
const GLubyte cubeVertexFaces[] = { 
    4, 5, 6, // Half of top face 
    4, 6, 7, // Other half of top face
    0, 1, 2, // Half of front face 
    0, 2, 3, // Other half of front face
    8, 9, 10, // Half of back face 
    8, 10, 11, // Other half of back face
    20, 21, 22, // Half of right face 
    20, 22, 23, // Other half of right face
    16, 17, 18, // Half of left face 
    16, 18, 19, // Other half of left face
    12, 13, 14, // Half of bottom face 
    12, 14, 15, // Other half of bottom face 
};

Other code remains the same. The key thing is just that in OpenGL you can't separately index vertex positions and texture coordinates. Ignoring the possibility of colours, normals, etc, the description of a vertex in OpenGL is one position with one texture coordinate.

iPhone OpenGL : Texturing a cube issue

2 Answers