Question

Im having trouble with texturing a cube with different textures per face. I can draw the cube with one texture on all the faces, but when I try use multiple textures it fails. The way im trying to do it is like so:

//my indexing array located in a header file
#define NUM_IMAGE_OBJECT_INDEX 36
static const unsigned short cubeIndices[NUM_IMAGE_OBJECT_INDEX] =
{
    0, 1, 2,   2, 3, 0,      // front
    4, 5, 6,   6, 7, 4,      // right
    8, 9,10,  10,11, 8,      // top
    12,13,14,  14,15,12,     // left
    16,17,18,  18,19,16,     // bottom
    20,21,22,  22,23,20      // back
};

now in my rendering function, this currently works for drawing the cube with a single texture

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, iconTextureID);

glDrawElements(GL_TRIANGLES, NUM_IMAGE_OBJECT_INDEX, GL_UNSIGNED_SHORT, 0);

this does not work

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, iconTextureID);

glDrawElements(GL_TRIANGLES, NUM_IMAGE_OBJECT_INDEX, GL_UNSIGNED_SHORT, (const GLvoid*)&cubeIndices[0]);

which should equate to the same thing, from looking at some other examples. Ultimately I would like to be doing this something like this:

for(int i = 0; i < 6; i++){

    iconTextureID = textureID[i];                
    glBindTexture(GL_TEXTURE_2D, iconTextureID);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (const GLvoid*)&cubeIndices[i*6]); //index 0-5 use texture 1, 6-11 use texture 2, etc

}

does anyone know what could be wrong with this indexing? ive basically copy pasted this code from an android project (which works), currently trying to do this on ios.

Answer 1

In OpenGL ES 2.0, index data can come from either buffer objects or pointers to client memory. Your code is obviously using a buffer object. Though you don’t show the creation of this buffer object, where you upload your client array of pointers, or where you call glBindBuffer(GL_ELEMENT_ARRAY_BUFFER) before rendering with it. It must be there or your code would have crashed. When a buffer is bound to GL_ELEMENT_ARRAY_BUFFER, OpenGL expects the “pointer” given to glDrawElements to be a byte offset into the buffer object, not a client-memory pointer.

This is why copy-and-paste coding is a bad idea. Where you copied from was probably using client memory; you are not.

If you want your looping code to work, you need to do the pointer arithmetic yourself:

for(int i = 0; i < 6; i++)
{
    iconTextureID = textureID[i];                
    glBindTexture(GL_TEXTURE_2D, iconTextureID);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, reinterpret_cast<void*>(i * 6 * sizeof(GLushort)));
}