Geometry Struct and glDrawElements

Question

currently I re-write my codes and change them to make they are better.

I have vertex struct like this:

struct SVertex
{
    float X, Y, Z;
};

struct STexCoord
{
    float U, V;
};

And I re-wrote it like this

struct SVertex
{
    int Flag, BoneID;
    float Pos[3];
    float UV[2];
};

When it comes to the render function, I just need to change it from:

glVertexPointer(3, GL_FLOAT, 0, &(Mesh[i].Vertex[0]));
glTexCoordPointer(2, GL_FLOAT, 0, &(Mesh[i].TexCoord[0]));

to:

glVertexPointer(3, GL_FLOAT, sizeof (SVertex), &(Mesh[i].Vertex[0].Pos));
glTexCoordPointer(2, GL_FLOAT, sizeof (SVertex), &(Mesh[i].Vertex[0].UV));

The problem comes from the triangle struct. I re-wrote the Triangle from

struct STriangle
{
    int X, Y, Z;
};

to

struct STriangle
{
    int Flag, Group;
    int V[3], N[3];
};

and I changed the glDrawElements from:

glDrawElements(GL_TRIANGLES, Mesh[i].TriangleTotal*3, GL_UNSIGNED_INT, &(Mesh[i].Triangle[0]));

to

glDrawElements(GL_TRIANGLES, Mesh[i].TriangleTotal*3, GL_UNSIGNED_INT, &(Mesh[i].Triangle[0].V));

And I got messed vertex with the new triangle struct. If I use the old ones, it is correct. How to pass the new triangle struct to the glDrawElements?

Answer 1

EDIT: There is actually no support for what you're trying to do. There is no equivalent function to glVertexPointer in OpenGL for the indexes. I earlier suggested glIndexPointer but it uses an array of colors, not indexes (a very misleading name, and my bad for posting it).

Unfortunately you will have to supply a contiguous array of indexes, or find another way to arrange your data in the structures. I searched for a while and stumbled upon this question in stackoverflow which is similar to what you are looking for, https://stackoverflow.com/questions/19057864/in-opengl-how-do-you-specify-indices-with-a-stride.

You should consider moving into modern OpenGL and start using VBOs. It's never too early to learn VBOs, it's probably best if you start learning VBOs sooner than later.

Timur's answer and Soapy's comment already have links to using VBOs and modern Open GL, do check them out.