Use vertex index

in initTriangle, making following changes

Vertex and index definition

There are now no duplicate vertices

    // two triangles : vertex data
    GLfloat verts[] = {
        -1.0f, 1.0f,  0.0f,// v0
        -1.0f, -1.0f, 0.0f,// v1
        1.0f, -1.0f,  0.0f, // v2
        1.0f, 1.0f,   0.0f,// v3
    };

    // indices of two triangles
    GLuint indices[] = { 0, 1, 2, 2, 3, 0};

We are moving from 2d vetex to 3d, so there is some change

The Vertex Position Attribute

The vertex position attribute is similar to that in LabA01, the difference is that now we are using 3d vertices.

    // set buffer data to triangle vertex and setting vertex attributes
    glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);

The Vertex Indices

After the code of vertex buffer, create triangle index buffer of type GL_ELEMENT_ARRAY_BUFFER

    // create index buffer
    GLuint idxBufID;
    glGenBuffers(1, &idxBufID);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, idxBufID);

    // set buffer data for triangle index
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

Draw triangles with indices

in drawTriangle, making following changes

    // glDrawArrays(GL_TRIANGLES, 0, 6);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

Now you get a rectangle

If you are using line drawing in drawTriangle(), you can see the two triangles

glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);