Question

I’ve been having a problem recently when getting to grips with Opengl VAOs.
Currently I have code which draws a triangle from a array of floats using a VBO.

Here is the code.

float vpp[] = { 0.75f, 0.75f, 0.0f,
                0.75f, -0.75f, 0.0f,
                -0.75f, -0.75f, 0.0f};

// Non Indexed
glGenBuffers(1, &m_mainVertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, m_mainVertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(vpp), vpp, GL_STATIC_DRAW);

glEnableVertexAttribArray(0);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glDrawArrays(GL_POINTS, 0, 3);
glDisableClientState(GL_VERTEX_ARRAY);
glBindBuffer(GL_ARRAY_BUFFER, 0);

Now when I try to implement a VAO, the program only draws out 1 point. (I would post a screen cap but I can’t).

And the code for this one.

float vp[] = { 0.0f, 0.75f, -0.75f };

//VAO
unsigned short sInds[9] = { 1, 1, 0,
                            1, 2, 0,
                            2, 2, 0};

//Indexed
glGenBuffers(1, &m_mainVertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, m_mainVertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(vp), vp, GL_STATIC_DRAW);

GLuint elBuf;
glGenBuffers(1, &elBuf);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elBuf);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, 9 * sizeof(unsigned short), sInds, GL_STATIC_DRAW);

glEnableVertexAttribArray(0);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glDrawElements(GL_POINTS, 3, GL_UNSIGNED_SHORT, sInds);
glDisableClientState(GL_VERTEX_ARRAY);


glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

Can anyone show me what I’m doing wrong?

Answer 1

Which OpenGL version do you use? So far your code uses only Vertex Buffer Objects, but there’s no Vertex Array Object support code there, yet. If you’re on OpenGL-4 core, you need to create a VAO first, otherwise nothing will draw.

If you don’t plan on using VAO abstraction, you can create a catch-all VAO right after creation of the context.

GLuint vaoID;
glGenVertexArrays(1, &vaoID);
glBindVertexArray(vaoID);

Update:

I didn’t it notice yesterday night, but Nicol Bolas did. Some part of your second code snippet doesn’t make sense. vp defines only 3 floats. It now completely depends on the semantics of the vertex shader (using generic vertex attributes mandates the use of shaders) how these are to be interpreted. Your call of glVertexAttribPointer with a element size of 3 tells OpenGL, that each element of that vertex attribute has 3 fields, which are addressed together.

float vp[] = { 0.0f, 0.75f, -0.75f };

              \------------------/ one attribute element

It would make more sense with your first definition vpp

float vpp[] = { 0.75f, 0.75f, 0.0f,

                \----------------/ attribute element 0

                0.75f, -0.75f, 0.0f,

                \-----------------/ attribute element 1

                -0.75f, -0.75f, 0.0f};

                \------------------/ attribute element 2

If you enable several vertex attributes, they together form a long vector of several attributes.

The indices supplied to glDrawElements address such vertex attribute vectors, not their individual fields.