Question

I’m trying to put together a very basic OpenGL 3.2 (core profile) application. In the following code, which is supposed to create a VBO containing the vertex positions for a triangle, the call to glVertexAttribPointer fails and raises the OpenGL error GL_INVALID_OPERATION. What does this mean, and how might I go about fixing it?

GLuint vbo, attribLocation = glGetAttribLocation(...);
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
GLfloat vertices[] = { 0, 1, 0, 1, 0, 0, -1, 0, 0 };
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glEnableVertexAttribArray(attribLocation);
// At this point, glGetError() returns GL_NO_ERROR.
glVertexAttribPointer(attribLocation, 3, GL_FLOAT, GL_FALSE, 0, 0);
// At this point, glGetError() returns GL_INVALID_OPERATION.

Answer 1

glEnableVertexAttribArray(program.getAttrib("in_Position"));
// A call to getGLError() at this point prints nothing.
glVertexAttribPointer(program.getAttrib("in_Position"), 3, GL_FLOAT, GL_FALSE, 0, 0);
// A call to getGLError() at this point prints "OpenGL error 1282".

First, there’s an obvious driver bug here, because glEnableVertexAttribArray should also have issued a GL_INVALID_OPERATION error. Or you made a mistake when you checked it.

Why should both functions error? Because you didn’t use a Vertex Array Object. glEnableVertexAttribArray sets state in the current VAO. There is no current VAO, so… error. Same goes for glVertexAttribPointer. It’s even in the list of errors for both on those pages.

You don’t need a VAO in a compatibility context, but you do in a core context. Which you asked for. So… you need one:

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

Put that somewhere in your setup and your program will work.

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As an aside, this:

glfwOpenWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);

is only necessary if you intend your code to run on MacOS’s GL 3.2+ implementation. Unless you have that as a goal, it is unneeded and can be disruptive, as a small number of features are available in a core context that are not part of forward compatibility (wide lines, for example).