Question

I am getting GL_INVALID_ENUM code when calling glGetError right after glew init has finished and before any other gl command was called.I am setting OpenGL 4.2 CORE ,forward compatible.Using GLFW for window/input and GLEW for gl context all this running on Windows 7 64bit.My graphic card is GeForce 550GT which has the latest drivers with OpenGL 4.2.Another strange thing is that I was using glDrawElements() which worked fine.But then I tried glDrawArrays(), which I needed for some specific task, and it didn’t work at all.I double rechecked all the gl syntax ,buffer creations shaders etc.Nothing.I do get GL_INVALID_OPERATION when calling glDrawArrays() and I have no Idea why.All this work is a mini render engine but currently the pipeline is really simple so I can’t understand what can be the problem.Here is how I initialize the context and window:

 void Engine::InitWithGLFW(){


    if(!glfwInit()){
        exit(EXIT_FAILURE);
    }
    glfwOpenWindowHint(GLFW_OPENGL_VERSION_MAJOR, 4);
    glfwOpenWindowHint(GLFW_OPENGL_VERSION_MINOR, 2);
    glfwOpenWindowHint(GLFW_OPENGL_FORWARD_COMPAT,GL_TRUE);
    glfwOpenWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwOpenWindowHint(GLFW_FSAA_SAMPLES,0);
    glfwDisable( GLFW_AUTO_POLL_EVENTS );

 #ifdef DEBUG
    glfwOpenWindowHint(GLFW_OPENGL_DEBUG_CONTEXT, GL_TRUE);
 #endif

    if(!glfwOpenWindow(_width,_height,8, 8, 8, 8, 24, 8,GLFW_WINDOW)){
        glfwTerminate();
        exit(EXIT_FAILURE);
    }

    glfwSetWindowTitle("XDEngine V-1.0");
    InitGlew();



}

void  Engine::InitGlew(){
    glewExperimental=GL_TRUE;
    GLenum err = glewInit(); 

    if (GLEW_OK != err)
    {
        /* Problem: glewInit failed, something is seriously wrong. */
        fprintf(stderr, "Error: %s\n", glewGetErrorString(err));

    }

    fprintf(stdout, "Status: Using GLEW %s\n", glewGetString(GLEW_VERSION));

    glEnable(GL_MULTISAMPLE);
    glEnable (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    glEnable(GL_DEPTH_TEST);
    glDepthMask(true);
    glDepthFunc(GL_LEQUAL);
    glDepthRange(0.0f, 1.0f);
    glEnable(GL_DEPTH_CLAMP);
    glEnable(GL_CULL_FACE);
    glCullFace(GL_BACK);

    glfwSetWindowSizeCallback(Reshape);
}

And this is the render loop for glDrawArrays():

     void Draw(){
            _material->Draw();///activate shader program
            glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glDisableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);
            _material->PostDraw();  //deactivate shader program

   }

As I said , all the buffers and shader programs are initialized ok.
UPDATE:
Ok , I found the answer for the first issue :

Seems like GL_INVALID_ENUM may happen after GlewInit and it is not supposed to screw the rendering.So I am left with the second problem…

UPDATE1:

I found the workaround for GL_INVALID_ENUM when calling glDrawArrays() with VBO.If I pack the VBO into VAO The error disappears and I get the rendering working.Does it mean that in forward compatible core profile we can’t draw using VBO directly?Never seen anything related to it in the official docs.

Answer 1

Vertex array objects are not optional in the core profile. The GL_INVALID_ENUM error is probably coming from somewhere else; you should be getting GL_INVALID_OPERATION. Remember: OpenGL errors are buffered, so you have to keep fetching GL errors until you come back with GL_NO_ERROR.

glfwOpenWindowHint(GLFW_OPENGL_FORWARD_COMPAT,GL_TRUE);

Please stop doing this. Forward compatibility is virtually meaningless in a core profile.