Creating a basic OpenGl context

Question

I'm reading the OpenGl red book, and I'm pretty much stuck at the first tutorial. Everything works fine if I use freeglut and glew, but I'd like to handle input and such myself. So I ditched freeglut and glew and wrote my own code. I've looked at some other tutorials and finished the code, but nothing is displayed. It seems like FreeGlut does some voodoo magic in the background, but I don't know what I'm missing. I've tried this:

int attributeListInt[19];
int pixelFormat[1];
unsigned int formatCount;
int result;
PIXELFORMATDESCRIPTOR pixelFormatDescriptor;
int attributeList[5];

context = GetDC (hwnd);
if (!context)
    return -1;

attributeListInt[0] = WGL_SUPPORT_OPENGL_ARB;
attributeListInt[1] = TRUE;
attributeListInt[2] = WGL_DRAW_TO_WINDOW_ARB;
attributeListInt[3] = TRUE;
attributeListInt[4] = WGL_ACCELERATION_ARB;
attributeListInt[5] = WGL_FULL_ACCELERATION_ARB;
attributeListInt[6] = WGL_COLOR_BITS_ARB;
attributeListInt[7] = 24;
attributeListInt[8] = WGL_DEPTH_BITS_ARB;
attributeListInt[9] = 24;
attributeListInt[10] = WGL_DOUBLE_BUFFER_ARB;
attributeListInt[11] = TRUE;
attributeListInt[12] = WGL_SWAP_METHOD_ARB;
attributeListInt[13] = WGL_SWAP_EXCHANGE_ARB;
attributeListInt[14] = WGL_PIXEL_TYPE_ARB;
attributeListInt[15] = WGL_TYPE_RGBA_ARB;
attributeListInt[16] = WGL_STENCIL_BITS_ARB;
attributeListInt[17] = 8;
attributeListInt[18] = 0;

result = wglChoosePixelFormatARB (context, attributeListInt, NULL, 1, pixelFormat, &formatCount);
if (result != 1)
    return -1;

result = SetPixelFormat (context, pixelFormat[0], &pixelFormatDescriptor);
if (result != 1)
    return -1;
attributeList[0] = WGL_CONTEXT_MAJOR_VERSION_ARB;
attributeList[1] = 4;
attributeList[2] = WGL_CONTEXT_MINOR_VERSION_ARB;
attributeList[3] = 2;
attributeList[4] = 0;

rendercontext = wglCreateContextAttribsARB (context, 0, attributeList);
if (rendercontext == NULL)
    return -1;

result = wglMakeCurrent (context, rendercontext);
if (result != 1)
    return -1;

glClearDepth (1.0f);
glFrontFace (GL_CCW);
glEnable (GL_CULL_FACE);
glCullFace (GL_BACK);

return 0;

This sets up a graphics context, but is apparently not enough to make everything work. The tutorial didn't include anything about view or projection matrices, so I'm not sure whether I should add anything like that. But the window remains black.

This is the tutorial code, adjusted to my code:

#define BUFFER_OFFSET(offset) ((void *)(offset))

bool init ();
bool mainloop ();

enum VAO_IDs { Triangles, NumVAOs };
enum Buffer_IDs { ArrayBuffer, NumBuffers };
enum Attrib_IDs { vPosition = 0 };
GLuint VAOs[NumVAOs];
GLuint Buffers[NumBuffers];
const GLuint NumVertices = 6;

int main (int argc, char** argv)
{
    Window w;

    w.init (&mainloop);
    if (!init ())
        return 0;

    w.run ();
    w.shutdown ();

    return 0;
}

bool init ()
{
    glGenVertexArrays (NumVAOs, VAOs);
    glBindVertexArray (VAOs[Triangles]);
    GLfloat vertices[NumVertices][2] = {
        {-0.90f, -0.90f}, // Triangle 1
        {0.85f, -0.90f},
        {-0.90f, 0.85f},
        {0.90f, -0.85f}, // Triangle 2
        {0.90f, 0.90f},
        {-0.85f, 0.90f}
    };

    glGenBuffers (NumBuffers, Buffers);
    glBindBuffer (GL_ARRAY_BUFFER, Buffers[ArrayBuffer]);
    glBufferData (GL_ARRAY_BUFFER, sizeof(vertices),
        vertices, GL_STATIC_DRAW);
    ShaderInfo shaders[] = {
        {GL_VERTEX_SHADER, "triangles.vert"},
        {GL_FRAGMENT_SHADER, "triangles.frag"},
        {GL_NONE, NULL}
    };

    GLuint program = LoadShaders (shaders);
    glUseProgram (program);
    glVertexAttribPointer (vPosition, 2, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET (0));
    glEnableVertexAttribArray (vPosition);

    return true;
}

bool mainloop ()
{
    glClear (GL_COLOR_BUFFER_BIT);
    glBindVertexArray (VAOs[Triangles]);
    glDrawArrays (GL_TRIANGLES, 0, NumVertices);
    glFlush ();

    return true;
}

For one thing, your triangles are wound counter-clockwise and you have the front-face set to clockwise. By culling the back-face (counter-clockwise) you are not going to see anything. Either change the front-face to CCW, cull the front, or disable culling. — Andon M. Coleman, Jun 01 '14 at 21:10
@AndonM.Coleman I have changed it to CCW, but it still doesn't show anything. — , Jun 01 '14 at 21:20
I don't think that doing everything yourself is a particularly sound solution. You won't learn faster that way. — Bartek Banachewicz, Jun 01 '14 at 21:30
Additionally, you have a double-buffered context, but rather than swpapping buffers (which is necessary and which ***implicitly*** invokes `glFlush (...)` or `glFinish (...)` depending on whether you are using a hardware ICD or GDI), you have called `glFlush (...)`. Replace that with `SwapBuffers ()` and also add `| GL_DEPTH_BUFFER_BIT` to your `glClear (...)` call (since you have a depth buffer in your pixel format). You don't have to clear the depth buffer since depth testing is disabled by default, but if you enable depth testing then things will start writing to the depth buffer. — Andon M. Coleman, Jun 01 '14 at 22:31
@AndonM.Coleman I've disabled the depth buffer, and with SwapBuffers everything works now, thanks! — , Jun 02 '14 at 13:43

datenwolf · Answer 1 · 2014-06-02T09:56:58.607

Creating a OpenGL context is not trivial. Especially if you want to use wglChoosePixelFormatARB which must be loaded through the OpenGL extension mechanism… which requires a functioning OpenGL context to work in the first place. I think you see that this is kind of a chicken-and-egg problem. In addition the Window you use to create an OpenGL context requires certain attributed to work reliably. For one the WndClass should have the CS_OWNDC style set and the window style should include WS_CLIPSIBLINGS | WS_CLIPCHILDREN

I recently approached the aforementioned chicken-and-egg problem with my small wglarb helper tools: https://github.com/datenwolf/wglarb

It also comes with a small test program that shows how to use it.

I suggest you use the functions provided by that. I wrote this little helper library in a way, that it is not negatively impacted if your program uses other extension loading mechanisms. ~~However it's not thread-safe yet; I'll have to add that eventually.~~ I took the time to make the code thread safe. You can now use the exposed functions without having to care about synchronization; it's all done internally in a reliable way.

Creating a basic OpenGl context

1 Answers1