创建一个基本的OpenGl上下文

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;
}

Answer 1

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 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.