在类中使用 OpenGL glutDisplayFunc

Using OpenGL glutDisplayFunc within class(在类中使用 OpenGL glutDisplayFunc)

问题描述

我创建了一个 C++ 类 (myPixmap) 来封装由 OpenGL GLUT 工具包执行的工作.类的 display() 成员函数包含设置 GLUT 所需的大部分代码.

void myPixmap::display()
{
    // open an OpenGL window if it hasn't already been opened
    if (!openedWindow)
    {
        // command-line arguments to appease glut
        char *argv[] = {"myPixmap"};
        int argc = 1;
        glutInit(&argc, argv);
        glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
        glutInitWindowSize(640, 480);
        glutInitWindowPosition(30, 30);
        glutCreateWindow("Experiment");
        glutDisplayFunc(draw);
        glClearColor(0.9f, 0.9f, 0.9f, 0.0);
        glClear(GL_COLOR_BUFFER_BIT);
        glutMainLoop();

        openedWindow = true;
    }  
}

void myPixmap::draw(void)
{
    glDrawPixels( m,n,GL_RGB,GL_UNSIGNED_BYTE, pixel );
}

<代码>void (myPixmap::)()"类型的参数与void (*)()"不匹配

有没有办法在类的成员函数内使用 GLUT 工具包，或者我必须在程序的 main 函数内调用所有的 GLUT 函数?

Is there a way to use the GLUT toolkit inside a member function of a class, or must I call all GLUT functions within the main function of the program?

推荐答案

问题是指向实例绑定成员函数的指针必须包含 this 指针.OpenGL 是一个 C API，对 this 指针一无所知.您必须使用静态成员函数(它不需要实例，因此不需要 this)，并设置一些静态数据成员(以访问实例)才能使用 glutDisplayFunc.

The problem is that a pointer to an instance bound member function has to include the this pointer. OpenGL is a C API, and knows nothing about this pointers. You'll have to use a static member function (which doesn't require an instance, and thus no this), and set some static data members (to access the instance) in order to use glutDisplayFunc.

class myPixmap
{
private:
  static myPixmap* currentInstance;

  static void drawCallback()
  {
    currentInstance->draw();
  }

  void setupDrawCallback()
  {
    currentInstance = this;
    ::glutDisplayFunc(myPixmap::drawCallback);
  }
};

您可能还会遇到 C 链接与 C++ 链接的问题，在这种情况下，您必须尝试使用​​ extern "C".如果是这样，您可能必须使用全局函数，而不是静态成员函数作为回调，并让 那个 调用 myPixmap::draw.类似的东西:

You may also have problems with C linkage vs C++ linkage, in which case you'll have to play around with extern "C". If so, you might have to use a global function, rather than a static member function as your callback, and have that call myPixmap::draw. Something like:

class myPixmap
{
public:
  void draw();

private:
  void setupDrawCallback();
};

myPixmap* g_CurrentInstance;

extern "C"
void drawCallback()
{
  g_CurrentInstance->draw();
}

void myPixmap::setupDrawCallback();
{
  ::g_CurrentInstance = this;
  ::glutDisplayFunc(::drawCallback);
}

考虑到所有这些，请尝试进行尽可能少的更改，因为这对于处理 OpenGL 作为 C API 来说确实有点麻烦.

With all of this, try to make as few changes as possible, since this is really kind of a kludge to deal w/ OpenGL being a C API.

如果你想要多个实例(我认为大多数使用 GLUT 的人不会创建多个实例，但也许你是)，你必须找出一个使用 std::map 来检索实例的解决方案:

If you want multiple instances (I don't think most people using GLUT make multiple instances, but maybe you are), you'll have to figure out a solution using a std::map to retrieve the instance:

static std::map<int, myPixmap> instanceMap;

从哪里获得 int 来解析 哪个 实例，我不确定:)

Where you'd get the int to resolve which instance, I am not sure :)

仅供参考，您应该以这种方式定义不带参数的函数:

FYI, you should define functions that take no parameters this way:

void some_function() { }

不是

void some_function(void) { }

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