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windows/mfc程序中使用OpenGL的多重采样功能

学过opengl基础的人都知道,打开混合功能并调用glEnable(GL_POINT_SMOOTH)/glEnable(GL_LINE_SMOOTH)可以实现模型点/线的反走样功能，非常简单且效果不错。

但是对于实心多边形的支持却没有那么简单，单纯的调用 glEnable(GL_POLYGON_SMOOTH)发现并没有效果。

OpenGL对于多边形的反走样有另一个多重采样的特性支持——glEnable(GL_MULTISAMPLE)
但是当调用后发现还是不起作用，查阅资料后才发现在Windows程序中要想使用OpenGL的多重采样，需要额外的一些操作。

一般情况下MFC程序创建OpenGL环境

普通的Windows OpenGL程序在OnCreat()函数中完成环境初始化比较简单

int OnCreate(LPCREATESTRUCT lpCreateStruct)
{ 

CWnd::OnCreate(lpCreateStruct);
PIXELFORMATDESCRIPTOR pfd;
pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_DRAW_TO_BITMAP | PFD_SUPPORT_OPENGL
| PFD_DOUBLEBUFFER/* PFD_SUPPORT_GDI*/ | PFD_STEREO_DONTCARE;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = 32;
pfd.cRedBits = 8;
pfd.cRedShift = 16;
pfd.cGreenBits = 8;
pfd.cGreenShift = 8;
pfd.cBlueBits = 8;
pfd.cBlueShift = 0;
pfd.cAlphaBits = 0;
pfd.cAlphaShift = 0;
pfd.cAccumBits = 64;
pfd.cAccumRedBits = 16;
pfd.cAccumGreenBits = 16;
pfd.cAccumBlueBits = 16;
pfd.cAccumAlphaBits = 0;
pfd.cDepthBits = 32;
pfd.cStencilBits = 8;
pfd.cAuxBuffers = 0;
pfd.iLayerType = PFD_MAIN_PLANE;
pfd.bReserved = 0;
pfd.dwLayerMask = 0;
pfd.dwVisibleMask = 0;
pfd.dwDamageMask = 0;
CDC* wnd_dc = GetDC();
if (!wnd_dc) return -1;
HDC dc_handle = wnd_dc->GetSafeHdc();
int choosed_pixel_format = ChoosePixelFormat(dc_handle, &pfd);
if (0 == choosed_pixel_format)
{ 

choosed_pixel_format = 1;
if (0 == DescribePixelFormat(dc_handle, choosed_pixel_format, sizeof(PIXELFORMATDESCRIPTOR), &pfd))
{ 

ReleaseDC(wnd_dc);
return -1;
}
}
if (FALSE == SetPixelFormat(dc_handle, choosed_pixel_format, &pfd))
{ 

ReleaseDC(wnd_dc);
return -1;
}
HGLRC render_context = wglCreateContext(dc_handle);
if (dc_handle && render_context && TRUE == wglMakeCurrent(dc_handle, render_context))
{ 

GLenum err = glewInit();
}
return 0；
}

如何使用OpenGL的多重采样

上面方式最主要的问题是PIXELFORMATDESCRIPTOR的数据结构是固定的，没有对多重采样的支持，要想使用此特性可以使用wglew库的wglChoosePixelFormatARB函数替换ChoosePixelFormat，但是使用wglChoosePixelFormatARB函数必须先调用glewInit()来初始化glew库，要初始化glew库则必须先得到窗口的渲染上下文，于是就必须先有一个临时窗口，但不能是真正的窗口。具体过程如下：

int OnCreate(LPCREATESTRUCT lpCreateStruct)
{ 

CWnd::OnCreate(lpCreateStruct);
CWnd tempWnd;    
tempWnd.Create(_T("STATIC"), _T("TMP"), WS_CHILD | WS_VISIBLE, CRect(), AfxGetMainWnd(), NULL);
HDC tempHdc = tempWnd.GetDC()->GetSafeHdc();
PIXELFORMATDESCRIPTOR pfd;
if (!SetPixelFormat(tempHdc, 1, &pfd))		//每个窗口只能设置一次 
return -1;
HGLRC temphRC = wglCreateContext(tempHdc);  //创建一个临时的环境为了初始化glew
wglMakeCurrent(tempHdc, temphRC);      
GLenum err = glewInit();                    //初始化glew库,一个程序中初始化一次就可以,不需要每个opengl环境都初始化
if (GLEW_OK != err) return -1;
wglMakeCurrent(NULL, NULL);
wglDeleteContext(temphRC);
tempWnd.DestroyWindow();    
float fPixAttribs[] = { 
 0, 0 };
int iPixAttribs[] = { 
 WGL_SUPPORT_OPENGL_ARB, GL_TRUE, // Must support OGL rendering
WGL_DRAW_TO_WINDOW_ARB, GL_TRUE, // pf that can run a window 
WGL_ACCELERATION_ARB,
WGL_FULL_ACCELERATION_ARB, // must be HW accelerated
WGL_COLOR_BITS_ARB, 32, // 8 bits of each R, G and B
WGL_DEPTH_BITS_ARB, 24, // 24 bits of depth precision for window
WGL_DOUBLE_BUFFER_ARB, GL_TRUE, // Double buffered context
WGL_PIXEL_TYPE_ARB, WGL_TYPE_RGBA_ARB, // pf should be RGBA type
WGL_STENCIL_BITS_ARB, 8,//开启模板缓冲区,模板缓冲区位数=8
WGL_SAMPLE_BUFFERS_ARB, GL_TRUE, // MSAA on,开启多重采样
WGL_SAMPLES_ARB, 4, // 4x MSAA ,多重采样样本数量为4
0 }; // NULL termination 
CDC* wnd_dc = GetDC();
if (!wnd_dc) return -1;
HDC dc_handle = wnd_dc->GetSafeHdc();
if (!wnd_dc) return -1;
int nPixelFormat = -1;
int nPixCount = 0;
wglChoosePixelFormatARB(dc_handle, iPixAttribs, fPixAttribs, 1, &nPixelFormat, (UINT*)&nPixCount);//新的查询像素格式的函数 
if (nPixelFormat == -1)    //多重采样时,如果硬件不支持就使用下面的代码关闭多重采样 
{ 
 // Try again without MSAA 
iPixAttribs[19] = 1;
wglChoosePixelFormatARB(dc_handle, iPixAttribs, fPixAttribs, 1, &nPixelFormat, (UINT*)&nPixCount);
} // Got a format, now set it as the current one
PIXELFORMATDESCRIPTOR pfd;
if (!SetPixelFormat(dc_handle, nPixelFormat, &pfd))
return -1;
GLint attribs[] = { 
 WGL_CONTEXT_MAJOR_VERSION_ARB, 4,//主版本4
WGL_CONTEXT_MINOR_VERSION_ARB, 3,//次版本号3
WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB,
0 };
render_context = wglCreateContextAttribsARB(dc_handle, 0, attribs);
if (m_gl_rendering_context == NULL)
{ 

TRACE0("!!! Could not create an OpenGL 4.3 context.\n");
attribs[1] = 1;
attribs[3] = 3;
render_context = wglCreateContextAttribsARB(dc_handle, 0, attribs);
if (render_context == NULL)
{ 

TRACE0("!!! Could not create an OpenGL 1.3 context.\n");
return -1;
}
}
if(!wglMakeCurrent(dc_handle, render_context))
return -1;
}

如此初始化后，再调用 glEnable(GL_MULTISAMPLE)多重采样就起作用啦，效果非常明显：