OpenGL于MFC使用汇总（三）——离屏渲染

有时直接创建OpenGL形式不适合，或者干脆不同意然后创建一个表单，正如我现在这个项目，创建窗体不显示，它仅限于主框架。而我只是ActiveX里做一些相关工作，那仅仅能用到OpenGL的离屏渲染技术了~即不直接绘制到窗体上，而是绘制到一张位图上。然后再次调用这张位图实现兴许的工作。

以下就总结怎么使用所谓的“离屏渲染”。

    const int WIDTH = 500;
    const int HEIGHT = 500;

    // Create a memory DC compatible with the screen
    HDC hdc = CreateCompatibleDC(0);
    if (hdc == 0) cout<<"Could not create memory device context";

    // Create a bitmap compatible with the DC
    // must use CreateDIBSection(), and this means all pixel ops must be synchronised
    // using calls to GdiFlush() (see CreateDIBSection() docs)
    BITMAPINFO bmi = {
        { sizeof(BITMAPINFOHEADER), WIDTH, HEIGHT, 1, 32, BI_RGB, 0, 0, 0, 0, 0 },
        { 0 }
    };
    unsigned char *pbits; // pointer to bitmap bits
    HBITMAP hbm = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, (void **) &pbits,
        0, 0);
    if (hbm == 0) cout<<"Could not create bitmap";

    //HDC hdcScreen = GetDC(0);
    //HBITMAP hbm = CreateCompatibleBitmap(hdcScreen,WIDTH,HEIGHT);

    // Select the bitmap into the DC
    HGDIOBJ r = SelectObject(hdc, hbm);
    if (r == 0) cout<<"Could not select bitmap into DC";

    // Choose the pixel format
    PIXELFORMATDESCRIPTOR pfd = {
        sizeof (PIXELFORMATDESCRIPTOR), // struct size
        1, // Version number
        PFD_DRAW_TO_BITMAP | PFD_SUPPORT_OPENGL, // use OpenGL drawing to BM
        PFD_TYPE_RGBA, // RGBA pixel values
        32, // color bits
        0, 0, 0, // RGB bits shift sizes...
        0, 0, 0, // Don't care about them
        0, 0, // No alpha buffer info
        0, 0, 0, 0, 0, // No accumulation buffer
        32, // depth buffer bits
        0, // No stencil buffer
        0, // No auxiliary buffers
        PFD_MAIN_PLANE, // Layer type
        0, // Reserved (must be 0)
        0, // No layer mask
        0, // No visible mask
        0, // No damage mask
    };
    int pfid = ChoosePixelFormat(hdc, &pfd);
    if (pfid == 0) cout<<"Pixel format selection failed";

    // Set the pixel format
    // - must be done *after* the bitmap is selected into DC
    BOOL b = SetPixelFormat(hdc, pfid, &pfd);
    if (!b) cout<<"Pixel format set failed";

    // Create the OpenGL resource context (RC) and make it current to the thread
    HGLRC hglrc = wglCreateContext(hdc);
    if (hglrc == 0) cout<<"OpenGL resource context creation failed";
    wglMakeCurrent(hdc, hglrc);

    // Draw using GL - remember to sync with GdiFlush()
    GdiFlush();
    /*
	详细的绘制函数~~~~~~~~~~~~~~我就不写了
    */

    // Clean up
    wglDeleteContext(hglrc); // Delete RC
    SelectObject(hdc, r); // Remove bitmap from DC
    DeleteObject(hbm); // Delete bitmap
    DeleteDC(hdc); // Delete DC

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