opengl離屏渲染（不需要和視窗繫結，僅當作一個可以渲染一張圖片的API使用）+ opencv顯示

#include <windows.h>
#include <iostream>
#include <gl/gl.h>
#include <gl/glu.h>
#include <string>
#include <time.h>

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

using namespace std;
using namespace cv;

void mGLRender()
{
    glClearColor(0.9f,0.9f,0.3f,1.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(30.0, 1.0, 1.0, 10.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0, 0, -5, 0, 0, 0, 0, 1, 0);
    glBegin(GL_TRIANGLES);
    glColor3d(1, 0, 0);
    glVertex3d(0, 1, 0);
    glColor3d(0, 1, 0);
    glVertex3d(-1, -1, 0);
    glColor3d(0, 0, 1);
    glVertex3d(1, -1, 0);
    glEnd();
    glFlush(); // remember to flush GL output!
}

void mGLRender1()
{
    glClearColor(0.3f,0.3f,0.3f,1.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(30.0, 1.0, 1.0, 10.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0, 0, -5, 0, 0, 0, 0, 1, 0);
    glBegin(GL_TRIANGLES);
    glColor3d(1, 0, 0);
    glVertex3d(0, 1, 0);
    glColor3d(0, 1, 0);
    glVertex3d(-1, -1, 0);
    glColor3d(0, 0, 1);
    glVertex3d(1, -1, 0);
    glEnd();
    glFlush(); // remember to flush GL output!
}


int main(int argc, char* argv[])
{
    clock_t  clockBegin, clockEnd;  
    const int WIDTH = 400;
    const int HEIGHT = 400;

    // 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()
    clockBegin = clock();  
    mGLRender();
    GdiFlush();
    //SaveBmp(hbm,"output.bmp");
    clockEnd = clock(); 
    printf("%d\n", clockEnd - clockBegin);

    clockBegin = clock();  
    mGLRender1();
    GdiFlush();
    //SaveBmp(hbm,"output1.bmp");
    clockEnd = clock(); 
    printf("%d\n", clockEnd - clockBegin);
    /*
    Examining the bitmap bits (pbits) at this point with a debugger will reveal
    that the colored triangle has been drawn.
    */

    //opencv show img
    Mat img(HEIGHT,WIDTH,CV_8UC4,(void *)pbits);
    imshow("img",img);
    waitKey();
    destroyWindow("img");

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

    system("pause");

    return 0;
}