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Do not call imshow if you use your own draw callback:

void on_opengl(void* userdata);

int main(void)
{
    VideoCapture webcam(CV_CAP_ANY);

    namedWindow("window", CV_WINDOW_OPENGL);

    Mat frame;
    ogl::Texture2D tex;

    setOpenGlDrawCallback("window", on_opengl, &tex);

    while(waitKey(30) < 0)
    {
        webcam >> frame;
        tex.copyFrom(frame);
        updateWindow("window");
    }

    setOpenGlDrawCallback("window", 0, 0);
    destroyAllWindows();

    return 0;
}

void on_opengl(void* userdata)
{
    ogl::Texture2D* pTex = static_cast<ogl::Texture2D*>(userdata);
    if (pTex->empty())
        return;

    glLoadIdentity();

    glTranslated(0.0, 0.0, 1.0);

    glRotatef( 55, 1, 0, 0 );
    glRotatef( 45, 0, 1, 0 );
    glRotatef( 0, 0, 0, 1 );

    static const int coords[6][4][3] = {
        { { +1, -1, -1 }, { -1, -1, -1 }, { -1, +1, -1 }, { +1, +1, -1 } },
        { { +1, +1, -1 }, { -1, +1, -1 }, { -1, +1, +1 }, { +1, +1, +1 } },
        { { +1, -1, +1 }, { +1, -1, -1 }, { +1, +1, -1 }, { +1, +1, +1 } },
        { { -1, -1, -1 }, { -1, -1, +1 }, { -1, +1, +1 }, { -1, +1, -1 } },
        { { +1, -1, +1 }, { -1, -1, +1 }, { -1, -1, -1 }, { +1, -1, -1 } },
        { { -1, -1, +1 }, { +1, -1, +1 }, { +1, +1, +1 }, { -1, +1, +1 } }
    };

    glEnable(GL_TEXTURE_2D);
    pTex->bind();

    for (int i = 0; i < 6; ++i) {
                glColor3ub( i*20, 100+i*10, i*42 );
                glBegin(GL_QUADS);
                for (int j = 0; j < 4; ++j) {
                        glVertex3d(0.2*coords[i][j][0], 0.2 * coords[i][j][1], 0.2*coords[i][j][2]);
                }
                glEnd();
    }
}