有没有人有PBO + SDL2.0 + OpenGL的例子

有没有人有PBO（像素缓冲区对象）+ SDL2.0（简单直接媒体层）+ OpenGL的工作示例？

原因是使用GLGetPixel下载异步GPU -> CPU，从而获得性能提升。

这是我的尝试。use_pbo = 假或真根本没有测量差异。我以前在同一台机器上使用过带有 GLUT 的 PBO，所以我知道我的硬件支持它。

我已经看了很多 http://www.songho.ca/opengl/gl_pbo.html 教程，但没有看到我做错了什么。

bool use_pbo = true; //remember to wait a bit for fps to pick up speed
if(SDL_Init(SDL_INIT_EVENTS) != 0) throw "SDL_Init";
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
sdl_window = SDL_CreateWindow("", 10, 20, window_width, window_height, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
sdl_gl_context = SDL_GL_CreateContext(sdl_window);
SDL_GL_MakeCurrent(sdl_window, sdl_gl_context);
vertical_sync(false);
{
    glGenBuffers(2, pbos);
    glBindBuffer(GL_PIXEL_PACK_BUFFER, pbos[0]);
    glBufferData(GL_PIXEL_PACK_BUFFER, pbo_width*pbo_height*4, 0, GL_STREAM_READ);
    glBindBuffer(GL_PIXEL_PACK_BUFFER, pbos[1]);
    glBufferData(GL_PIXEL_PACK_BUFFER, pbo_width*pbo_height*4, 0, GL_STREAM_READ);
    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
}
//render to default frame buffer
{
    glDrawBuffer(GL_BACK);
    glClearColor(0.1, 0.2, 0.3, 0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    glUseProgram(simple_shader);
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, transparent_texture);
    glBegin(GL_TRIANGLE_STRIP);
    //textcoord, color, position
    glVertexAttrib2f(2, 0, 1); glVertexAttrib4f(1, 1, 0, 0, 1); glVertexAttrib2f(0, -1, -1); //bottom left
    glVertexAttrib2f(2, 1, 1); glVertexAttrib4f(1, 0, 1, 0, 1); glVertexAttrib2f(0, +1, -1); //bottom right
    glVertexAttrib2f(2, 0, 0); glVertexAttrib4f(1, 0, 0, 1, 1); glVertexAttrib2f(0, -1, +1); //top left
    glVertexAttrib2f(2, 1, 0); glVertexAttrib4f(1, 1, 1, 0, 1); glVertexAttrib2f(0, +1, +1); //top right
    glEnd();
    glBindTexture(GL_TEXTURE_2D, 0);
    glUseProgram(0);
    glDisable(GL_BLEND);
    while(true)
    {
        if(use_pbo)
        {
            glBindBuffer(GL_PIXEL_PACK_BUFFER, pbos[1]);
            GLvoid* map_buffer_data = glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
            if(map_buffer_data)
            {
                //glRasterPos2i(-1, -1); //default, left bottom
                glRasterPos2i(0, -1);
                glDrawPixels(pbo_width, pbo_height, GL_BGRA, GL_UNSIGNED_BYTE, map_buffer_data);
                glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
            }
            glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
        }
        else
        {       
            glRasterPos2i(0, -1); //right bottom
            glDrawPixels(pbo_width, pbo_height, GL_BGRA, GL_UNSIGNED_BYTE, t.c);
        }
    }
}
SDL_GL_SwapWindow(sdl_window);
//cycle buffers
{
    GLuint t = pbos[1];
    pbos[1] = pbos[0];
    pbos[0] = t;
}