像素缓冲区对象（PBO）的异步Read-Back 源码解析

接这篇文章 OpenGL深入探索——像素缓冲区对象 （PBO）（附完整工程代码地址）

原理示意图如下：

关键代码如下：

      int main(int argc, char **argv)
      {
         initSharedMem();
         // register exit callback
         atexit(exitCB);
         // init GLUT and GL
         initGLUT(argc, argv);
         initGL();
         // get OpenGL info
          glInfo glInfo;
          glInfo.getInfo();
          glInfo.printSelf();
      #ifdef _WIN32
         // 检查视频显卡是否支持 PBO
         if (glInfo.isExtensionSupported("GL_ARB_pixel_buffer_object"))
          {
             // get pointers to GL functions
              glGenBuffersARB = (PFNGLGENBUFFERSARBPROC)wglGetProcAddress("glGenBuffersARB");
              glBindBufferARB = (PFNGLBINDBUFFERARBPROC)wglGetProcAddress("glBindBufferARB");
              glBufferDataARB = (PFNGLBUFFERDATAARBPROC)wglGetProcAddress("glBufferDataARB");
              glBufferSubDataARB = (PFNGLBUFFERSUBDATAARBPROC)wglGetProcAddress("glBufferSubDataARB");
              glDeleteBuffersARB = (PFNGLDELETEBUFFERSARBPROC)wglGetProcAddress("glDeleteBuffersARB");
              glGetBufferParameterivARB = (PFNGLGETBUFFERPARAMETERIVARBPROC)wglGetProcAddress("glGetBufferParameterivARB");
              glMapBufferARB = (PFNGLMAPBUFFERARBPROC)wglGetProcAddress("glMapBufferARB");
              glUnmapBufferARB = (PFNGLUNMAPBUFFERARBPROC)wglGetProcAddress("glUnmapBufferARB");
             // check once again PBO extension
             if (glGenBuffersARB && glBindBufferARB && glBufferDataARB && glBufferSubDataARB &&
                      glMapBufferARB && glUnmapBufferARB && glDeleteBuffersARB && glGetBufferParameterivARB)
              {
                  pboSupported = pboUsed = true;
                  std::cout << "Video card supports GL_ARB_pixel_buffer_object." << std::endl;
              }
             else
              {
                  pboSupported = pboUsed = false;
                  std::cout << "Video card does NOT support GL_ARB_pixel_buffer_object." << std::endl;
              }
          }
         // check EXT_swap_control is supported
         if (glInfo.isExtensionSupported("WGL_EXT_swap_control"))
          {
             // get pointers to WGL functions
              wglSwapIntervalEXT = (PFNWGLSWAPINTERVALEXTPROC)wglGetProcAddress("wglSwapIntervalEXT");
              wglGetSwapIntervalEXT = (PFNWGLGETSWAPINTERVALEXTPROC)wglGetProcAddress("wglGetSwapIntervalEXT");
             if (wglSwapIntervalEXT && wglGetSwapIntervalEXT)
              {
                 // disable v-sync
                 wglSwapIntervalEXT(0);
                  std::cout << "Video card supports WGL_EXT_swap_control." << std::endl;
              }
          }
      #else // for linux, do not need to get function pointers, it is up-to-date
         if (glInfo.isExtensionSupported("GL_ARB_pixel_buffer_object"))
          {
              pboSupported = pboUsed = true;
              std::cout << "Video card supports GL_ARB_pixel_buffer_object." << std::endl;
          }
         else
          {
              pboSupported = pboUsed = false;
              std::cout << "Video card does NOT support GL_ARB_pixel_buffer_object." << std::endl;
          }
      #endif
         if (pboSupported)
          {
             // create 2 pixel buffer objects, you need to delete them when program exits.
     		//（创建两个 PBO）
             // glBufferDataARB with NULL pointer reserves only memory space.
             glGenBuffersARB(PBO_COUNT, pboIds);
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[0]);
             glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, DATA_SIZE, 0, GL_STREAM_READ_ARB);
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[1]);
             glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, DATA_SIZE, 0, GL_STREAM_READ_ARB);
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
          }
         // start timer, the elapsed time will be used for updateVertices()
          timer.start();
         // the last GLUT call (LOOP)
         // window will be shown and display callback is triggered by events
         // NOTE: this call never return main().
         glutMainLoop(); /* Start GLUT event-processing loop */
         return 0;
      }
  
 

      void displayCB()
      {
         static int shift = 0;
         static int index = 0;
         int nextIndex = 0;                  // pbo index used for next frame
         // brightness shift amount
          shift = ++shift % 200;
         // increment current index first then get the next index（增加当前的index，再获得 nextIndex）
         // "index" is used to read pixels from a framebuffer to a PBO（从 FB 中读取像素到 index 指定的 PBO 中）
         // "nextIndex" is used to process pixels in the other PBO（nextIndex 指定 PBO 中待处理的像素[先前从 FB 中读取]）
          index = (index + 1) % 2;	// 两个 PBO 交替
          nextIndex = (index + 1) % 2;
         // set the framebuffer to read（设置当前读取的 FB）
         glReadBuffer(GL_FRONT);
         if (pboUsed) // with PBO
          {
             // read framebuffer ///
              t1.start();  // 启动计时器，计算读取 FB 到 PBO 的时间
             // copy pixels from framebuffer to PBO （从 FB 中拷贝像素到 index指定的 PBO 中）
             // Use offset instead of ponter.（注意glReadPixels最后一个参数不是指针，而是偏移量）
             // OpenGL should perform asynch DMA transfer, so glReadPixels() will return immediately.
     		//（OpenGL 将执行一个异步的DMA[Direcct Memory Access]，所以 glReadPixels方法会立刻返回，不会阻塞CPU时间）
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[index]);
             glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, 0);
             // measure the time reading framebuffer
              t1.stop();
              readTime = t1.getElapsedTimeInMilliSec();
             ///
             // process pixel data /
              t1.start(); // 启动计时器，计算处理 PBO中像素的时间
             // map the PBO that contain framebuffer pixels before processing it
     		//（映射存储着先前 FB 像素的 PBO ，便于处理像素）
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[nextIndex]);
     		// 后续的 add() 并没有改变 PBO 中的像素值，计算的结果保存在 colorBuffer 中，所以标记为只读
              GLubyte *src = (GLubyte *)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
             if (src)
              {
                 // change brightness
                 add(src, SCREEN_WIDTH, SCREEN_HEIGHT, shift, colorBuffer);
                 glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB);     // release pointer to the mapped buffer
              }
             // measure the time processing the pixels of PBO
              t1.stop();
              processTime = t1.getElapsedTimeInMilliSec();
             ///
             glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
          }
         else        // without PBO
          {
             // read framebuffer ///
              t1.start();
     		// 不使用 PBO 的情况下，最后一个参数就是保存读取数据的指针
             glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, colorBuffer);
             // measure the time reading framebuffer
              t1.stop();
              readTime = t1.getElapsedTimeInMilliSec();
             ///
             // covert to greyscale 
              t1.start();
             // change brightness
             add(colorBuffer, SCREEN_WIDTH, SCREEN_HEIGHT, shift, colorBuffer);
             // measure the time reading framebuffer
              t1.stop();
              processTime = t1.getElapsedTimeInMilliSec();
             ///
          }
         // render to the framebuffer //
         glDrawBuffer(GL_BACK);	// 设定绘制在后缓冲区
         toPerspective(); // set to perspective on the left side of the window（当前窗口左侧的投影矩阵）
         // clear buffer
         glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
         // tramsform camera
         glTranslatef(0, 0, -cameraDistance);
         glRotatef(cameraAngleX, 1, 0, 0);   // pitch
         glRotatef(cameraAngleY, 0, 1, 0);   // heading
         // draw a cube（在左侧绘制普通的立方体）
         glPushMatrix();
         draw();
         glPopMatrix();
         // draw the read color buffer to the right side of the window
     	//（在右侧绘制之前处理的 colorBuffer）
         toOrtho();      // set to orthographic on the right side of the window（窗口右侧的正交矩阵[想象为把左侧的图处理一下直接贴上去]）
         glRasterPos2i(0, 0);	// 设置字体光栅的位置
         glDrawPixels(SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, colorBuffer);
         // draw info messages
         showInfo();
         printTransferRate();
         glutSwapBuffers();// 切换前后缓冲区
      }
  
 

程序的执行对比情况：

 可见通过 PBO的异步read-back技术，FPS 还是有明显提高的（我的显卡是 GeForce GTX 970，CPU 是 i7-6700 HQ，主频为2.6GHz）。

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原文链接：panda1234lee.blog.csdn.net/article/details/53270443