像素缓冲区对象（PBO）的异步Read-Back 源码解析
【摘要】
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(exitC...
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：
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int main(int argc, char **argv)
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{
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initSharedMem();
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// register exit callback
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atexit(exitCB);
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// init GLUT and GL
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initGLUT(argc, argv);
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initGL();
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// get OpenGL info
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glInfo glInfo;
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glInfo.getInfo();
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glInfo.printSelf();
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#ifdef _WIN32
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// 检查视频显卡是否支持 PBO
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if (glInfo.isExtensionSupported("GL_ARB_pixel_buffer_object"))
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{
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// get pointers to GL functions
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glGenBuffersARB = (PFNGLGENBUFFERSARBPROC)wglGetProcAddress("glGenBuffersARB");
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glBindBufferARB = (PFNGLBINDBUFFERARBPROC)wglGetProcAddress("glBindBufferARB");
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glBufferDataARB = (PFNGLBUFFERDATAARBPROC)wglGetProcAddress("glBufferDataARB");
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glBufferSubDataARB = (PFNGLBUFFERSUBDATAARBPROC)wglGetProcAddress("glBufferSubDataARB");
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glDeleteBuffersARB = (PFNGLDELETEBUFFERSARBPROC)wglGetProcAddress("glDeleteBuffersARB");
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glGetBufferParameterivARB = (PFNGLGETBUFFERPARAMETERIVARBPROC)wglGetProcAddress("glGetBufferParameterivARB");
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glMapBufferARB = (PFNGLMAPBUFFERARBPROC)wglGetProcAddress("glMapBufferARB");
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glUnmapBufferARB = (PFNGLUNMAPBUFFERARBPROC)wglGetProcAddress("glUnmapBufferARB");
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// check once again PBO extension
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if (glGenBuffersARB && glBindBufferARB && glBufferDataARB && glBufferSubDataARB &&
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glMapBufferARB && glUnmapBufferARB && glDeleteBuffersARB && glGetBufferParameterivARB)
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{
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pboSupported = pboUsed = true;
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std::cout << "Video card supports GL_ARB_pixel_buffer_object." << std::endl;
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}
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else
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{
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pboSupported = pboUsed = false;
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std::cout << "Video card does NOT support GL_ARB_pixel_buffer_object." << std::endl;
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}
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}
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// check EXT_swap_control is supported
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if (glInfo.isExtensionSupported("WGL_EXT_swap_control"))
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{
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// get pointers to WGL functions
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wglSwapIntervalEXT = (PFNWGLSWAPINTERVALEXTPROC)wglGetProcAddress("wglSwapIntervalEXT");
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wglGetSwapIntervalEXT = (PFNWGLGETSWAPINTERVALEXTPROC)wglGetProcAddress("wglGetSwapIntervalEXT");
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if (wglSwapIntervalEXT && wglGetSwapIntervalEXT)
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{
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// disable v-sync
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wglSwapIntervalEXT(0);
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std::cout << "Video card supports WGL_EXT_swap_control." << std::endl;
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}
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}
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#else // for linux, do not need to get function pointers, it is up-to-date
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if (glInfo.isExtensionSupported("GL_ARB_pixel_buffer_object"))
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{
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pboSupported = pboUsed = true;
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std::cout << "Video card supports GL_ARB_pixel_buffer_object." << std::endl;
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}
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else
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{
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pboSupported = pboUsed = false;
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std::cout << "Video card does NOT support GL_ARB_pixel_buffer_object." << std::endl;
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}
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#endif
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if (pboSupported)
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{
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// create 2 pixel buffer objects, you need to delete them when program exits.
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//（创建两个 PBO）
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// glBufferDataARB with NULL pointer reserves only memory space.
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glGenBuffersARB(PBO_COUNT, pboIds);
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[0]);
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glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, DATA_SIZE, 0, GL_STREAM_READ_ARB);
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[1]);
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glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, DATA_SIZE, 0, GL_STREAM_READ_ARB);
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
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}
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// start timer, the elapsed time will be used for updateVertices()
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timer.start();
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// the last GLUT call (LOOP)
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// window will be shown and display callback is triggered by events
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// NOTE: this call never return main().
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glutMainLoop(); /* Start GLUT event-processing loop */
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return 0;
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}
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void displayCB()
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{
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static int shift = 0;
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static int index = 0;
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int nextIndex = 0; // pbo index used for next frame
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// brightness shift amount
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shift = ++shift % 200;
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// increment current index first then get the next index（增加当前的index，再获得 nextIndex）
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// "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）
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// "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]）
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index = (index + 1) % 2; // 两个 PBO 交替
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nextIndex = (index + 1) % 2;
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// set the framebuffer to read（设置当前读取的 FB）
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glReadBuffer(GL_FRONT);
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if (pboUsed) // with PBO
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{
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// read framebuffer ///
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t1.start(); // 启动计时器，计算读取 FB 到 PBO 的时间
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// copy pixels from framebuffer to PBO （从 FB 中拷贝像素到 index指定的 PBO 中）
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// Use offset instead of ponter.（注意glReadPixels最后一个参数不是指针，而是偏移量）
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// OpenGL should perform asynch DMA transfer, so glReadPixels() will return immediately.
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//（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）
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[index]);
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glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, 0);
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// measure the time reading framebuffer
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t1.stop();
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readTime = t1.getElapsedTimeInMilliSec();
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///
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// process pixel data /
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t1.start(); // 启动计时器，计算处理 PBO中像素的时间
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// map the PBO that contain framebuffer pixels before processing it
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//（映射存储着先前 FB 像素的 PBO ，便于处理像素）
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboIds[nextIndex]);
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// 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
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GLubyte *src = (GLubyte *)glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
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if (src)
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{
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// change brightness
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add(src, SCREEN_WIDTH, SCREEN_HEIGHT, shift, colorBuffer);
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glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB); // release pointer to the mapped buffer
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}
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// measure the time processing the pixels of PBO
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t1.stop();
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processTime = t1.getElapsedTimeInMilliSec();
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///
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glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
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}
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else // without PBO
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{
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// read framebuffer ///
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t1.start();
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// 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
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glReadPixels(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, colorBuffer);
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// measure the time reading framebuffer
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t1.stop();
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readTime = t1.getElapsedTimeInMilliSec();
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///
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// covert to greyscale
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t1.start();
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// change brightness
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add(colorBuffer, SCREEN_WIDTH, SCREEN_HEIGHT, shift, colorBuffer);
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// measure the time reading framebuffer
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t1.stop();
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processTime = t1.getElapsedTimeInMilliSec();
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///
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}
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// render to the framebuffer //
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glDrawBuffer(GL_BACK); // 设定绘制在后缓冲区
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toPerspective(); // set to perspective on the left side of the window（当前窗口左侧的投影矩阵）
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// clear buffer
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glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
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// tramsform camera
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glTranslatef(0, 0, -cameraDistance);
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glRotatef(cameraAngleX, 1, 0, 0); // pitch
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glRotatef(cameraAngleY, 0, 1, 0); // heading
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// draw a cube（在左侧绘制普通的立方体）
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glPushMatrix();
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draw();
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glPopMatrix();
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// draw the read color buffer to the right side of the window
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//（在右侧绘制之前处理的 colorBuffer）
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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]）
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glRasterPos2i(0, 0); // 设置字体光栅的位置
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glDrawPixels(SCREEN_WIDTH, SCREEN_HEIGHT, PIXEL_FORMAT, GL_UNSIGNED_BYTE, colorBuffer);
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// draw info messages
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showInfo();
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printTransferRate();
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glutSwapBuffers();// 切换前后缓冲区
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}
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）。
文章来源: panda1234lee.blog.csdn.net，作者：panda1234lee，版权归原作者所有，如需转载，请联系作者。
原文链接：panda1234lee.blog.csdn.net/article/details/53270443
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