像素缓冲区对象（PBO）的异步Read-Back 源码解析 _缓冲区

知识为进步之母,而进步又为富强之源泉。这篇文章主要讲述像素缓冲区对象（PBO）的异步Read-Back 源码解析相关的知识，希望能为你提供帮助。

原理示意图如下：

文章图片

【像素缓冲区对象（PBO）的异步Read-Back 源码解析】

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;

省略掉中间其他一些无关代码，剩下关键方法：

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 = https://www.songbingjia.com/android/(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