最简单的PBO（异步Read-Back）Demo 完整代码示例

本Demo 主要是参考了 http://www.songho.ca/opengl/gl_pbo.html 的 pboPack.zip，

但是我是基于FreeGlut 、Glew 库和 Soil 库（Soil库是负责从本地加载纹理），并删减了一些和主题不相关的代码。

原理示意图：

完整代码及详细解释如下：

#include <time.h>#include <gl/glew.h>#include <gl/freeglut.h>#include<soil.h>#define WIN_W 640#define WIN_H 480#define WIN_X 0// ☆#define WIN_Y 0#define CAPTURE 1//#define IO 2// PBO开关#define PBO_TEST 1GLuint  texName;#if PBO_TESTGLuint  pboName[2];#endifvoid add(unsigned char *src, int width, int height, int shift, unsigned char *dst);// 自定义的方法bool saveBMP(const char *lpFileName){    GLint viewport[4];    glGetIntegerv(GL_VIEWPORT, viewport);    int width = viewport[2];    int height = viewport[3];    // 设置解包像素的对齐格式——Word对齐(4字节)    glPixelStorei(GL_PACK_ALIGNMENT, 4);    // 计算对齐后的真实宽度    int nAlignWidth = (width * 24 + 31) / 32;#if PBO_TEST    static int index = 0;    index = (index + 1) % 2;    int next_index = (index + 1) % 2;    // PBO    glReadBuffer(GL_FRONT);    glBindBuffer(GL_PIXEL_PACK_BUFFER, pboName[index]);    glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, 0);    glBindBuffer(GL_PIXEL_PACK_BUFFER, pboName[next_index]);    unsigned char *src = (unsigned char *)glMapBufferRange(GL_PIXEL_PACK_BUFFER, 0, nAlignWidth * height * 4, GL_MAP_READ_BIT);    unsigned char *pdata = NULL;    if (src)    {        pdata = new unsigned char[nAlignWidth * height * 4];        memcpy(pdata, src, nAlignWidth * height * 4);        glUnmapBuffer(GL_PIXEL_PACK_BUFFER);    }    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);#else    // 分配缓冲区    unsigned char *pdata = new unsigned char[nAlignWidth * height * 4];    memset(pdata, 0, nAlignWidth * height * 4);    //从当前绑定的 frame buffer 读取 pixels    glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, pdata);#endif    // ============================================    if (!pdata)    {        printf("pdata is NULL! \n");        return FALSE;    }unsigned char *dst = new unsigned char[nAlignWidth * height * 4];// 耗时操作add(pdata, nAlignWidth, height, 0, dst);#ifdef IO    // 以下就是为了 BMP 格式 做准备    //由RGB变BGR    for (int i = 0; i < width * height * 3; i += 3)    {        unsigned char tmpRGB;        tmpRGB = pdata[i];        pdata[i] = pdata[i + 2];        pdata[i + 2] = tmpRGB;    }    // 设置 BMP 的文件头    BITMAPFILEHEADER Header;    BITMAPINFOHEADER HeaderInfo;    Header.bfType = 0x4D42;    Header.bfReserved1 = 0;    Header.bfReserved2 = 0;    Header.bfOffBits = (DWORD)(                           sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER));    Header.bfSize = (DWORD)(                        sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)                        + nAlignWidth * height * 4);    HeaderInfo.biSize = sizeof(BITMAPINFOHEADER);    HeaderInfo.biWidth = width;    HeaderInfo.biHeight = height;    HeaderInfo.biPlanes = 1;    HeaderInfo.biBitCount = 24;    HeaderInfo.biCompression = 0;    HeaderInfo.biSizeImage = 4 * nAlignWidth * height;    HeaderInfo.biXPelsPerMeter = 0;    HeaderInfo.biYPelsPerMeter = 0;    HeaderInfo.biClrUsed = 0;    HeaderInfo.biClrImportant = 0;    // 写入字节数据    FILE *pfile;    if (!(pfile = fopen(lpFileName, "wb+")))    {        printf("保存图像失败!\n");        return FALSE;    }    fwrite(&Header, 1, sizeof(BITMAPFILEHEADER), pfile);    fwrite(&HeaderInfo, 1, sizeof(BITMAPINFOHEADER), pfile);    fwrite(pdata, 1, HeaderInfo.biSizeImage, pfile);    fclose(pfile);#endif#ifndef PBO_TEST    delete[] pdata;#else    pdata = NULL;#endifdelete[] dst;    return TRUE;}GLuint loadGLTexture(const char *file_path){    // 切记要初始化 OpenGL Context 和 GLEW ！    /* load an image file directly as a new OpenGL texture */    GLuint  tex_2d = SOIL_load_OGL_texture                     (                         file_path,                         SOIL_LOAD_AUTO,                         SOIL_CREATE_NEW_ID,                         SOIL_FLAG_MIPMAPS | SOIL_FLAG_INVERT_Y | SOIL_FLAG_NTSC_SAFE_RGB | SOIL_FLAG_COMPRESS_TO_DXT                     );    /* check for an error during the load process */    if (0 == tex_2d)    {        printf("SOIL loading error: '%s'\n", SOIL_last_result());    }    return tex_2d;}void init(void){    // ------------------------------------    GLenum err = glewInit();    if (GLEW_OK != err)    {        /* Problem: glewInit failed, something is seriously wrong. */        fprintf(stderr, "Error: %s\n", glewGetErrorString(err));    }    fprintf(stdout, "Status: Using GLEW %s\n", glewGetString(GLEW_VERSION));    // ------------------------------------    glClearColor(0.0, 0.0, 0.0, 0.0);    glShadeModel(GL_FLAT);    glEnable(GL_DEPTH_TEST);    // ------------------------------------    texName = loadGLTexture("../micky.png");    // 按字节对齐    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);    // 区别1    //glGenTextures(1, &texName);    glBindTexture(GL_TEXTURE_2D, texName);    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);    // 区别2    //glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, imageRec->sizeX, imageRec->sizeX,    //0, GL_RGB, GL_UNSIGNED_BYTE, imageRec->data);    // ☆#if PBO_TEST    glGenBuffers(2, pboName);    int nAlignWidth = (WIN_W * 24 + 31) / 32;    glBindBuffer(GL_PIXEL_PACK_BUFFER, pboName[0]);    glBufferData(GL_PIXEL_PACK_BUFFER, nAlignWidth * WIN_H * 4, NULL, GL_DYNAMIC_READ);    glBindBuffer(GL_PIXEL_PACK_BUFFER, pboName[1]);    glBufferData(GL_PIXEL_PACK_BUFFER, nAlignWidth * WIN_H * 4, NULL, GL_DYNAMIC_READ);    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);#endif}// 居然要 200+msvoid saveScreenShot(const char *img_path, int x, int y, int width, int height){    int save_result = SOIL_save_screenshot                      (                          img_path,                          SOIL_SAVE_TYPE_BMP,                          x, y, width, height                      );}void display(void){    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glEnable(GL_TEXTURE_2D);    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);    glBindTexture(GL_TEXTURE_2D, texName);    glBegin(GL_QUADS);    // 左面    glTexCoord2f(0.0, 0.0);    glVertex3f(-2.0, -1.0, 0.0);    glTexCoord2f(0.0, 1.0);    glVertex3f(-2.0, 1.0, 0.0);    glTexCoord2f(1.0, 1.0);    glVertex3f(0.0, 1.0, 0.0);    glTexCoord2f(1.0, 0.0);    glVertex3f(0.0, -1.0, 0.0);    // 右面    glTexCoord2f(0.0, 0.0);    glVertex3f(1.0, -1.0, 0.0);    glTexCoord2f(0.0, 1.0);    glVertex3f(1.0, 1.0, 0.0);    glTexCoord2f(1.0, 1.0);    glVertex3f(2.41421, 1.0, -1.41421);    glTexCoord2f(1.0, 0.0);    glVertex3f(2.41421, -1.0, -1.41421);    glEnd();#if CAPTUREstatic int count = 1;    // 根据时间来命名位图    clock_t time = clock();    char img_path[100];    sprintf(img_path, "../bmp/%ld.bmp", time);    saveBMP(img_path);    //saveScreenShot(img_path, WIN_X, WIN_Y, WIN_W, WIN_H);// 太慢    clock_t elapse = clock() - time;    //printf("elapse = %dms\n", elapse);static long sum = 0.;sum += elapse;float avg = 1. * sum / count;count++;printf("average = %fms\n", avg);# endif    glFlush();// 与单缓冲对应    // glDisable(GL_TEXTURE_2D);    glBindTexture(GL_TEXTURE_2D, 0);}void reshape(int w, int h){    glViewport(0, 0, (GLsizei)w, (GLsizei)h);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective(60.0, (GLfloat)w / (GLfloat)h, 1.0, 30.0);// ☆    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    glTranslatef(0.0, 0.0, -3.6);}void processKeyboard(unsigned char key, int x, int y){    // http://blog.sina.com.cn/s/blog_4a9aa55c0100p36b.html    if (key == 27)    {        exit(0);    }    else if (key == 'r')    {        int mod = glutGetModifiers();        if (mod == GLUT_ACTIVE_ALT)            printf("Alt + r ! \n");        else if(mod == GLUT_ACTIVE_CTRL)            printf("Ctrl+ r ! \n");        else if (mod == GLUT_ACTIVE_SHIFT)            printf("Shift+ r ! \n");    }    else if (key == GLUT_KEY_F1)    {        int mod = glutGetModifiers();        if (mod == (GLUT_ACTIVE_CTRL | GLUT_ACTIVE_ALT))        {            printf("Ctrl + Shift+ F1 ! \n");        }    }}// 测试：耗时操作void add(unsigned char *src, int width, int height, int shift, unsigned char *dst){    if (!src || !dst)        return;    int value;    for (int i = 0; i < height; ++i)    {        for (int j = 0; j < width; ++j)        {//printf("(%d, %d)\n", i, j);            value = *src + shift;            if (value > 255) *dst = (unsigned char)255;            else            *dst = (unsigned char)value;            ++src;            ++dst;            value = *src + shift;            if (value > 255) *dst = (unsigned char)255;            else            *dst = (unsigned char)value;            ++src;            ++dst;            value = *src + shift;            if (value > 255) *dst = (unsigned char)255;            else            *dst = (unsigned char)value;            ++src;            ++dst;            ++src;    // skip alpha            ++dst;        }    }}int main(int argc, char **argv){    glutInit(&argc, argv);    // 单缓冲    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);    glutInitWindowSize(WIN_W, WIN_H);    glutInitWindowPosition(WIN_X, WIN_Y);    glutCreateWindow(argv[0]);    init();    glutDisplayFunc(display);    glutIdleFunc(display);    glutKeyboardFunc(&processKeyboard);    glutReshapeFunc(reshape);    glutMainLoop();    return 0;}

未使用 PBO 的平均耗时：

使用了 PBO 的平均耗时：

显然，开启PBO能够一定程度上提高程序性能。

相关阅读：

OpenGL深入探索——像素缓冲区对象 （PBO）

像素缓冲区对象（PBO) 的Streaming-Texture上传 源码解析

像素缓冲区对象（PBO）的异步Read-Back 源码解析