openGL之API学习（三十七）如何从FBO中读取颜色、深度信息

方法一、保存成图片

QImage* img=new QImage(WINDOW_WIDTH,WINDOW_HEIGHT,QImage::Format_ARGB32);
    uchar* tmpBIT = img->bits();


    //从颜色缓冲区中读取数据
    int tmpPixelSize = WINDOW_WIDTH*WINDOW_HEIGHT * 4;
    char* tmpPixelsBuffer = (char*)malloc(tmpPixelSize);
    glReadPixels(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT,GL_RGBA, GL_UNSIGNED_BYTE, tmpPixelsBuffer);
    for (int y = WINDOW_HEIGHT-1; y >=0 ; y--)
    {
        for (int x = 0; x < WINDOW_WIDTH; x++)
        {

            //蓝色
            tmpBIT[0] = tmpPixelsBuffer[(y*WINDOW_WIDTH + x) * 4 + 2];
            //绿色
            tmpBIT[1] = tmpPixelsBuffer[(y*WINDOW_WIDTH + x) * 4 + 1];
            //红色
            tmpBIT[2] = tmpPixelsBuffer[(y*WINDOW_WIDTH + x) * 4 + 0];
            tmpBIT[3] = 100;//不起作用
            tmpBIT += 4;
        }
    }

    //从深度缓冲区读取数据
    int tmpPixelSize = WINDOW_WIDTH*WINDOW_HEIGHT;
    float* tmpPixelsBuffer = new float[ tmpPixelSize ];
    //GLfloat* tmpPixelsBuffer = new GLfloat[ tmpPixelSize ];
    //glReadBuffer(GL_NONE);
    //glReadBuffer(GL_BACK);
    glReadPixels(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT,GL_DEPTH_COMPONENT, GL_FLOAT, tmpPixelsBuffer);

    GLenum aaa = glGetError();
    for (int y = WINDOW_HEIGHT-1; y >=0 ; y--)
    {
        for (int x = 0; x < WINDOW_WIDTH; x++)
        {
            float iii=tmpPixelsBuffer[(y*WINDOW_WIDTH + x)  + 0];
            iii*=255;
            //蓝色
            tmpBIT[0] = iii;
            //绿色
            tmpBIT[1] = iii;
            //红色
            tmpBIT[2] = iii;
            tmpBIT[3] = 255;//不起作用
            tmpBIT += 4;
        }
    }


    img->save("a.jpg");

方法二、输出到纹理

1、配置FBO，使颜色、深度输出到纹理

    glGenFramebuffers(1, &m_fbo);

    // Создаем буфер глубины
    glGenTextures(1, &m_shadowMap);
    glBindTexture(GL_TEXTURE_2D, m_shadowMap);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, WindowWidth, WindowHeight, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, m_fbo);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D,
                           m_shadowMap, 0);


    //增加一个颜色缓冲区
    glGenTextures(1, &m_shadowColor);
    glBindTexture(GL_TEXTURE_2D, m_shadowColor);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, WindowWidth, WindowHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, m_fbo);
    glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
                           m_shadowColor, 0);
    glDrawBuffer(GL_COLOR_ATTACHMENT0);



    // 不允许读写颜色缓冲区，也会禁用片元着色器
    //glDrawBuffer(GL_NONE);
    //glReadBuffer(GL_NONE);

    GLenum Status = glCheckFramebufferStatus(GL_FRAMEBUFFER);

    if (Status != GL_FRAMEBUFFER_COMPLETE){
        printf("FB error, status: 0x%x\n", Status);
        return false;
    }
    return true;

2、在默认帧缓冲区（或其他帧缓冲区）使用纹理

    glActiveTexture(TextureUnit);
    glBindTexture(GL_TEXTURE_2D, m_shadowMap);