当glReadPixels为空时,如何生成屏幕截图？

Question

我有一个使用OpenGL在窗口中运行的程序(带有freeglut 2.8.1的VS2012).基本上在每一个步骤(通过glutPostRedisplay从我的glutIdleFunc钩子调用运行)我调用我自己的draw函数,然后调用glFlush以显示结果.然后我调用我自己的screenShot函数,它使用glReadPixels函数将像素转储到tga文件.

此设置的问题是当窗口最小化时文件为空.也就是说,输出glReadPixels是空的; 我怎么能避免这个？

这是screenShot我正在使用的功能的副本(我不是版权所有者):

//////////////////////////////////////////////////
// Grab the OpenGL screen and save it as a .tga //
// Copyright (C) Marius Andra 2001              //
// http://cone3d.gz.ee  EMAIL: cone3d@hot.ee    //
//////////////////////////////////////////////////
// (modified by me a little)
int screenShot(int const num)
{
    typedef unsigned char uchar;
    // we will store the image data here
    uchar *pixels;
    // the thingy we use to write files
    FILE * shot;
    // we get the width/height of the screen into this array
    int screenStats[4];

    // get the width/height of the window
    glGetIntegerv(GL_VIEWPORT, screenStats);

    // generate an array large enough to hold the pixel data 
    // (width*height*bytesPerPixel)
    pixels = new unsigned char[screenStats[2]*screenStats[3]*3];
    // read in the pixel data, TGA's pixels are BGR aligned
    glReadPixels(0, 0, screenStats[2], screenStats[3], 0x80E0, 
    GL_UNSIGNED_BYTE, pixels);

    // open the file for writing. If unsucessful, return 1
    std::string filename = kScreenShotFileNamePrefix + Function::Num2Str(num) + ".tga";

    shot=fopen(filename.c_str(), "wb");

    if (shot == NULL)
        return 1;

    // this is the tga header it must be in the beginning of 
    // every (uncompressed) .tga
    uchar TGAheader[12]={0,0,2,0,0,0,0,0,0,0,0,0};
    // the header that is used to get the dimensions of the .tga
    // header[1]*256+header[0] - width
    // header[3]*256+header[2] - height
    // header[4] - bits per pixel
    // header[5] - ?
    uchar header[6]={((int)(screenStats[2]%256)),
    ((int)(screenStats[2]/256)),
    ((int)(screenStats[3]%256)),
    ((int)(screenStats[3]/256)),24,0};

    // write out the TGA header
    fwrite(TGAheader, sizeof(uchar), 12, shot);
    // write out the header
    fwrite(header, sizeof(uchar), 6, shot);
    // write the pixels
    fwrite(pixels, sizeof(uchar), 
    screenStats[2]*screenStats[3]*3, shot);

    // close the file
    fclose(shot);
    // free the memory
    delete [] pixels;

    // return success
    return 0;
}

那么无论Windows是否决定在显示器上实际显示内容,我如何将屏幕截图打印到TGA文件？

注意:因为我试图保持模拟进度的可视化记录,所以我需要打印每一帧,无论它是否被渲染.我意识到最后的陈述有点矛盾,因为我需要渲染帧以产生screengrab.改写; 我需要glReadPixels(或一些替代功能)在每一步产生我的程序的更新状态,以便我可以将它打印到文件,无论窗口是否会选择显示它.

Answer 1

听起来你正在与像素所有权问题发生冲突.

渲染到FBO并用于glReadPixels()从中取出图像而不是前缓冲区.