How can I generate a screenshot when glReadPixels is empty?

Question

I have a program which runs in a window using OpenGL (VS2012 with freeglut 2.8.1). Basically at every time step (run via a call to glutPostRedisplay from my glutIdleFunc hook) I call my own draw function followed by a call to glFlush to display the result. Then I call my own screenShot function which uses the glReadPixels function to dump the pixels to a tga file.

The problem with this setup is that the files are empty when the window gets minimised. That is to say, the output from glReadPixels is empty; How can I avoid this?

Here is a copy of the screenShot function I am using (I am not the copyright holder):

//////////////////////////////////////////////////
// Grab the OpenGL screen and save it as a .tga //
// Copyright (C) Marius Andra 2001              //
// http://cone3d.gz.ee  EMAIL: [email protected]    //
//////////////////////////////////////////////////
// (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;
}

So how can I print the screenshot to a TGA file regardless of whether Windows decides to actually display the content on the monitor?

Note: Because I am trying to keep a visual record of the progress of a simulation, I need to print every frame, regardless of whether it is being rendered. I realise that last statement is a bit of a contradiction, since I need to render the frame in order to produce the screengrab. To rephrase; I need glReadPixels (or some alternative function) to produce the updated state of my program at every step so that I can print it to a file, regardless of whether windows will choose to display it.

Answer 1

Sounds like you're running afoul of the pixel ownership problem.

Render to a FBO and use glReadPixels() to slurp images out of that instead of the front buffer.