Take screenshot with openGL and save it as png

Question

I'm trying to take a screenshot of full screen and save it as a png. I found a code here and modified it a bit. For the screenshot I use openGL and Glut and for the saving in png the gd library for c. All I'm getting is a black png and I can't figure out why. I searched in stackoverflow and found some posts, but unfortunately they didn't help. One of them was to use glReadBuffer( GL_FRONT); instead of glReadBuffer(GL_BACK); I tryed with both of them with no success. Here is my code:

int SVimage2file(char *filename){
    int width = glutGet(GLUT_SCREEN_WIDTH);
    int height = glutGet( GLUT_SCREEN_HEIGHT);
    FILE *png;
    GLubyte *OpenGLimage, *p;
    gdImagePtr image;
    unsigned int r, g, b;
    int i,j,rgb;

    png = fopen(filename, "wb");

    if (png == NULL) {
        printf("*** warning:  unable to write to %s\n",filename);
        return 1;
    }

    OpenGLimage = (GLubyte *) malloc(width * height * sizeof(GLubyte) * 3);
    if(OpenGLimage == NULL){
        printf("error allocating image:%s\n",filename);
        exit(1);
    }

    printf("Saving to: %s .\n",filename);
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glReadBuffer( GL_FRONT);
    glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, OpenGLimage);
    p = OpenGLimage;
    image = gdImageCreateTrueColor(width,height);

    for (i = height-1 ; i>=0; i--) {
        for(j=0;j<width;j++){
                r=*p++; g=*p++; b=*p++;
                rgb = (r<<16)|(g<<8)|b;
                //printf("the rgb color %d\n", rgb );
                gdImageSetPixel(image,j,i,rgb);
        }
    }

    gdImagePng(image,png);
    fclose(png);
    gdImageDestroy(image);
}

What is it that I'm missing?

Answer 1

You could use the devil image library and take a screeshot with:

void takeScreenshot(const char* screenshotFile)
{
    ILuint imageID = ilGenImage();
    ilBindImage(imageID);
    ilutGLScreen();
    ilEnable(IL_FILE_OVERWRITE);
    ilSaveImage(screenshotFile);
    ilDeleteImage(imageID);
    printf("Screenshot saved to: %s\n", screenshotFile);
}

takeScreenshot("screenshot.png");

Answer 2

If you don't reject to use C++ library, you should try PNGwriter! It write the picture pixel by pixel and their RGB values. Since the PNGwriter start form left-up corner while the glReadPixels() start from left-bottom, your code while like:

GLfloat* OpenGLimage = new GLfloat[nPixels];
glReadPixels(0.0, 0.0, width, height,GL_RGB, GL_FLOAT, OpenGLimage);
pngwriter PNG(width, height, 1.0, fileName);
size_t x = 1;   // start the top and leftmost point of the window
size_t y = 1;
double R, G, B;
for(size_t i=0; i<npixels; i++)
{
      switch(i%3) //the OpenGLimage array look like [R1, G1, B1, R2, G2, B2,...]
     {
           case 2:
                 B = (double) pixels[i]; break;
           case 1:
                 G = (double) pixels[i]; break;
           case 0:
                 R = (double) pixels[i];
                 PNG.plot(x, y, R, G, B);
                 if( x == width )
                 {
                       x=1;
                       y++;
                  }
                  else
                  { x++; }
                  break;
     }
}
PNG.close();

PS. I had also try libgd, but it seems only convert one image file (in the hard disk or in memory) to another format of image. But I think it still useful while you want to convert many PNG file to GIF format to create a GIF animate.