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OpenGL problem with simple screen capture to ppm

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hello, For some time I have implemented screen captures in my OpenGL programs using the following function (which a kind soul gave me in another forum)
int capture_to_ppm(void)
	int width, height, colorDepth, maxColorValue,y;
	unsigned char	*pixels;
	int fd;
	char sbuf[256]; /* for sprintf() */

	/* open output file: you can name it as you like */
	fd = open(picfile,O_CREAT|O_TRUNC|O_WRONLY,
	if(fd == -1) return -1;

	/* width & height of the window */
	width = glutGet(GLUT_WINDOW_WIDTH);
	height = glutGet(GLUT_WINDOW_HEIGHT);

	/* maxColorValue is 255 in most cases... */
	colorDepth = glutGet(GLUT_WINDOW_RED_SIZE);
	maxColorValue = (1 << colorDepth) - 1;

	/* allocate pixels[]: 3 is for RGB */
	pixels = malloc(3*width*height);
	if( !pixels ) return -2;

	/* get RGB values from the frame buffer into pixels[] */
	glReadBuffer(GL_FRONT); /* if you are using "double buffer" */

	/* write ppm file header */
	sprintf(sbuf,"P6 %d %d %d\n",width,height,maxColorValue);
	/* write ppm RGB data: we must invert upside down */
	for(y = height-1; y >= 0; --y) {
		write(fd, pixels+3*width*y, 3*width);

	return 0;

It has always worked very nicely, provided that my OpenGL program does not open subwindows. I can open any number of windows, and get a perfect window grab of one of them using a glutSetWindow() call. Now this same code is failing when I use it to grab a subwindow. I have a program that opens a main window with 3 subwindows inside. I need to grab one of the subwindows. I call the capture window this way:

where modelwin is the subwindow to be grabbed. The problem is I get a skewed image (the pixels' x coordinate is displaced from its expected position), and without colors. I have been trying to read about glReadPixels(), where I think the problem resides, but I still do not know how to correct it (I read something about setting GL_UNPACK_ALIGNMENT differently than default, but whatever I did had no effect). Could anyone give me an idea what I need to do? Thanks a million, mc61

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GL_UNPACK_ALIGNMENT is for transferring pixel data to OpenGL. Notice the word, "unpack".

For reading data from OpenGL to your application, GL_PACK_ALIGNMENT should be used. Try to set this state to 1 if it fixes your problem.

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Thank you, it worked!!!

Naively I thought I was "unpacking" the pixels, while I was actually packing them it seems [smile].

Since you have been so kind, I have one more related question, just to show how little I really know about OpenGL. I set this by placing glPixelStoref(GL_PACK_ALIGNMENT,1) inside my capture_to_ppm function. However, it seems to me that this should be set once, and not every time the screen capture is invoked. Where should it go?

Thanks again for your help,


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