Problem with glReadPixels

OpenGL OpenGL: Basic Coding
1

Hi guys

I trying to take a snapshot from the scene(which is basically a teapot) and then store it in a file on this format:
x y r g b

The program reads the pixels but the resulting picture is not coming right.

Do I have to call any functions before the glReadPixels function?
Also, If i want to take a picture of the whole scene which hight and width should I pass to the glReadPixels function: the viewport or the window ?


	int WIDTH=512;
	int HEIGHT=512;
	float imageData[WIDTH*HEIGHT*3] ; 
	glReadBuffer(GL_BACK);
	glReadPixels(0, 0, WIDTH, HEIGHT, GL_RGB, GL_FLOAT, imageData);
	int pixelNb = WIDTH*HEIGHT;
	for( int i = 0; (i< pixelNb) && readFrame; ++i )
	{
		if(imageData[ i ]!=1)
		myfile  << i%HEIGHT <<"	"<<i/HEIGHT<< "	"<< int(imageData[ i ] *255 )<< "	"<< int(imageData[ i + 1 ]*255 )<< "	"<<int(imageData[ i + 2 ]*255) << endl;
	
	}
2

“picture is not coming right”
Is it black ?
random noise ?
mixed colors from the teapot ?
… something else ?

width,height should be the viewport size.

RGB is not 4 byte aligned, look for pitfall “7. Watch Your Pixel Store Alignment” here :
http://www.opengl.org/resources/features/KilgardTechniques/oglpitfall/
(that page is old, but parts are still true)

3

the image comes as parallel lines in a shape of a circle.

4

This is the actual picture

This is the picture produced by the text file resulting from the program

5

It could also be a problem with your non-OpenGL textfile-to-image decoder.

6

yes, that’s some weird image output.
check the image debugger http://billbaxter.com/projects/imdebug/
make sure your image is at least copied correct to system memory.

7

Call glFinish() before glReadPixels. Be sure that packing alignment is correct (As Zbuffer said). In most cases, byte row alignment is used whereas the default one is word alignment:

glPixelStorei( GL_PACK_ALIGNMENT, 1 );