blob detection in C++

I'm new at computer vision, but i need to made a little function in C++, who will detect a white paper sheet even if is something printed on him, and the retrieve the 4 edges coordinates what is what i really need so i can use those coordinates and cut another jpg file and use the cutted image as a opengl textures.
I dont know how to detect the paper.
Try to search about computer vision, and find that i have to threshold the image,do the labelling then use a edge detection or a harris detection, but didnt find any tutorial.
Can any one help me with this, or show me some tutorial who can help me?

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