I read tutorial 6 in opengl-tutorial.org. Here's the link.

http://www.opengl-tutorial.org/begin...ard-and-mouse/

But I don't understand with this part:
Code :
horizontalAngle += mouseSpeed * deltaTime * float(1024/2 - xpos );
verticalAngle   += mouseSpeed * deltaTime * float( 768/2 - ypos );

Why is the vector becomes an increment of horizontal and vertical angle???