The viewport subroutine specifies, in window coordinates, the area of the window that displays an image. By default, when a window is opened on the screen, its viewport is set to cover the whole window. Its parameters (left, right, bottom, top) define a rectangular area on the window by specifying the left, right, bottom, and top coordinates. The portion of modeling space that the window, ortho, or perspective subroutine describes is mapped into the viewport. The syntax is as follows:
void viewport(Screencoord left, Screencoord right, Screencoord bottom, Screencoord top)