|x, y||Specify the lower left corner of the viewport rectangle, in pixels. The initial value is (0,0).|
|width, height||Specify the width and height of the viewport. When a GL context is first attached to a window, width and height are set to the dimensions of that window.|
glViewport specifies the affine transformation of x and y from normalized device coordinates to window coordinates. Let (xnd, ynd) be normalized device coordinates. Then the window coordinates (xw, yw) are computed as follows:
xw = (xnd+1)(width / 2) + x
yw = (ynd+1)(height / 2) + y
Viewport width and height are silently clamped
to a range that depends on the implementation.
To query this range, call glGet with argument
GL_INVALID_VALUE is generated if either width or height is negative.
GL_INVALID_OPERATION is generated if glViewport is executed between the execution of glBegin and the corresponding execution of glEnd.
glGet with argument GL_VIEWPORT
glGet with argument GL_MAX_VIEWPORT_DIMS