glViewport — set the viewport
Specify the lower left corner of the viewport rectangle, in pixels. The initial value is (0,0).
Specify the width and height
of the viewport.
When a GL context is first attached to a window,
height are set to the dimensions of that
glViewport specifies the affine transformation of
normalized device coordinates to window coordinates.
be normalized device coordinates.
Then the window coordinates
are computed as follows:
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
height is negative.
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