glScissor - define the scissor box

glScissor
	glScissor(x, y, width, height) -> None

void glScissor( GLint	x,
                GLint	y,
                GLsizei width,
                GLsizei height )

x, y Specify the lower left corner of	the scissor box.
     Initially (0, 0).

width, height
     Specify the width and height of the scissor box.	 When
     a GL context is first attached to a window, width and
     height are set to the dimensions	of that	window.

glScissor defines a rectangle, called	the scissor box, in
window coordinates.  The first two arguments,	x and y,
specify the lower left corner	of the box.  width and height
specify the width and	height of the box.

To enable and	disable	the scissor test, call glEnable	and
glDisable with argument GL_SCISSOR_TEST. The test is
initially disabled. While the	test is	enabled, only pixels
that lie within the scissor box can be modified by drawing
commands.  Window coordinates	have integer values at the
shared corners of frame buffer pixels.  glScissor(0,0,1,1)
allows modification of only the lower	left pixel in the
window, and glScissor(0,0,0,0) doesn't allow modification of
any pixels in	the window.

When the scissor test	is disabled, it	is as though the
scissor box includes the entire window.

GL_INVALID_VALUE is generated	if either width	or height is
negative.

GL_INVALID_OPERATION is generated if glScissor is executed
between the execution	of glBegin and the corresponding
execution of glEnd.

glGet	with argument GL_SCISSOR_BOX
glIsEnabled with argument GL_SCISSOR_TEST

glEnable, glViewport