glScissor — define the scissor box
Specify the lower left corner of the scissor box. Initially (0, 0).
Specify the width and height of the scissor box. When a GL context is first attached to a window,
height are set to the dimensions of that window.
glScissor defines a rectangle, called the scissor box, in window coordinates. The first two arguments,
y, specify the lower left corner of the box.
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.
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