OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Scissor Test

The first additional test you can enable to control fragment visibility is the scissor test. The scissor box is a rectangular portion of your window and restricts all drawing to its region. You specify the scissor box using the glScissor() command, and enable the test by specifying GL_SCISSOR_TEST with glEnable(). If a fragment lies inside the rectangle, it passes the scissor test.

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

Sets the location and size of the scissor rectangle (also known as the scissor box). The parameters define the lower left corner (x, y) and the width and height of the rectangle. Pixels that lie inside the rectangle pass the scissor test. Scissoring is enabled and disabled by passing GL_SCISSOR_TEST ...