OpenGL Programming Guide

Chapter 10The Framebuffer

Testing and Operating on Fragments

Scissor Test

You can define a rectangular portion of your window and restrict drawing to take place within it by using the glScissor() command. 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. 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 to glEnable() and glDisable(). By default, the rectangle matches the size of the window and scissoring is disabled.

The scissor test is just a version of a stencil test using a rectangular region of the screen. It's fairly easy to create a blindingly fast hardware implementation of scissoring, while a given system might be much slower at stenciling - perhaps because the stenciling is performed in software.

Advanced

As an advanced use of scissoring, you might use it to perform nonlinear projection. First, divide the window into a regular grid of subregions, specifying viewport and scissor parameters that limit rendering to one region at a time. Then project the entire scene to each region using a different projection matrix.

You can use GL_SCISSOR_TEST with glIsEnabled() and GL_SCISSOR_BOX with glGetIntegerv() to determine whether scissoring is enabled and to obtain the values that define the scissor rectangle.

OpenGL Programming Guide