3D graphics eBook - Course Materials Repository

Stencil buffer 195
Stencil buffer
A stencil buffer is an extra buffer, in addition to the
color buffer (pixel buffer) and depth buffer
(z-buffering) found on modern computer graphics
hardware. The buffer is per pixel, and works on integer
values, usually with a depth of one byte per pixel. The
depth buffer and stencil buffer often share the same
area in the RAM of the graphics hardware.
In the simplest case, the stencil buffer is used to limit
the area of rendering (stenciling). More advanced usage
of the stencil buffer makes use of the strong connection
between the depth buffer and the stencil buffer in the
rendering pipeline. For example, stencil values can be
automatically increased/decreased for every pixel that
fails or passes the depth test.
The simple combination of depth test and stencil
modifiers make a vast number of effects possible (such
In this program the stencil buffer is filled with 1s wherever a white
stripe is drawn and 0s elsewhere. Two versions of each oval, square,
or triangle are then drawn. A black colored shape is drawn where the
stencil buffer is 1, and a white shape is drawn where the buffer is 0.
as shadows, outline drawing or highlighting of intersections between complex primitives) though they often require
several rendering passes and, therefore, can put a heavy load on the graphics hardware.
The most typical application is still to add shadows to 3D applications. It is also used for planar reflections.
Other rendering techniques, such as portal rendering, use the stencil buffer in other ways; for example, it can be used
to find the area of the screen obscured by a portal and re-render those pixels correctly.
The stencil buffer and its modifiers can be accessed in computer graphics APIs like OpenGL and Direct3D.