Stencil buffer - AbsoluteAstronomy.com

A stencil
Stencil
A stencil is a thin sheet of material, such as paper, plastic, or metal, with letters or a design cut from it, used to produce the letters or design on an underlying surface by applying pigment through the cut-out holes in the material. The key advantage of a stencil is that it can be reused to...

buffer is an extra buffer, in addition to the color buffer (pixel buffer

Pixel buffer

Pixel buffer or pBuffer is an extension to OpenGL which allows to do off-screen rendering. It expands upon Vertex Buffer Objects in order to store not only vertex data but also pixel data into the buffer objects....

) and depth buffer (z-buffering

Z-buffering

In computer graphics, z-buffering is the management of image depth coordinates in three-dimensional graphics, usually done in hardware, sometimes in software. It is one solution to the visibility problem, which is the problem of deciding which elements of a rendered scene are visible, and which...

) found on modern computer graphics

Computer graphics

Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....

hardware

Hardware

Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

. The buffer is per pixel, and works on integer

Integer

The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

values, usually with a depth of one byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

per pixel. The depth buffer

Z-buffering

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

Rendering (computer graphics)

Rendering is the process of generating an image from a model , by means of computer programs. A scene file contains objects in a strictly defined language or data structure; it would contain geometry, viewpoint, texture, lighting, and shading information as a description of the virtual scene...

(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

Graphics pipeline

In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current state of the art method of rasterization-based rendering as supported by commodity graphics hardware. The graphics pipeline typically accepts some representation of a three-dimensional...

. 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 as shadow

Shadow

A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the...

s, outline drawing or highlighting of intersections between complex primitives

Geometric primitive

The term geometric primitive in computer graphics and CAD systems is used in various senses, with the common meaning of the simplest geometric objects that the system can handle . Sometimes the subroutines that draw the corresponding objects are called "geometric primitives" as well...

) 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

3D computer graphics

3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...

. It is also used for planar reflections.

Other rendering techniques, such as portal rendering

Portal rendering

In computer-generated imagery and real-time 3D computer graphics, portal rendering is an algorithm for visibility determination. For example, consider a 3D computer game environment, which may contain many polygons, only a few of which may be visible on screen at a given time...

, 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

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

like OpenGL

OpenGL

OpenGL is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL...

and Direct3D

Direct3D

Direct3D is part of Microsoft's DirectX application programming interface . Direct3D is available for Microsoft Windows operating systems , and for other platforms through the open source software Wine. It is the base for the graphics API on the Xbox and Xbox 360 console systems...

See also