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Z-buffering 248
Z-buffering
In computer graphics, z-buffering is the management of image depth
coordinates in three-dimensional (3-D) 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 are hidden. The painter's
algorithm is another common solution which, though less efficient, can
also handle non-opaque scene elements. Z-buffering is also known as
depth buffering.
When an object is rendered by a 3D graphics card, the depth of a
generated pixel (z coordinate) is stored in a buffer (the z-buffer or
depth buffer). This buffer is usually arranged as a two-dimensional
array (x-y) with one element for each screen pixel. If another object of
the scene must be rendered in the same pixel, the graphics card
compares the two depths and chooses the one closer to the observer.
The chosen depth is then saved to the z-buffer, replacing the old one.
In the end, the z-buffer will allow the graphics card to correctly
Z-buffer data
reproduce the usual depth perception: a close object hides a farther one. This is called z-culling.
The granularity of a z-buffer has a great influence on the scene quality: a 16-bit z-buffer can result in artifacts (called
"z-fighting") when two objects are very close to each other. A 24-bit or 32-bit z-buffer behaves much better,
although the problem cannot be entirely eliminated without additional algorithms. An 8-bit z-buffer is almost never
used since it has too little precision.
Uses
Z-buffer data in the area of video editing permits one to combine 2D video elements in 3D space, permitting virtual
sets, "ghostly passing through wall" effects, and complex effects like mapping of video on surfaces. An application
for Maya, called IPR, permits one to perform post-rendering texturing on objects, utilizing multiple buffers like
z-buffers, alpha, object id, UV coordinates and any data deemed as useful to the post-production process, saving time
otherwise wasted in re-rendering of the video.
Z-buffer data obtained from rendering a surface from a light's POV permits the creation of shadows in a scanline
renderer, by projecting the z-buffer data onto the ground and affected surfaces below the object. This is the same
process used in non-raytracing modes by the free and open sourced 3D application Blender.
Developments
Even with small enough granularity, quality problems may arise when precision in the z-buffer's distance values is
not spread evenly over distance. Nearer values are much more precise (and hence can display closer objects better)
than values which are farther away. Generally, this is desirable, but sometimes it will cause artifacts to appear as
objects become more distant. A variation on z-buffering which results in more evenly distributed precision is called
w-buffering (see below).
At the start of a new scene, the z-buffer must be cleared to a defined value, usually 1.0, because this value is the
upper limit (on a scale of 0 to 1) of depth, meaning that no object is present at this point through the viewing
frustum.