Motion blur is the apparent streaking of rapidly moving objects in a still image
or a sequence of images such as a movie or animation. It results when the image
being recorded changes during the recording of a single exposure, either
due to rapid movement or long exposure. Applications of motion blur
= Photography = When a camera creates an image, that
image does not represent a single instant of time. Because of
technological constraints or artistic requirements, the image may represent
the scene over a period of time. Most often this exposure time is brief enough
that the image captured by the camera appears to capture an instantaneous
moment, but this is not always so, and a fast moving object or a longer exposure
time may result in blurring artifacts which make this apparent. As objects in
a scene move, an image of that scene must represent an integration of all
positions of those objects, as well as the camera's viewpoint, over the period
of exposure determined by the shutter speed. In such an image, any object
moving with respect to the camera will look blurred or smeared along the
direction of relative motion. This smearing may occur on an object that is
moving or on a static background if the camera is moving. In a film or
television image, this looks natural because the human eye behaves in much
the same way. Because the effect is caused by the
relative motion between the camera, and the objects and scene, motion blur may
be avoided by panning the camera to track those moving objects. In this
case, even with long exposure times, the objects will appear sharper, and the
background more blurred. = Animation =
In computer animation this effect must be simulated as a virtual camera
actually does capture a discrete moment in time. This simulated motion blur is
typically applied when either the camera or objects in the scene move rapidly.
Without this simulated effect each frame shows a perfect instant in time, with
zero motion blur. This is why a video game with a frame rate of 25-30 frames
per second will seem staggered, while natural motion filmed at the same frame
rate appears rather more continuous. Many modern video games feature motion
blur, especially vehicle simulation games.
Some of the better-known games that utilise this are the recent Need for
Speed titles, Unreal Tournament III, The Legend of Zelda: Majora's Mask, among
many others. There are two main methods used in video games to achieve motion
blur: cheaper full-screen effects, which typically only take camera movement into
mind, and more "selective" or "per-object" motion blur, which
typically uses a shader to create a velocity buffer to mark motion intensity
for a motion blurring effect to be applied to or uses a shader to perform
geometry extrusion. In pre-rendered computer animation, such
as CGI movies, realistic motion blur can be drawn because the renderer has more
time to draw each frame. Temporal anti-aliasing produces frames as a
composite of many instants. Frames are not points in time, they are periods of
time. If an object makes a trip at a linear speed along a path from 0% to
100% in four time periods, and if those time periods are considered frames, then
the object would exhibit motion blur streaks in each frame that are 25% of
the path length. If the shutter speed is shortened to less than the duration of a
frame, and it may be so shortened as to approach zero time in duration, then the
computer animator must choose which portion of the quarter paths she wishes
to feature as "open shutter" times. She may choose to render the beginnings of
each frame, in which case she will never see the arrival of the object at the end
of the path, or she may choose to render the beginnings of each frame, in which
case she will miss the starting point of the trip. Most computer animations
systems make the classic "fence-post error" in the way they handle time,
confusing the periods of time of an animation with the instantaneous moments
that delimit them. Thus most computer animation systems will incorrectly place
an object on a four frame trip along a path at 0%, 0.33%, 0.66%, and 1.0% and
when called upon to render motion blur will have to cut one or more frames
short, or look beyond the boundaries of the animation, compromises that real
cameras don't do and synthetic cameras needn't do.
Motion lines in cel animation are drawn in the same direction as motion blur and
perform much the same duty. Go motion is a variant of stop motion animation that
moves the models during the exposure to create a less staggered effect.
= Computer graphics = In 2D computer graphics, motion blur is
an artistic filter that converts the digital imageraster image in order to
simulate the effect. Many graphical software products offer simple motion
blur filters. However, for advanced motion blur filtering including curves
or non-uniform speed adjustment, specialized software products are
necessary. = Biology =
When an animal's eye is in motion, the image will suffer from motion blur,
resulting in an inability to resolve details. To cope with this, humans
generally alternate between saccades and fixation. Saccadic masking makes motion
blur during a saccade invisible. Similarly, smooth pursuit allows the eye
to track a target in rapid motion, eliminating motion blur of that target
instead of the scene. Negative effects of motion blur
In televised sports, where conventional cameras expose pictures 25 or 30 times
per second, motion blur can be inconvenient because it obscures the
exact position of a projectile or athlete in slow motion. For this reason
special cameras are often used which eliminate motion blurring by taking
rapid exposures on the order of 1/1000 of a second, and then transmitting them
over the course of the next 1/25 or 1/30 of a second. Although this gives sharper
slow motion replays it can look strange at normal speed because the eye expects
to see motion blurring and is not provided with blurred images.
Conversely, extra motion blur can unavoidably occur on displays when it is
not desired. This occurs with some video displays that exhibits motion blur
during fast motion. This can lead to more perceived motion blurring above and
beyond the pre-existing motion blur in the video material. See display motion
blur. Sometimes, motion blur can be removed
from images with the help of deconvolution.
Some video game players claim that artificial motion blur causes headaches.
For some games, it is recommended to disable motion blur, use a high refresh
rate screen, and play with a high frame rate. That way, it becomes more natural
to pinpoint objects on the screen. Some players argue that motion blur should
come naturally from the eyes, and screens shouldn't need to simulate that
effect. Restoration
An example of blurred image restoration with Wiener deconvolution:
References Gallery
See also Motion lines, a technique found in
comics which conveys a sense of motion