Motion blur

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Earth's rotation causes motion blur in long-exposure photos of the night sky. This diurnal motion leaves star trails in exposures like this one taken at La Silla Observatory. Star Rain in the Desert.jpg
Earth's rotation causes motion blur in long-exposure photos of the night sky. This diurnal motion leaves star trails in exposures like this one taken at La Silla Observatory.

Motion blur is the apparent streaking of moving objects in a photograph or a sequence of frames, such as a film or animation. It results when the image being recorded changes during the recording of a single exposure, due to rapid movement or long exposure.

Contents

Usages / Effects of motion blur

Photography

An example of motion blur showing a London bus passing a telephone box in London London bus and telephone box on Haymarket.jpg
An example of motion blur showing a London bus passing a telephone box in London
1920s example of motion blur Dog Leaping.jpg
1920s example of motion blur

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 manipulated by panning the camera to track those moving objects. In this case, even with long exposure times, the moving objects will appear sharper while the background will become more blurred, with the resulting image conveying a sense of movement and speed. [2]

Animation

Two animations rotating around a figure, with motion blur (left) and without Figure-Animation2.gif
Two animations rotating around a figure, with motion blur (left) and without

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 (analogous to a camera with an infinitely fast shutter), 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 (and sometimes how fast the camera is moving in 3-D Space to create a radial blur) 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. Classic "motion blur" effects prior to modern per-pixel shading pipelines often simply drew successive frames on top of each other with slight transparency, which is strictly speaking a form of video feedback.

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 (in our 4 frame example) they wish to feature as "open shutter" times. They may choose to render the beginnings of each frame, in which case they will never see the arrival of the object at the end of the path, or they may choose to render the ends of each frame, in which case they 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 image [3] /bitmap [4] /raster image in order to simulate the effect. Many graphical software products (e.g. Adobe Photoshop or GIMP) offer simple motion blur filters. However, for advanced motion blur filtering including curves or non-uniform speed adjustment, specialized software products (e.g. VirtualRig Studio) are necessary. [5]

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 (quick eye movements) and fixation (focusing on a single point). 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

A taxicab starting to drive off blurred the girl's faces in the image. Three women in a taxi in Manhattan who said hello.jpg
A taxicab starting to drive off blurred the girl's faces in the image.

Television

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 millisecond, and then transmitting them over the course of the next 30 to 40 milliseconds. Although this gives sharper slow motion replays, it can look strange at normal speeds 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 (especially LCD) that exhibits motion blur during fast motion. This can lead to more perceived motion blurring above and beyond the preexisting motion blur in the video material. See display motion blur.

Sometimes, motion blur can be removed from images with the help of deconvolution.

Video games

In video games the use or not of motion blur is somewhat controversial. [6] Some gamers claim that the blur actually makes gaming worse since it does blur images, making more difficult to recognize objects, especially in fast-paced moments. This does become noticeable the lower the frame rate is. Improvements in the visual quality of modern post-process motion blur shaders [7] as well as a tendency towards higher frame rates have lessened the visual detriment of undersampled motion blur effects.

Wind turbines

Birds cannot properly see the swirling blades of wind turbines and can get struck by them fatally. A newly published report from Norway suggests that painting one of the three blades with a black tip makes the blades more visible and hence more avoidable. This reduces the motion blur of the unpainted blades and cuts bird deaths by up to 70 percent. [8]

Restoration

An example of blurred image restoration with Wiener deconvolution:

Image restoration (motion blur, Wiener filtering).png
From left: original image, blurred image and de-blurred image. Notice some artifacts in the de-blurred image.

See also

Related Research Articles

Frame rate is the frequency (rate) at which consecutive images called frames appear on a display. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be called the frame frequency, and be expressed in hertz.

Camera Optical device for recording images

A camera is an optical instrument used to capture an image. At their most basic, cameras are sealed boxes with a small hole that allow light in to capture an image on a light-sensitive surface. Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera, the size of the aperture can be widened or narrowed to let more or less light into the camera, and a shutter mechanism determines the amount of time the photo-sensitive surface is exposed to the light.

Interlaced video

Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon.

Shutter speed The length of time when the film or digital sensor inside a camera is exposed to light

In photography, shutter speed or exposure time is the length of time when the film or digital sensor inside the camera is exposed to light, also when a camera's shutter is open when taking a photograph. The amount of light that reaches the film or image sensor is proportional to the exposure time. ​1500 of a second will let half as much light in as ​1250.

Bullet time is a visual effect or visual impression of detaching the time and space of a camera from those of its visible subject. It is a depth enhanced simulation of variable-speed action and performance found in films, broadcast advertisements, and realtime graphics within video games and other special media. It is characterized both by its extreme transformation of time and space. This is almost impossible with conventional slow motion, as the physical camera would have to move implausibly fast; the concept implies that only a "virtual camera", often illustrated within the confines of a computer-generated environment such as a virtual world or virtual reality, would be capable of "filming" bullet-time types of moments. Technical and historical variations of this effect have been referred to as time slicing, view morphing, temps mort and virtual cinematography.

Go motion is a variation of stop motion animation which incorporates motion blur into each frame involving motion. It was co-developed by Industrial Light & Magic and Phil Tippett. Stop motion animation can create a disorienting, and distinctive staccato effect, because the animated object is perfectly sharp in every frame, since each frame of the animation was actually shot when the object was perfectly still. Real moving objects in similar scenes of the same movie will have motion blur, because they moved while the shutter of the camera was open. Filmmakers use a variety of techniques to simulate motion blur, such as moving the model slightly during the exposure of each film frame or using a glass plate smeared with petroleum jelly in front of the camera lens to blur the moving areas.

The science of photography is the use of chemistry and physics in all aspects of photography. This applies to the camera, its lenses, physical operation of the camera, electronic camera internals, and the process of developing film in order to take and develop pictures properly.

Time-lapse photography

Time-lapse photography is a technique whereby the frequency at which film frames are captured is much more spread out than the frequency used to view the sequence. When played at normal speed, time appears to be moving faster and thus lapsing. For example, an image of a scene may be captured at 1 frame per second, but then played back at 30 frames per second; the result is an apparent 30 times speed increase. In a similar manner, film can also be played at a much lower rate than at which it was captured, slowing down an otherwise fast action, as in slow motion or high-speed photography.

Rotary disc shutter

A rotary disc shutter is a type of shutter. It is notably used in motion picture cameras. Rotary shutters are semicircular discs that spin in front of the film gate, alternately allowing light from the lens to strike the film, or blocking it.

Shutter (photography)

In photography, a shutter is a device that allows light to pass for a determined period, exposing photographic film or a photosensitive digital sensor to light in order to capture a permanent image of a scene. A shutter can also be used to allow pulses of light to pass outwards, as seen in a movie projector or a signal lamp. A shutter of variable speed is used to control exposure time of the film. The shutter is constructed so that it automatically closes after a certain required time interval. The speed of the shutter is controlled by a ring outside the camera, on which various timings are marked.

Neutral-density filter

In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter, and is denoted by Wratten number 96. The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens. Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures. This is done to achieve effects such as a shallower depth of field or motion blur of a subject in a wider range of situations and atmospheric conditions.

High-speed photography Photography genre

High-speed photography is the science of taking pictures of very fast phenomena. In 1948, the Society of Motion Picture and Television Engineers (SMPTE) defined high-speed photography as any set of photographs captured by a camera capable of 69 frames per second or greater, and of at least three consecutive frames. High-speed photography can be considered to be the opposite of time-lapse photography.

High-motion is the characteristic of video or film footage displayed possessing a sufficiently high frame rate that moving images do not blur or strobe even when tracked closely by the eye. The most common forms of high motion are NTSC and PAL video at their native display rates. Movie film does not portray high motion even when shown on television monitors.

Image stabilization

Image stabilization (IS) is a family of techniques that reduce blurring associated with the motion of a camera or other imaging device during exposure.

3D rendering

3D rendering is the 3D computer graphics process of converting 3D models into 2D images on a computer. 3D renders may include photorealistic effects or non-photorealistic styles.

Temporal anti-aliasing (TAA) seeks to reduce or remove the effects of temporal aliasing. Temporal aliasing is caused by the sampling rate of a scene being too low compared to the transformation speed of objects inside of the scene; this causes objects to appear to jump or appear at a location instead of giving the impression of smoothly moving towards them. To avoid aliasing artifacts altogether, the sampling rate of a scene must be at least twice as high as the fastest moving object. The shutter behavior of the sampling system strongly influences aliasing, as the overall shape of the exposure over time determines the band-limiting of the system before sampling, an important factor in aliasing. A temporal anti-aliasing filter can be applied to a camera to achieve better band-limiting. A common example of temporal aliasing in film is the appearance of vehicle wheels travelling backwards, the so-called wagon-wheel effect. Temporal anti-aliasing can also help to reduce jaggies, making images appear softer.

Long-exposure photography Photography using a long-duration shutter speed

Long-exposure, time-exposure, or slow-shutter photography involves using a long-duration shutter speed to sharply capture the stationary elements of images while blurring, smearing, or obscuring the moving elements. Long-exposure photography captures one element that conventional photography does not: an extended period of time.

Display motion blur, also called HDTV blur and LCD motion blur, refers to several visual artifacts that are frequently found on modern consumer high-definition television sets and flat panel displays for computers.

Rolling shutter

Rolling shutter is a method of image capture in which a still picture or each frame of a video is captured not by taking a snapshot of the entire scene at a single instant in time but rather by scanning across the scene rapidly, either vertically or horizontally. In other words, not all parts of the image of the scene are recorded at exactly the same instant. This produces predictable distortions of fast-moving objects or rapid flashes of light. This is in contrast with "global shutter" in which the entire frame is captured at the same instant.

Strip photography Type of photographic technique

Strip photography, or slit photography, is a photographic technique of capturing a 2-dimensional image as a sequence of 1-dimensional images over time, rather than a single 2-dimensional at one point in time. As one moves across, one moves in time in addition to moving in space. The image can be loosely interpreted as a collection of thin vertical or horizontal strips patched together, hence the name. This is correct if the strips are discrete, as in a digital sensor that captures one line at a time, but in film photography, the image is produced continuously, and thus the "strips" are infinitesimal – a smooth gradation.

References

  1. "Star Rain in the Desert". eso.org. European Southern Observatory . Retrieved 26 August 2014.
  2. Motion blur in photography, Click and Learn Photography
  3. Motion Blur Effect, TutorialsRoom
  4. Photoshop - Motion Blur, tizag.com
  5. Traditional motion blur methods, virtualrig-studio.com
  6. "Motion Blur == Headache" . Retrieved 4 December 2012.
  7. "A Fast and Stable Feature-Aware Motion Blur Filter" (PDF). Retrieved 17 May 2018.
  8. "Black turbine blade 'can cut bird deaths'". BBC News. 2020-08-26. Retrieved 2020-12-20.