Frame rate

From Wikipedia, the free encyclopedia
  (Redirected from Framerate)
Jump to: navigation, search

Frame rate, also known as frame frequency and frames per second (FPS), is the frequency (rate) at which an imaging device produces unique consecutive images called frames. The term applies equally well to film and video cameras, computer graphics, and motion capture systems. Frame rate is most often expressed in frames per second (FPS) and is also expressed in progressive scan monitors as hertz (Hz).

Background[edit]

The threshold of human visual perception varies depending on what is being measured. When looking at a lighted display, people begin to notice a brief interruption of darkness if it is about 16 milliseconds or longer.[1] Observers can recall one specific image in an unbroken series of different images, each of which lasts as little as 13 milliseconds.[2] When given very short single-millisecond visual stimulus people report a duration of between 100 ms and 400 ms due to persistence of vision in the visual cortex. This may cause images perceived in this duration to appear as one stimulus, such as a 10 ms green flash of light immediately followed by a 10 ms red flash of light perceived as a single yellow flash of light.[3] Persistence of vision may also create an illusion of continuity, allowing a sequence of still images to give the impression of motion.

Early silent films had stated frame rates anywhere from 16 to 24 FPS,[4] but since the cameras were hand-cranked, the rate often changed during the scene to fit the mood. Projectionists could also change the frame rate in the theater by adjusting a rheostat controlling the voltage powering the film-carrying mechanism in the projector.[5] Silent films were often intended to be shown at higher frame rates than those used during filming.[6] These frame rates were enough for the sense of motion, but it was perceived as jerky motion. By using projectors with dual- and triple-blade shutters, the rate was multiplied two or three times as seen by the audience. Thomas Edison said that 46 frames per second was the minimum need by the visual cortex: "Anything less will strain the eye."[7][8] In the mid to late 1920s, the frame rate for silent films increased to between 20 and 26 FPS.[7]

When sound film was introduced in 1926, variations in film speed were no longer tolerated as the human ear is more sensitive to changes in audio frequency. Many theaters had shown silent films at 22 to 26 FPS which is why 24 FPS was chosen for sound. From 1927 to 1930, as various studios updated equipment, the rate of 24 FPS became standard for 35 mm sound film.[9] At 24 FPS the film travels through the projector at a rate of 456 millimetres (18.0 in) per second. This allowed for simple two-blade shutters to give a projected series of images at 48 per second, satisfying Edison's recommendation. Many modern 35 mm film projectors use three-blade shutters to give 72 images per second—each frame is flashed on screen three times.[7]

Motion picture film[edit]

In the motion picture industry, where traditional film stock is used, the industry standard filming and projection formats are 24 frames per second (fps). Historically, 25 fps was used in some European countries. Shooting at a slower frame rate would create fast motion when projected, while shooting at a frame rate higher than 24 fps would create slow motion when projected. Other examples of historical experiments in frame rates that were not widely accepted were Maxivision 48 and Showscan, developed by 2001: A Space Odyssey special effects creator Douglas Trumbull.

Digital video and television[edit]

There are three main frame rate standards in the TV and digital cinema business: 24p, 25p, and 30p. However, there are many variations on these as well as newer emerging standards.

  • 24p is a progressive format and is now widely adopted by those planning on transferring a video signal to film. Film and video makers use 24p even if they are not going to transfer their productions to film, simply because of the on-screen "look" of the (low) frame rate, which matches native film. When transferred to NTSC television, the rate is effectively slowed to 23.976 FPS (24×1000÷1001 to be exact), and when transferred to PAL or SECAM it is sped up to 25 FPS. 35 mm movie cameras use a standard exposure rate of 24 FPS, though many cameras offer rates of 23.976 FPS for NTSC television and 25 FPS for PAL/SECAM. The 24 FPS rate became the de facto standard for sound motion pictures in the mid-1920s.[7] Practically all hand-drawn animation is designed to be played at 24 FPS. Actually hand-drawing 24 unique frames per second ("1's") is costly. Even in big budget films, usually hand-draw animation is done shooting on "2's" (one hand-drawn frame is shown twice, so only 12 unique frames per second)[10] and some animation is even drawn on "4's" (one hand-drawn frame is shown four times, so only six unique frames per second).
  • 25p is a progressive format and runs 25 progressive frames per second. This frame rate derives from the PAL television standard of 50i (or 50 interlaced fields per second). Film and television companies use this rate in 50 Hz regions for direct compatibility with television field and frame rates. Conversion for 60 Hz countries is enabled by slowing down the media to 24p then converting to 60 Hz systems using pulldown. While 25p captures half the temporal resolution or motion that normal 50i PAL registers, it yields a higher vertical spatial resolution per frame. Like 24p, 25p is often used to achieve "cine"-look, albeit with virtually the same motion artifacts. It is also better suited to progressive-scan output (e.g., on LCD displays, computer monitors and projectors) because the interlacing is absent.
  • 30p is a progressive format and produces video at 30 frames per second. Progressive (noninterlaced) scanning mimics a film camera's frame-by-frame image capture. The effects of inter-frame judder are less noticeable than 24p yet retains a cinematic-like appearance. Shooting video in 30p mode gives no interlace artifacts but can introduce judder on image movement and on some camera pans. The widescreen film process Todd-AO used this frame rate in 1954–1956.[11]
  • 48p is a progressive format and is currently being trialled in the film industry. At twice the traditional rate of 24p, this frame rate attempts to reduce motion blur and flicker found in films. Director James Cameron stated his intention to film the two sequels to his film Avatar higher than 24 frames per second to add a heightened sense of reality.[12] The first film to be filmed at 48 FPS was The Hobbit: An Unexpected Journey, a decision made by its director Peter Jackson.[13] At a preview screening at CinemaCon, the audience's reaction was mixed after being shown some of the film's footage at 48p, with some arguing that the feel of the footage was too lifelike (thus breaking the suspension of disbelief).[14]
  • 50i is an interlaced format and is the standard video field rate per second for PAL and SECAM television.
  • 60i is an interlaced format and is the standard video field rate per second for NTSC television (e.g., in the US), whether from a broadcast signal, DVD, or home camcorder. This interlaced field rate was developed separately by Farnsworth and Zworykin in 1934,[15] and was part of the NTSC television standards mandated by the FCC in 1941. When NTSC color was introduced in 1953, the older rate of 60 fields per second was reduced by a factor of 1000/1001 to avoid interference between the chroma subcarrier and the broadcast sound carrier. (Hence the usual designation "29.97 fps" = 30 frames (60 fields)/1.001)
  • 50p/60p is a progressive format and is used in high-end HDTV systems. While it is not technically part of the ATSC or DVB broadcast standards yet, reports suggest that higher progressive frame rates will be a feature of the next-generation high-definition television broadcast standards.[16] In Europe, the EBU considers 1080p50 the next step future proof system for TV broadcasts and is encouraging broadcasters to upgrade their equipment for the future.[17]
  • 72p is a progressive format and is currently in experimental stages. Major institutions such as Snell have demonstrated 720p72 pictures as a result of earlier analogue experiments, where 768 line television at 75 FPS looked subjectively better than 1150 line 50 FPS progressive pictures with higher shutter speeds available (and a corresponding lower data rate).[18] Modern cameras such as the Red One can use this frame rate to produce slow motion replays at 24 FPS. Douglas Trumbull, who undertook experiments with different frame rates that led to the Showscan film format, found that emotional impact peaked at 72 FPS for viewers.[citation needed] 72 FPS is the maximum rate available in the WMV video file format.
  • 120p (120.00 Hz exactly) is a progressive format and is standardized for UHDTV by the ITU-R BT.2020 recommendation. It will be the single global "double-precision" frame rate for UHDTV (instead of using 100 Hz for PAL-based countries and 119.88 Hz for NTSC-based countries).
  • 300 FPS, interpolated 300 FPS along with other high frame rates, have been tested by BBC Research for use in sports broadcasts.[19] 300 FPS can be converted to both 50 and 60 FPS transmission formats without major issues.

Video games[edit]

Frame rates in video games refer to the speed at which the image is refreshed (typically in frames per second, or FPS). Many underlying processes, such as collision detection and network processing, run at different or inconsistent frequencies or in different physical components of a computer. FPS affects the gaming experience in two ways: low FPS does not effectively give the illusion of motion and affects the user's capacity to interact with the game, while FPS that varies substantially from one second to the next depending on computational load produces uneven, "choppy" movement or animation.

The first 3D first-person game for a personal computer, 3D Monster Maze, had a frame rate of approximately 6 FPS, and was still a success. In modern action-oriented games where players must visually track animated objects and react quickly, frame rates of between 30 and 60 FPS are considered acceptable by most, though this can vary significantly from game to game. Modern action games, including popular console games such as Halo 3, are locked at 30 FPS maximum, while others, such as Unreal Tournament 3, can run well in excess of 100 FPS on sufficient hardware. Additionally some games such as Quake 3 Arena perform physics, AI, networking, and other calculations in sync with the rendered frame rate - this can result in inconsistencies with movement and network prediction code if players are unable to maintain the designed maximum frame rate of 125 FPS. The frame rate within games varies considerably depending upon the complexity of the individual frame to render, or with the hardware configuration (especially in PC games). When the computation of a frame consumes more time than intended, the frame rate decreases. This instability causes stuttering and screen tearing.

Frame rate in video games is a delicate trade off between picture fidelity and render time (which translates into FPS). Unlike movies, video games depend on the interaction between consumer and game. Lower frame rates affect this feedback loop with choppy motion and additional input delay. However a trend to low frame rate high fidelity titles was perceivable in the last years. This is mostly because of the mismatch between console platforms steady stated computational power and the demand of higher fidelity graphics. For many game studios the shift to 30FPS is a notable gain considering that lower frame rates are less apparent for the untraind eye than reduced picture quality.

A culture of competition has arisen among game enthusiasts with regard to frame rates, with players striving to obtain the highest FPS possible, due to their utility in demonstrating a system's power and efficiency. Indeed, many benchmarks (such as 3DMark) released by the marketing departments of hardware manufacturers and published in hardware reviews focus on the FPS measurement. LCD monitors of today are built with three major refresh rate in mind. The most common is 60 Hz, which can be used at any resolution without requiring high quality computer systems to render, and then 120 Hz and 144 Hz. The 120 Hz standard also supports what is known as 'lightboost' technology in some monitors, where strobing lights behind the monitor reduce ghosting at high FPS rates.

Beyond measurement and bragging rights, such exercises do have practical bearing in some cases. A certain amount of discarded “headroom” frames are beneficial for the elimination of uneven (“choppy” or “jumpy”) output, and to prevent FPS from plummeting during the intense sequences when players need smooth feedback most.

Without realistic motion blurring, video games and computer animations do not look as fluid as film. When a fast moving object is present on two consecutive frames, a gap between the images on the two frames contributes to a noticeable separation of the object and its afterimage in the eye. Motion blurring mitigates this effect, since it tends to reduce the image gap when the two frames are strung together. The effect of motion blurring is essentially superimposing multiple images of the fast-moving object on a single frame. Motion blurring makes the motion more fluid for some people, even as the image of the object becomes blurry on each individual frame. Motion blur can also induce headaches when people play a game that requires concentration.[20]

A high frame rate still does not guarantee fluid movements, especially on hardware with more than one GPU. This effect is known as micro stuttering.

See also[edit]

References[edit]

  1. ^ Andrew B. Watson (1986), "Temporal sensitivity", Handbook of Perception and Human Performance (Wiley) 
  2. ^ http://link.springer.com/article/10.3758%2Fs13414-013-0605-z
  3. ^ Robert Efron. "Conservation of temporal information by perceptual systems". Perception & Psychophysics 14 (3): 518–530. doi:10.3758/bf03211193. 
  4. ^ Brown, Julie (2014). "Audio-visual Palimpsests: Resynchronizing Silent Films with 'Special' Music". In David Neumeyer. The Oxford Handbook of Film Music Studies. Oxford University Press. p. 588. ISBN 0195328493. 
  5. ^ Kerr, Walter (1975). Silent Clowns. Knopf. p. 36. ISBN 0394469070. 
  6. ^ Card, James (1994). Seductive cinema: the art of silent film. Knopf. p. 53. ISBN 0394572181. 
  7. ^ a b c d Brownlow, Kevin (Summer 1980). "Silent Films: What Was the Right Speed?". Sight & Sound 49 (3): 164–167. Archived from the original on 8 July 2011. Retrieved 2 May 2012. 
  8. ^ Thomas Elsaesser, Thomas Elsaesser; Barker, Adam (1990). Early cinema: space, frame, narrative. BFI Publishing. p. 284. ISBN 0-85170-244-9. 
  9. ^ Read, Paul; Meyer, Mark-Paul; Gamma Group (2000). Restoration of motion picture film. Conservation and Museology. Butterworth-Heinemann. pp. 24–26. ISBN 0-7506-2793-X. 
  10. ^ "How many cels does a typical cartoon yield?"
  11. ^ Todd-AO Specifications at a Glance, Widescreen Museum.
  12. ^ Giardina, Carolyn (March 30, 2011). "James Cameron 'Fully Intends' to Make 'Avatar 2 and 3' at Higher Frame Rates". The Hollywood Reporter. Retrieved April 4, 2010. 
  13. ^ Jackson, Peter (12 April 2011). "48 Frames Per Second". Peter Jackson's Facebook page. Facebook. Retrieved 12 April 2011. 
  14. ^ Walters, Florence (25 April 2012). "The Hobbit previews to mixed reactions". The Daily Telegraph (London). Retrieved 30 April 2012. 
  15. ^ Gary Edgerton, The Columbia History of American Television, Columbia University Press, 2009, p. 51–52. ISBN 978-0-231-12165-1.
  16. ^ Hoffmann, Hans; Takebumi Itagaki; David Wood; Alois Bock (December 2006). "Studies on the Bit Rate Requirements for a HDTV Format With 1920 × 1080 pixel Resolution, Progressive Scanning at 50 Hz Frame Rate Targeting Large Flat Panel Displays". IEEE Transactions on Broadcasting 52 (4): 420–434. doi:10.1109/tbc.2006.884735. 
  17. ^ "10 Things You Need to Know about 1080p50". EBU Technical. 
  18. ^ Snell & Willcox
  19. ^ High Frame-Rate Television, BBC White Paper WHP 169, September 2008, M Armstrong, D Flynn, M Hammond, S Jolly, R Salmon
  20. ^ "Motion Blur == Headache". Retrieved 4 December 2012. 

Further reading[edit]

External links[edit]