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Explanation of refreshing, flickering, interlacing

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Presentation on theme: "Explanation of refreshing, flickering, interlacing"— Presentation transcript:

1 Explanation of refreshing, flickering, interlacing
C A E D C Computer Aided Engineering Design Centre

2 Explanation of refreshing

3 What is Refreshing??? Refreshing is the transfer of data between two types of the same storage medium so there are no bitrate (the number of bits that are conveyed or processed per unit of time. ) changes or alteration of data. For example, transferring census data from an old preservation CD to a new one. This strategy may need to be combined with migration when the software or hardware required to read the data is no longer available or is unable to understand the format of the data. Refreshing will likely always be necessary due to the deterioration of physical media.

4 Explanation of flickering

5 What is Flicker (screen)???
Flicker is visible fading between cycles displayed on video displays, especially the refresh interval on cathode ray tube (CRT) based computer screens.

6 When Flicker occurs??? Flicker occurs on CRTs when they are driven at a low refresh rate (The refresh rate is the number of times in a second that display hardware draws the data. ) allowing the screen's phosphors (A phosphor is a substance that exhibits the phenomenon of phosphorescence (sustained glowing after exposure to energized particles such as electrons or ultraviolet photons) ) to lose their excitation (afterglow) between sweeps of the electron gun. A similar effect occurs in PDPs (plasma display panel) during their refresh cycles.

7 Flickering For example, if a CRT computer monitor's vertical refresh rate is set to 60 Hz, most monitors will produce a visible "flickering" effect, unless they use phosphor with long afterglow. Most people find that refresh rates of Hz and above enable flicker-free viewing on CRTs. Use of refresh rates above 120 Hz is uncommon, as they provide little noticeable flicker reduction and limit available resolution. The lighting used in film projectors is typically an incandescent lamp or arc lamp, which does not flicker, but some degree of flicker is desirable to help decrease the flicker fusion threshold comfortably below film's typical framerate of 24 fps. This is usually accomplished with a shutter which causes the lamplight to apparently strobe on and off at a multiple of the framerate, most often Hz.

8 How to prevent Flicker??? The exact refresh rate necessary to prevent the perception of flicker varies greatly based on the viewing environment. In a completely dark room, a sufficiently dim display can run as low as 30 Hz without visible flicker. At normal room and TV brightness this same display rate would produce flicker so severe as to be unwatchable. Another factor in detecting flicker is peripheral vision. The human eye is most sensitive to flicker at the edges of our field of view, and least sensitive at the center of gaze (the area being focused on). As a result, the greater the portion of our field of view that is occupied by a display, the greater is the need for high refresh rates. This is why computer monitor CRTs usually run at 70 to 90 Hz, while TVs, which are viewed from further away, are seen as acceptable at 60 or 50 Hz.

9 Explanation of interlacing

10 What is Interlace??? Interlace is a technique of improving the picture quality of a video signal without consuming extra bandwidth. Interlaced video was designed for display on CRT (cathode ray tube) televisions. Interlace is still used for most standard definition TVs, and the 1080i HDTV broadcast standard, but not for LCD, micromirror (DLP), or plasma displays (A plasma display panel (PDP) is a type of flat panel display common to large TV displays (80 cm or larger)). These displays do not use a raster scan to create an image, and so cannot benefit from interlacing: in practice, they have to be driven with a progressive scan (Progressive or noninterlaced scanning is a method for displaying, storing or transmitting moving images in which all the lines of each frameare drawn in sequence) signal.

11 Continue… The deinterlacing circuitry to get progressive scan from a normal interlaced broadcast television signal can add to the cost of a television set using such displays. Currently, progressive displays dominate the HDTV market. Interlaced scan refers to one of two common methods for "painting" a video image on an electronic display screen (the other being progressive scan) by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame. One field contains all the odd lines in the image. The other contains all the even lines of the image. A PAL based television display, for example, scans 50 fields every second (25 odd and 25 even).

12 Continue… The two sets of 25 fields work together to create a full frame every 1/25th of a second, resulting in a display of 25 frames per second. When interlaced video is watched on a progressive monitor without deinterlacing, it exhibits combing when there is movement between two fields of one frame.

13 What is Interlacing (bitmaps)???
Interlacing is a method of encoding a bitmap image such that a person who has partially received it sees a degraded copy of the entire image. An illustration of Adam7 interlacing over a 16×16 image When communicating over a slow communications link, this is often preferable to seeing a perfectly clear copy of one part of the image, as it helps the viewer decide more quickly whether to abort or continue the transmission.

14 Continue… Interlacing is also known as "progressive" encoding, because the image becomes progressively clearer as it is received. This terminology is different than the interlaced and progressive scan terminology of video encoding. Interlacing is supported by the following formats, where it is optional: GIF interlacing stores the lines in the order 0, 8, 16, ...(8n), 4, 12, ...(8n+4), 2, 6, 10, 14, ...(4n+2), 1, 3, 5, 7, 9, ...(2n+1). PNG uses the Adam7 algorithm, which interlaces in both the vertical and horizontal direction. JPEG, JPEG 2000, and JPEG XR (actually using a frequency decomposition hierarchy rather than interlacing of pixel values) PGF (also using a frequency decomposition)

15 Continue… Interlaced graphics were once widely used in web design and before that in the distribution of graphics files over bulletin board systems and other low-speed communications methods. The practice is much less common today, as common broadband internet connections allow most images to be downloaded to the user's screen nearly instantaneously, and interlacing is an inefficient method of encoding images.

16 Applications of interlacing
Interlacing is used by all the analogue TV broadcast systems in current use: PAL: 50 fields per second, 625 lines, odd lines drawn first SECAM: 50 fields per second, 625 lines, odd lines drawn first NTSC: fields per second, 525 lines, even lines drawn first PAL-M: fields per second, 525 lines, even lines drawn first

17 Benefits of interlacing
One of the most important factors is bandwidth, measured in megahertz (for analog video), or bit rate (for digital video). The greater the bandwidth, the more expensive and complex the entire system (camera, storage systems such as tape recorders or hard disks, transmission systems such as cable television systems or respectively internet, and displays such as television monitors or digital monitors). Interlaced video reduces the signal bandwidth by a factor of two, for a given line count and refresh rate. Interlace artifacts aren't usually objectionable when viewed at the intended field rate, on an interlaced video display. For a given bandwidth and refresh rate, interlaced video can be used to provide a higher spatial resolution than progressive scan.

18 Problems caused by interlacing
Interlaced video is designed to be captured, transmitted or stored, and displayed in the same interlaced format. Because each frame of interlaced video is composed of two fields that are captured at different moments in time, interlaced video frames will exhibit motion artifacts known as "interlacing effects", or "combing", if the recorded objects are moving fast enough to be in different positions when each individual field is captured. These artifacts may be more visible when interlaced video is displayed at a slower speed than it was captured or when still frames are presented.

19 Continue… Using progressive-scan monitors with interlaced video
Because modern computer video displays are progressive scan systems, interlaced video will have visible artifacts when it is displayed on computer systems. Computer systems are frequently used to edit video and this disparity between computer video display systems and television signal formats means that the video content being edited cannot be viewed properly unless separate video display hardware is utilized. To minimize the artifacts caused by interlaced video display on a progressive scan monitor, a process called deinterlacing (Deinterlacing is the process of converting interlaced video, such as common analog television signals or 1080i format HDTV signals, into a non-interlaced form.) can be utilized. This process is not perfect, and it generally results in a lower resolution, particularly in areas with objects in motion. Deinterlacing systems are integrated into progressive scan television displays in order to provide the best possible picture quality for interlaced video signals.

20 Continue… Interline twitter
Interlace introduces a potential problem called interline twitter. This aliasing effect only shows up under certain circumstances, when the subject being shot contains vertical detail that approaches the horizontal resolution of the video format. For instance, a person on television wearing a shirt with fine dark and light stripes may appear on a video monitor as if the stripes on the shirt are "twittering". Television professionals are taught to avoid wearing clothing with fine striped patterns to avoid this problem. High-end video cameras or Computer Generated Imagery systems apply a low-pass filter to the vertical resolution of the signal in order to prevent possible problems with interline twitter.

21 Continue… This animation demonstrates the interline twitter effect.

22 Continue… On the left are two progressive scan images.
Center are two interlaced images. Right are two images with line doublers. Top are original resolution, bottom are with anti-aliasing. The two interlaced images use half the bandwidth of the progressive one. The interlaced scan (right) precisely duplicates the pixels of the progressive image (left), but interlace causes details to twitter. A line doubler operating in "bob" (interpolation) mode would produce the images at far right. Real interlaced video blurs such details to prevent twitter, as seen in the bottom row, but such softening (or anti-aliasing) comes at the cost of resolution. But even the best line doubler could never restore the bottom center image to the full resolution of the progressive image.

23 Continue… Note – Because the frame rate has been slowed down by a factor of 3, you will notice additional flicker in simulated interlaced portions of this image. Interline twitter is the primary reason that interlacing is unacceptable for a computer display. Each scanline on a high-resolution computer monitor is typically used to display discrete pixels that do not span the scanlines above or below. When the overall interlaced framerate is 30 frames per second, a pixel that spans only one scanline is visible for 1/30th of a second followed by 1/30th of a second of darkness, reducing the per-line/per-pixel framerate to 15 frames per second.

24 Continue… To avoid this problem, sharp detail is typically never displayed on standard interlaced television. When computer graphics are shown on a standard television, the screen is treated as if it were half the resolution of what it actually is or even lower. If text is displayed, it will be large enough so that horizontal lines are never just one scanline wide. Most fonts used in television programming have wide, fat strokes, and do not include fine-detail serifs that would make the twittering more visible. Despite arguments against it and the calls by many prominent technological companies such as Microsoft to leave interlacing to history, interlacing continues to be supported by the television standard setting organizations and is still being included in new digital video transmission formats such as DV, DVB (including its HD modifications), and ATSC.

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