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2.3 Digital Video

We have experienced a digital media revolution in the last couple of decades. TV and cinema have gone all-digital and high-definition, and most movies and some TV broadcasts are now in 3D format. High-definition digital video has landed on laptops, tablets, and cellular phones with high-quality media streaming over the Internet. Apart from the more robust form of the digital signal, the main advantage of digital representation and transmission is that they make it easier to provide a diverse range of services over the same network. Digital video brings broadcasting, cinema, computers, and communications industries together in a truly revolutionary manner, where telephone, cable TV, and Internet service providers have become fierce competitors. A single device can serve as a personal computer, a high-definition TV, and a videophone. We can now capture live video on a mobile device, apply digital processing on a laptop or tablet, and/or print still frames at a local printer. Other applications of digital video include medical imaging, surveillance for military and law enforcement, and intelligent highway systems.

2.3.1 Spatial Resolution and Frame Rate

Digital-video systems use component color representation. Digital color cameras provide individual RGB component outputs. Component color video avoids the artifacts that result from analog composite encoding. In digital video, there is no need for blanking or sync pulses, since it is clear where a new line starts given the number of pixels per line.

The horizontal and vertical resolution of digital video is related to the pixel sampling density, i.e., the number of pixels per unit distance. The number of pixels per line and the number of lines per frame is used to classify video as standard, high, or ultra-high definition, as depicted in Figure 2.7. In low-resolution digital video, pixellation (aliasing) artifact arises due to lack of sufficient spatial resolution. It manifests itself as jagged edges resulting from individual pixels becoming visible. The visibility of pixellation artifacts varies with the size of the display and the viewing distance. This is quite different from analog video where the lack of spatial-resolution results in blurring of image in the respective direction.

Figure 2.7

Figure 2.7 Digital-video spatial-resolution formats.

The frame/field rate is typically 50/60 Hz, although some displays use frame interpolation to display at 100/120, 200 or even 400 Hz. The notation 50i (or 60i) indicates interlaced video with 50 (60) fields/sec, which corresponds to 25 (30) pictures/sec obtained by weaving the two fields together. On the other hand, 50p (60p) denotes 50 (60) full progressive frames/sec.

The arrangement of pixels and lines in a contiguous region of the memory is called a bitmap. There are five key parameters of a bitmap: the starting address in the memory, the number of pixels per line, the pitch value, the number of lines, and the number of bits per pixel. The pitch value specifies the distance in memory from the start of one line to the next. The most common use of pitch different from the number of pixels per line is to set pitch to the next highest power of 2, which may help certain applications run faster. Also, when dealing with interlaced inputs, setting the pitch to double the number of pixels per line facilitates writing lines from each field alternately in memory. This will form a “weaved frame” in a contiguous region of the memory.

2.3.2 Color, Dynamic Range, and Bit-Depth

This section addresses color representation, dynamic range, and bit-depth in digital images/video.

Color Capture and Display

Color cameras can be the three-sensor type or single-sensor type. Three-sensor cameras capture R, G, and B components using different CCD panels, using an optical beam splitter; however, they may suffer from synchronicity problems and high cost, while single-sensor cameras often have to compromise spatial resolution. This is because a color filter array is used so that each CCD element captures one of R, G, or B pixels in some periodic pattern. A commonly used color filter pattern is the Bayer array, shown in Figure 2.8, where two out of every four pixels are green, one is red, and one is blue, since green signal contributes the most to the luminance channel. The missing pixel values in each color channel are computed by linear or adaptive interpolation filters, which may result in some aliasing artifacts. Similar color filter array patterns are also employed in LCD/LED displays, where the human eye performs low-pass filtering to perceive a full-colored image.

Figure 2.8

Figure 2.8 Bayer color-filter array pattern.

Dynamic Range

The dynamic range of a capture device (e.g., a camera or scanner) or a display device is the ratio between the maximum and minimum light intensities that can be represented. The luminance levels in the environment range from –4 log cd/m2 (starlight) to 6 log cd/m2 (sun light); i.e., the dynamic range is about 10 log units [Fer 01]. The human eye has complex fast and slow adaptation schemes to cope with this large dynamic range. However, a typical imaging device (camera or display) has a maximum dynamic range of 300:1, which corresponds to 2.5 log units. Hence, our ability to capture and display a foreground object subject to strong backlighting with proper contrast is limited. High dynamic range (HDR) imaging aims to remedy this problem.

HDR Image Capture

HDR image capture with a standard dynamic range camera requires taking a sequence of pictures at different exposure levels, where raw pixel exposure data (linear in exposure time) are combined by weighted averaging to obtain a single HDR image [Gra 10]. There are two possible ways to display HDR images: i) employ new higher dynamic range display technologies, or ii) employ local tone-mapping algorithms for dynamic range compression (see Chapter 3) to better render details in bright or dark areas on a standard display [Rei 07].

HDR Displays

Recently, new display technologies that are capable of up to 50,000:1 or 4.7 log units dynamic range with maximum intensity 8500 cd/m2, compared to standard displays with contrast ratio 2 log units and maximum intensity 300 cd/m2, have been proposed [See 04]. This high dynamic range matches the human eye’s short time-scale (fast) adaptation capability well, which enables our eyes to capture approximately 5 log units of dynamic range at the same time.

Bit-Depth

Image-intensity values at each sample are quantized for a finite-precision representation. Today, each color component signal is typically represented with 8 bits per pixel, which can capture 255:1 dynamic range for a total of 24 bits/pixel and 224 distinct colors to avoid “contouring artifacts.” Contouring results in slowly varying regions of image intensity due to insufficient bit resolution. Some applications, such as medical imaging and post-production editing of motion pictures may require 10, 12, or more bits/pixel/color. In high dynamic range imaging, 16 bits/pixel/color is required to capture a 50,000:1 dynamic range, which is now supported in JPEG.

Digital video requires much higher data rates and transmission bandwidths as compared to digital audio. CD-quality digital audio is represented with 16 bits/sample, and the required sampling rate is 44 kHz. Thus, the resulting data rate is approximately 700 kbits/sec (kbps). This is multiplied by 2 for stereo audio. In comparison, a high-definition TV signal has 1920 pixels/line and 1080 lines for each luminance frame, and 960 pixels/line and 540 lines for each chrominance frame. Since we have 25 frames/sec and 8 bits/pixel/color, the resulting data rate exceeds 700 Mbps, which testifies to the statement that a picture is worth 1000 words! Thus, the feasibility of digital video is dependent on image-compression technology.

2.3.3 Color Image Processing

Color images/video are captured and displayed in the RGB format. However, they are often converted to an intermediate representation for efficient compression and processing. We review the luminance-chrominance (for compression and filtering) and the normalized RGB and hue-saturation-intensity (HSI) (for color-specific processing) representations in the following.

Luminance-Chrominance

The luminance-chrominance color model was used to develop an analog color TV transmission system that is backwards compatible with the legacy analog black and white TV systems. The luminance component, denoted by Y, corresponds to the gray-level representation of video, while the two chrominance components, denoted by U and V for analog video or Cr and Cb for digital video, represent the deviation of color from the gray level on blue–yellow and red–cyan axes. It has been observed that the human visual system is less sensitive to variations (higher frequencies) in chrominance components (see Figure 2.4(b)). This has resulted in the subsampled chrominance formats, such as 4:2:2 and 4:2:0. In the 4:2.2 format, the chrominance components are subsampled only in the horizontal direction, while in 4:2:0 they are subsampled in both directions as illustrated in Figure 2.9. The luminance-chrominance representation offers higher compression efficiency, compared to the RGB representation due to this subsampling.

Figure 2.9

Figure 2.9 Chrominance subsampling formats: (a) no subsampling; (b) 4:2:2; (c) 4:2:0 format.

ITU-R BT.709 defines the conversion between RGB and YCrCb representations as:

which states that the human visual system perceives the contribution of R-G-B to image intensity approximately with a 3-6-1 ratio, i.e., red is weighted by 0.3, green by 0.6 and blue by 0.1.

The inverse conversion is given by

The resulting R, G, and B values must be truncated to the range (0, 255) if they fall outside. We note that Y-Cr-Cb is not a color space. It is a way of encoding the RGB information, and actual colors displayed depends on the specific RGB space used.

A common practice in color image processing, such as edge detection, enhancement, denoising, restoration, etc., in the luminance-chrominance domain is to process only the luminance (Y) component of the image. There are two main reasons for this: i) processing R, G, and B components independently may alter the color balance of the image, and ii) the human visual system is not very sensitive to high frequencies in the chrominance components. Therefore, we first convert a color image into Y-Cr-Cb color space, then perform image enhancement, denoising, restoration, etc., on the Y channel only. We then transform the processed Y channel and unprocessed Cr and Cb channels back to the R-G-B domain for display.

Normalized rgb

Normalized rgb components aim to reduce the dependency of color represented by the RGB values on image brightness. They are defined by

The normalized r, g, b values are always within the range 0 to 1, and

Hence, they can be specified by any two components, typically by (r, g) and the third component can be obtained from Eqn. (2.9). The normalized rgb domain is often used in color-based object detection, such as skin-color or face detection.

Hue-Saturation-Intensity (HSI)

Color features that best correlate with human perception of color are hue, saturation, and intensity. Hue relates to the dominant wavelength, saturation relates to the spread of power about this wavelength (purity of the color), and intensity relates to the perceived luminance (similar to the Y channel). There is a family of color spaces that specify colors in terms of hue, saturation, and intensity, known as HSI spaces. Conversion to HSI where each component is in the range [0,1] can be performed from the scaled RGB, where each component is divided by 255 so they are in the range [0,1]. The HSI space specifies color in cylindrical coordinates and conversion formulas (2.10) are nonlinear [Gon 07].

Note that HSI is not a perceptually uniform color space, i.e., equal perturbations in the component values do not result in perceptually equal color variations across the range of component values. The CIE has also standardized some perceptually uniform color spaces, such as L*, u*, v* and L*, a*, b* (CIELAB).

2.3.4 Digital-Video Standards

Exchange of digital video between different products, devices, and applications requires digital-video standards. We can group digital-video standards as video-format (resolution) standards, video-interface standards, and image/video compression standards. In the early days of analog TV, cinema (film), and cameras (cassette), the computer, TV, and consumer electronics industries established different display resolutions and scanning standards. Because digital video has brought cinema, TV, consumer electronics, and computer industries ever closer, standardization across industries has started. This section introduces recent standards and standardization efforts.

Video-Format Standards

Historically, standardization of digital-video formats originated from different sources: ITU-R driven by the TV industry, SMPTE driven by the motion picture industry, and computer/consumer electronics associations.

Digital video was in use in broadcast TV studios even in the days of analog TV, where editing and special effects were performed on digitized video because it is easier to manipulate digital images. Working with digital video avoids artifacts that would otherwise be caused by repeated analog recording of video on tapes during various production stages. Digitization of analog video has also been needed for conversion between different analog standards, such as from PAL to NTSC, and vice versa. ITU-R (formerly CCIR) Recommendation BT.601 defines a standard definition TV (SDTV) digital-video format for 525-line and 625-line TV systems, also known as digital studio standard, which is originally intended to digitize analog TV signals to permit digital post-processing as well as international exchange of programs. This recommendation is based on component video with one luminance (Y) and two chrominance (Cr and Cb) signals. The sampling frequency for analog-to-digital (A/D) conversion is selected to be an integer multiple of the horizontal sweep frequencies (line rates) fh,525 = 525 = 29.97 = 15,734 and fh,625 = 625 × 25 = 15,625 in both 525- and 625-line systems, which was discussed in Section 2.2.3. Thus, for the luminance

  • fs,lum = 858 fh,525 = 864 fh,625 = 13.5 MHz

i.e., 525 and 625 line systems have 858 and 864 samples/line, respectively, and for chrominance

  • fs,chr = fs,lum/2 = 6.75 MHz

ITU-R BT.601 standards for both 525- and 625-line SDTV systems employ interlaced scan, where the raw data rate is 165.9 Mbps. The parameters of both formats are shown in Table 2.1. Historically, interlaced SDTV was displayed on analog cathode ray tube (CRT) monitors, which employ interlaced scanning at 50/60 Hz. Today, flat-panel displays and projectors can display video at 100/120 Hz interlace or progressive mode, which requires scan-rate conversion and de-interlacing of the 50i/60i ITU-R BT.601 [ITU 11] broadcast signals.

Table 2.1 ITU-R TV Broadcast Standards

Standard

Pixels

Lines

Interlace/Progressive,Picture Rate

Aspect Ratio

BT.601-7 480i

720

486

2:1 Interlace, 30 Hz (60 fields/s)

4:3, 16:9

BT.601-7 576i

720

576

2:1 Interlace, 25 Hz (50 fields/s)

4:3, 16:9

BT.709-5 720p

1280

720

Progressive, 50 Hz, 60 Hz

16:9

BT.709-5 1080i

1920

1080

2:1 Interlace, 25 Hz, 30 Hz

16:9

BT.709-5 1080p

1920

1080

Progressive

16:9

BT.2020 2160p

3840

2160

Progressive

16:9

BT.2020 4320p

7680

4320

Progressive

16:9

Recognizing that the resolution of SDTV is well behind today’s technology, a new high-definition TV (HDTV) standard, ITU-R BT.709-5 [ITU 02], which doubles the resolution of SDTV in both horizontal and vertical directions, has been approved with three picture formats: 720p, 1080i, and 1080p. Table 2.1 shows their parameters. Today broadcasters use either 720p/50/60 (called HD) or 1080i/25/29.97 (called FullHD). There are no broadcasts in 1080p format at this time. Note that many 1080i/25 broadcasts use horizontal sub-sampling to 1440 pixels/line to save bitrate. 720p/50 format has full temporal resolution 50 progressive frames per second (with 720 lines). Note that most international HDTV events are captured in either 1080i/25 or 1080i/29.97 (for 60 Hz countries) and presenting 1080i/29.97 in 50 Hz countries or vice versa requires scan rate conversion. For 1080i/25 content, 720p/50 broadcasters will need to de-interlace the signal before transmission, and for 1080i/29.97 content, both de-interlacing and frame-rate conversion is required. Furthermore, newer 1920 × 1080 progressive scan consumer displays require up-scaling 1280 × 720 pixel HD broadcast and 1440 × 1080i/25 sub-sampled FullHD broadcasts.

In the computer and consumer electronics industry, standards for video-display resolutions are set by a consortia of organizations such as Video Electronics Standards Association (VESA) and Consumer Electronics Association (CEA). The display standards can be grouped as Video Graphics Array (VGA) and its variants and Extended Graphics Array (XGA) and its variants. The favorite aspect ratio of the display industry has shifted from the earlier 4:3 to 16:10 and 16:9. Some of these standards are shown in Table 2.2. The refresh rate was an important parameter for CRT monitors. Since activated LCD pixels do not flash on/off between frames, LCD monitors do not exhibit refresh-induced flicker. The only part of an LCD monitor that can produce CRT-like flicker is its backlight, which typically operates at 200 Hz.

Table 2.2 Display Standards

Standard

Pixels

Lines

Aspect Ratio

VGA

640

480

4:3

WSVGA

1024

576

16:9

XGA

1024

768

4:3

WXGA

1366

768

16:9

SXGA

1280

1024

5:4

UXGA

1600

1200

4:3

FHD

1920

1080

16:9

WUXGA

1920

1200

16:10

HXGA

4096

3072

4:3

WQUXGA

3840

2400

16:10

WHUXGA

7680

4800

16:10

Recently, standardization across TV, consumer electronics, and computer industries has started, resulting in the so-called convergence enabled by digital video. For example, some laptops and cellular phones now feature 1920 × 1080 progressive mode, which is a format jointly supported by TV, consumer electronics, and computer industries.

Ultra-high definition television (UHDTV) is the most recent standard proposed by NHK Japan and approved as ITU-R BT.2020 [ITU 12]. It supports the 4K (2160p) and 8K (4320p) digital-video formats shown in Table 2.1. The Consumer Electronics Association announced that “ultra high-definition” or “ultra HD” or “UHD” would be used for displays that have an aspect ratio of at least 16:9 and at least one digital input capable of carrying and presenting native video at a minimum resolution of 3,840 × 2,160 pixels. The ultra-HD format is very similar to 4K digital cinema format (see Section 2.5.2) and may become an across industries standard in the near future.

Video-Interface Standards

Digital-video interface standards enable exchange of uncompressed video between various consumer electronics devices, including digital TV monitors, computer monitors, blu-ray devices, and video projectors over cable. Two such standards are Digital Visual Interface (DVI) and High-Definition Multimedia Interface (HDMI). HDMI is the most popular interface that enables transfer of video and audio on a single cable. It is backward compatible with DVI-D or DVI-I. HDMI 1.4 and higher support 2160p digital cinema and 3D stereo transfer.

Image- and Video-Compression Standards

Various digital-video applications, e.g., SDTV, HDTV, 3DTV, video on demand, interactive games, and videoconferencing, reach potential users over either broadcast channels or the Internet. Digital cinema content must be transmitted to movie theatres over satellite links or must be shipped in harddisks. Raw (uncompressed) data rates for digital video are prohibitive, since uncompressed broadcast HDTV requires over 700 Mbits/s and 2K digital cinema data exceeds 5 Gbits/sec in uncompressed form. Hence, digital video must be stored and transmitted in compressed form, which leads to compression standards.

Video compression is a key enabling technology for digital video. Standardization of image and video compression is required to ensure compatibility of digital-video products and hardware by different vendors. As a result, several video-compression standards have been developed, and work for even more efficient compression is ongoing. Major standards for image and video compression are listed in Table 2.3.

Table 2.3 International Standards for Image/Video Compression

Standard

Application

ITU-T (formerly CCITT) G3/G4

FAX, Binary images

ISO JBIG

Binary/halftone, gray-scale images

ISO JPEG

Still images

ISO JPEG2000

Digital cinema

ISO MPEG2

Digital video, SDTV, HDTV

ISO MPEG4 AVC/ITU-T H.264

Digital video

ISO HEVC/ ITU-T H.265

HD video, HDTV, UHDTV

Historically, standardization in digital-image communication started with the ITU-T (formerly CCITT) digital fax standards. The ITU-T Recommendation T.4 using 1D coding for digital fax transmission was ratified in 1980. Later, a more efficient 2D compression technique was added as an option to the ITU-T recommendation T.30 and ISO JBIG was developed to fix some of the problems with the ITU-T Group 3 and 4 codes, mainly in the transmission of half-tone images.

JPEG was the first color still-image compression standard. It has also found some use in frame-by-frame video compression, called motion JPEG, mostly because of its wide availability in hardware. Later JPEG2000 was developed as a more efficient alternative especially at low bit rates. However, it has mainly found use in the digital cinema standards.

The first commercially successful video-compression standard was MPEG-1 for video storage on CD, which is now obsolete. MPEG-2 was developed for compression of SDTV and HDTV as well as video storage in DVD and was the enabling technology of digital TV. MPEG-4 AVC and HEVC were later developed as more efficient compression standards especially for HDTV and UHDTV as well as video on blu-ray discs. We discuss image- and video-compression technologies and standards in detail in Chapter 7 and Chapter 8, respectively.

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