Video Indexing and Retrieval

Advancement in digitalization, video technology and communication networks made Internet as a huge repository for text, audio and video documents. In the years to come searching video content in Internet and playing it on a mobile is going to be the norm. Implementation of video search engine poses big challenge because of two reasons. First, size of the video is very big and the next reason is structure of video is not as explicit as text. A text document can be easily broken down i.e. parsed into paragraphs, lines, and words. Much of the video search engine concepts have evolved from text search engine. 

Video Analytics – I

    Today every nook and corner of public place is under surveillance camera. They work round the clock and for 365 days. Thus huge amount of video data is generated. Monitoring a video for more than 20 minutes is tedious to human beings and security officers mostly fail to detect abnormal activities. Thus there is a requirement for “machine assistance” to security officers. Watching video via Internet has become a norm. Popular video servers like YouTube, Dailymotion, and Metacafe provide video free of charge. Netflix is subscription based video server. Sixty hours of video are uploaded every minute in YouTube video servers alone [1]. Thus one can imagine the quantum of video content available in Internet. In the years to come more than half of Internet traffic will be due to video. We encounter problem choosing 'right' or cherry-pick the video from the huge pile of video scattered in Internet. Here to we require some form of “machine assistance” to ordinary viewer.

Video is made up of consecutive sequences of frames or images. Each image contain large amount of pixel information. Images offer very little prior structure to work with [2]. Earlier video databases were small and manual annotation was a possible solution. Today it is not a feasible solution. Present day computing power will manage huge size of data. But automatic analysis of video requires artificial intelligence. Video analytics is the baby step in that direction. Video analytics deals with extraction of information from video with the aid of machine assistance. Video processing means performing some image processing (like resampling or colour correction) on the video content. Most of the time extracted information is overlaid on the video for better human interpretation. Big data analytics is latest buzz word in technical world. Video analytics is considered as a subset of big data analytics.

Eighty years of entertainment video

Let us begin the post with a piece of information. In the Internet more than 50 percentage of traffic is due to video transmission. Colour movies emerged in early 1930s and it was the only source of video at that time. TV emerged in 1950s and added huge collection video to the world. The evolution of video from 1934 to 2014 (80 years) is to be discussed in this post. 

Video Classification

For a general public, a movie stored in video cassette or Video Compact Disc (VCD) is considered as a video. But a graduate in electronics engineering will quantify video as “sequence of still pictures (frames) that are displayed on screen in a very small time interval.” Video can be classified into three major categories namely; entertainment video, Industrial video and Surveillance video. As name implies entertainment video encompasses movies and TV programmes. As per the definition Black and White (B&W) silent movies come under the category of video.  But in this post it is restricted to colour talkies (colour movie with soundtrack). In manufacturing industries video cameras are extensively used to capture the production processes. Videos are analysed and features are extracted. These features are used as feedback in production processes. Machines fitted with camera replace human beings. They work 24x7, seven days a week without a holiday. As they don't require any pay rise, there is no strike in factories. The video content generated by machine vision systems can be branded as Industrial video. Most of the public places are monitored by a surveillance system. This cost effective method reduces the requirement of Beat Officers (police who are on patrol), aftermath of crime recorded video acts as clues to nail the culprits and acts as prosecution evidence in the court of law. Video content is increasing leaps and bounds. 

Gigapixel Images

A 1.8 gigapixel video surveillance camera named ARGUS was built jointly by DARPA (Defense Advanced Research Projects Agency) and the US Army. It is capable to pick out a sleeping dog in the Earth from the altitude of 20,000 feet (6 km).  In other words, it can resolve details as close to six inches. Quite amazing! The ARGUS can be attached to drone (unmanned aeroplane remotely controlled) and taken to a height of 20,000 feet to observe 25 square kilometres at any instant. Thus entire New York City can be brought into surveillance by two ARGUS attached drones to hover over the city.  The entire Manhattan is under observation 24x7.  

Like ARGUS, AWARE-2 is another gigapixel camera. It was used to study the behaviour of tundra swans present in Pungo Lake, USA [1]. With AWARE-2 camera gigapixel snapshot was taken [1] and found 656 swans swimming in the lake and 27 flying above the lake at that instant. Scientists can use the snapshot to track individual swan or study the swans' flock (group) behaviour.  Existing scanning panoramic camera cannot achieve this feat. Use of gigapixel is not confined to ecology alone. It can be applied in fields like urban planning, traffic control, forestry, archaeology and so on.

Gigapixel snapshot of Budapest city, Hungary. Inset: two landmarks building in the city. Zooming operation  on gigapixel image helps us find the buildings. Image courtesy www.photographyblog.com [7]

Digital Society

Today we are living in digital society.  Due to rise of digital technology basic fabric of society is almost changed.  For example, in India twenty years back, cellular phones were bulky and were a status symbol. Users seldom revealed their cell phone numbers, as incoming calls were also metered or charged. Now sleek cell phones are the norm and each Indian possesses a cell phone. The word “Privacy” is unknown to Indians earlier, now they practice it. Thus digital technology has started shaping the society. Earlier posts like tribal, agrarian and industrial society discussed length and breadth of technology-society association. In this post emphasis will on the digital technology.

Technology has so advanced that by click of a button we get any information we want. Is this means we are far superior than our ancestors? The answer is Yes and No. Internet has become giant virtual library. Internet contains lot of text information and videos which are structured, static and suited for one way communication (author to reader or viewers). We have honed our skills to surf and sift information from Internet. By this reason, yes is the answer.  We lack the ability to communicate, leave alone extracting information from fellow humans. This is because human to human interaction is much unstructured, dynamic and two way (author to listener, and listener to author). Communication is complex and lot of training is required to excel [1].  We had fishermen, who observed the cloud and predicted the weather and other vital information required for fishing. In agrarian society humans acted as an information repository.  

Today society banks more on man-machine interaction rather than human-human interaction. This may result in assorted individuals (Man, woman, boy and girl) instead of a family (Father, mother, son and daughter) staying in a house. Thus our challenge is to make the digital devices to assist us in improving human-human interaction.

Digital Devices

We come across devices like digital clock, mp3 player, audio compact disc player, Video Compact Disc (VCD) player, Digital Video Disc (DVD) player, computer, cellular phone, Tablet and so on. These diverse devices can be brought under umbrella of digital devices. Analog devices operate on analog signals which are continuous in time as well as in value. Digital signals are discrete in time and can hold only two values i.e. ones and zeros. Dominance of digital devices over analog counter parts is due to the axis of IC fabrication, Computer and Internet.

Figure 1. Digital Clock | Image courtesy Wikipedia

Pre-Digital Society

In earlier posts tribal, agrarian and industrial society were discussed. In this post, post-industrial society will be discussed.  One may wonder about the title and they may have an urge rename it as “information society.” Lot of definitions exists for information society [1]. Industrialization of a nation is gauged by amount of petroleum or energy usage. Likewise two metrics can be used for information society. First is extent of information usage. Next is economy of a country should hang on information production (for example movies, music videos, books, information equipments etc). So it is better to leave the discussion on information society to economists and social scientists. The present digital society may lead us to information society. 

       This is one part of series of articles dealing with evolution of societies. In the present society we encounter lot of displays. These articles try to find out the answer for “rise of display and smart devices.” I firmly believe engineering, technology, economics and end user behaviour has to be studied as a whole and not in isolation.  That is why article like this appear in this blog which is dedicated to Digital Image Processing. 

The post-industrial society can be broadly classified into pre-digital and digital society. In Industrial society electricity was used a form energy like coal and petroleum. In digital society, electricity is used to transport information. In digital society, information production, distribution and consumption happens in binary form i.e. ones and zeros. In pre-digital society analog signals were used and optional use of electronic components.

Smart devices and Learning - Part II

In the earlier post we tried to understand what is learning. We had a glimpse of tribal and agrarian society. This post deals with industrial society. Characteristics of industrial society are usage of coal, petroleum, steam engine and urban dwelling.

The objective of this three-part post is to understand the motives behind the veneration of gadgets (i.e. smart phones) usage, and education that emphasize reading and writing skills. 

Genesis of industrial society

Extensive use of coal and steam engine by the society can be regarded as the starting point industrialization. Earlier times steam engines were used to draw water from iron mines.  Their efficiency improved and used in locomotives. First successful locomotive using steam engine was devised by James Watt. 

Britain was in the forefront in industrial revolution as iron and coal was extensively available. Climate conditions in England helped to start textile industry. People used to spun, and then weave it to make cloth at home. It was a labour intensive task.  In a textile factory, conversion of cotton into cloth was carried out with machines with the help of few workers. Textile mills were very productive and were able to produce cloth economically. The machines like spinning jenny, water frame, flying shuttle, spinning mule required huge space to house them. Thus buildings were exclusively built. Thus first time in history mass production of goods was carried out. Till then it was production of goods by masses.  (Above statement is an adaptation from Mahatma Gandhi's famous quote “We don't require mass production but production by masses”).

A mill engine from Stott Park Bobbin Mill, Cumbria, England.  Image source: Wikipedia

Smart devices and Learning - Part I

Today smart phones are fashion icons. It is regarded as a height of accomplishment for citizens of information society. Living without broadband access is unthinkable. Any time and any where is the mantra of information-citizens. Few questions arisen in my mind. Whether I use electronic gadgets to enhance my cognitive ability (like use of knife) or to overcome my inability (use of crutches)? Are we smarter than our ancestors? A person who adapts to the environment and at the same time achieves his or her goal is termed as smart. In today’s environment, accessing electronic gadgets is associated with smartness. Adaptation is otherwise called as learning.

    We spend much time on how to display information in mobile phones, tablets, computer screens and TV.  We seldom think about "Why we have to display the content." This post deals the evolution and need to display information.

Learning and artifacts

Any living being (bacteria to human beings) adapt to environment. It means environment is observed, changes in stimuli are noted and responses modified to suit environment. Put is simple way “living beings learn.” What is learned by yester generation living beings are passed to future generation is passed down. It may be in the form genetic mutation or in bevavioural change. Present day bacteria are   resistant to anti-biotic medicines due to mutations (for more information google “drug resistant Tuberculosis”). Wild grasses due to genetic mutations become rice, wheat and so on. Long long ago few wolves started interacting with human beings. The relationship was beneficial to both of them. Slowly aggressiveness of wolves mellowed, and gave birth to new species called “Dogs.” In the stated case change in behaviour has resulted in change in anatomy. Adaptation to environment makes changes in brain connections in other cases.  Reorientation in neuron changes makes marked changes in behaviour.  Storage of information in brain is not inherently permanent. Unless efforts are taken information stored in brain becomes unrecoverable. Our ancestors somehow learned to store the information in external objects. This is blessing as well as curse for us.  

The biggest difference between humans and other living beings is that they are able to create objects.  The entire human history can be divided into four categories based on amount of storage of information on external objects: tribal society, agrarian society, industrial society and information society. 

Figure 1. (Left) Tribal society (right) Ploughing - root for agrarian society

Abstract DIP Model - Part III (Rise of Display Devices)

An abstract model for DIP came across my mind. It contains four sections viz; Acquire, Transfer, Display and Interpret. This post is belongs to Display section of abstract model. Refer earlier posts for Acquire and Transfer sections. This article deals with the rise of display devices and social factors that helped for the phenomenal rise. This post does not elaborate the principles of display technologies. 

Display systems are one of the wonders of the 21st century. Origins of this wonder are blood soaked. Brutal World War II killed and maimed millions of people and destroyed many cities. Scientists of fighting nations, worked day and night to improvise their countries' war machine with new inventions. Two such inventions namely, computing machines and radar display units helped display systems to blossom into a multi-billion dollar industry. After the war, radar display units were modified to show pictures. This resulted in the birth of Television. After the war bulky computers paved the way to personal computers that possess Video Display Unit (VDU) as output section.

In earlier days Cathode Ray Tube (CRT) was “the display technology.” The shadow-mask colour CRT came to the market way back in 1950. Yet most information displays and imaging applications used monochromatic displays until 1970. Microprocessors in arrived in the market mid 1970s and facilitated the introduction of CRT colour displays for computers. The processing power of microprocessors enabled to encode, manipulate colour in these devices. In the next twenty five years, exponential growth in terms of technology and application of colour occurred [1]. 

An ideal display system is expected to possess the following features, namely, high contrast, maximum brightness, high resolution and lower cost. Moreover, consumer expects larger display, larger colour gamut and better colour saturation [1]. Customer expectations for color display have risen in rapid pace, driving the development of display technologies and allied colour control and image processing algorithms [1]. Till date no display technology possesses such venerated features. Thus   diverse colour display technologies have evolved to support wide variety of applications. It is imperative to build taxonomy (i.e. classification) of display systems and technologies to comprehend them.

VDU classification

Classification of Visual display units can be based on duration, number of viewers, technology incorporated and targeted applications. Movie screens are viewed for minimum 90 minutes and in another extreme digital signage are viewed for a maximum of 90 seconds. VDU can be broadly divided into public-viewer, multi-viewer and mono-viewer displays based on number of simultaneous viewers. Display systems can be segmented into the following categories based on underlying technologies used. They are CRT, Digital Light Processing (DLP), Plasma Display Panel (PDP), Liquid Crystal Display (LCD), Light Emitting Diode (LED), e-ink (electronic ink). Targeted applications like entertainment, consumer, automotive, informative etc can be the basis of classification. I strongly feel classification like this is very fuzzy.  

As categorization based on number of viewers is not standardized, let us define in the following way. Public-viewer displays are capable of exhibiting the content to more than 100 people at a time. The movie theatre projection systems perfectly fall into this category. Public address systems are used to reach large gathering of audience via loud speaker. Likewise public-viewer display reaches large viewer via projection systems. Multi-viewer display is suited for few people at a time. Television is the best example.  As the name suggests mono-viewer displays are suited for single viewers. Instrument panels, automotive panels, Personal Digital Assistant (PDA), e-readers, tablets, mp3 player, cellphone and digital camera display falls into this category.

Display Characteristics 

Comparison between display technologies can be performed based on the following parameters; brightness, contrast, field of view, colour gamut, resolution, physical dimension and cost [2]. Brightness is light perceived by the human eye and luminance refers to the amount of light emitted by a source say VDU. Luminance is measured in Candela per meter squared (cd/m2). Dynamic range refers to the luminance difference between white and black pixels [2]. Contrast also refers to the difference in range but with respect to images. But Contrast and dynamic range is used interchangeablely in every day use. I believe contrast is connected to perception of human eye and dynamic connected to luminance (i.e. measurable quantity).  Field of view measured in degrees, determines the number of people can view the display device simultaneously. Colour gamut is an enclosed triangle in chromacity diagram. Area of triangle represents the VDU unit's ability to display range of colours. For details refer to the earlier post of this blog [3]. Resolution refers to the number of pixels and provides rough estimates of the height and width of image.

Public-viewer Display

Rapid proliferation of digital technology has affected conventional projection of movies through films.  Today digital projection systems are widely used. It is a combination of projector and cinema screen which is made up of silver halide. Digital projectors can be broadly classified into three categories based on technology; DLP, Liquid Crystal on Silicon (LCoS) and Grating Light Valve (GLV). DLP is made up of tiny mirrors which can move -10 to 10 degrees [4]. Small rotation of mirror causes grey scale values. LCoS is similar to LCD and acts as a 'Window Blinds.' Thus passage of light is controlled by LCoS. Size of silver screen will be in the range of 14 meters to 20 meters (diagonal). The lamps used to display pictures on the screens consume around 1kW of power. The resolution of ranges starts from 2K, 4K and reaches 8K. The 2K resolution is equivalent to High Definition Television (HDTV) resolution. The luminance and dynamic range is very much higher than Television systems. Movies are viewed in dark environment. These reasons make 2K resolution is sufficient for the average movie patron. If a patron sit in the front rows and watch movies then he or she can spot the difference between 2K and 4K systems. Patron sitting in last row cannot distinguish the difference unless he or she is a digital cinema expert [citation required]. 

Figure 1. Display device classification. (a) Public-viewer display - Movie screen (b) Multi-viewer display (c) Mono-viewer display - smart phone       Image Courtesy: Wikipedia

Multi-viewer Display

Television (TV) is the apt example for multi-viewer display. At present, the preferred technology for production of TV set is CRT. But within a decade, other technologies like LCD or PDP may take over.

•  Merits of CRT technology are as follows. (i) It is a century old technology. So, it is very matured. (ii) Display is bright. So, it is not very much affected by stray external lights. (iii) Long life and have high reliability. (iv) Inexpensive. Cost per pixel is lowest among the display technologies. (v) In 1980s, 640 x 480 was high resolution. Today HDTV (1920x1080?) resolution CRTs are available. (vi) Viewing angle is high. This enables multiple people to view the TV comfortably.

• The demerits of CRT technology are as follows. (i) It is very bulky i.e. voluminous. Weight is also high compared to other display technologies. (ii) Size of the screen is limited. Seeing 36” and above is rarity or even a technical wonder. (iii) Consumes high power. Minimum power consumption is 100W per hour. (iv) Emits electromagnetic radiation.

 Introduction of HDTV standards (in USA, Japan) created a need for larger display. Next proliferation of digital satellite broadcasting helped to get crisp video signals. Satellite TV channels earmarked for sports and movies created a need for large screen. Conventional large CRT screens spoilt the aesthetics of the room in which it was housed. Slim form factor of new technologies resembled wall hanging paintings. Thus Techno-savvy people preferred to shift their loyalty to LCD and PDP technologies. They were ready to shell out extra money for aesthetics.

The other merits of new technologies are as follows, (i) Rich colour (ii)  Consumes less power (particularly LCD) (iii) Suited for high resolution (iv) Viewing angle is  high for PDP and relative low for LCD (v) No electromagnetic radiation (vi) Suited for mobile environment (like in car, van, lorry i.e. truck)

Mono-viewer Display

In earlier days, computer screens were the dominant mono-viewer display.  Automotive and industrial instruments used mechanical dials only. Rapid automation and digitization of the industry paved way for electronic displays. Rapid increase in oil (i.e. gasoline) price made automobile manufacturers to opt for fuel efficiency. All mechanical controls were replaced with electronic controls and electronic displays were introduced to assist the drivers with timely information (for example GPS enabled car).

Prices of computers were falling and after the advent of Web, non-programmers started using the computers extensively. A huge pool of volunteers put up required content for the Web. The content is in digital form. Internet connects geographically separated computers and enables seamless flow of digital data between them. Thus digital delivery favoured display of information, rather than print form of information [5].

Parallelly, cell phones which were built to transfer speech were improvised to transfer digital data and act as low-end digital cameras. In earlier days, most of the mobile displays were expected to display alphanumeric and limited graphical icons. This was satisfied with monochromatic screens. The requirement for colour screens emerged because of the need of view finder in digital camera and on-board monitors to display captured pictures. The requirement was emboldened by picture phones and embedded digital camera in cell phone [1]. As the mobile phone user base far exceeds the Personal computer user base, large amount of small-screen display are manufactured. Internet enabled smart phones were introduced and size of the screen increased to 5” (pocket size of an adult). These screens have very good resolution to facilitate reading.

Seven inch electronic readers help to house electronic books. E-readers can hold as much as e-books as memory can hold. Font size of text can be customized. Search facility is possible [6]. It is a very handy method for globe trotters to take their favourite book shelf.  Thus E-readers made a niche market for itself. The E-readers extensively rely on e-ink technology. It is a bistable technology. So power is not required to display content. Power is used only when a page is turned (i.e. previous or next). It consumes very low power. Present day battery technologies augment e-reader dominance.  Full colour e-ink is emerging. Present day e-readers are Sony reader, Amazon Kindle and Barnes & Noble Reader. 

Mobile devices need display units that are capable of visibility under diverse illumination environments, small form factor, less power consumption and longer battery life. Under this conditions LCD is the dominant technology. Organic Light Emitting Diode (OLED) is a promising technology [1].

Display of high quality image is a great engineering challenge.  The colour gamut of mobile colour displays is compromised compared to TV and computer screens. This is due to the limitations of mobile computing power. Mobile devices have limited processing resources. They are expected to provide ability to handle out-of-gamut colours, contrast stretching and saturation enhancement. This has to be carried out by image processing algorithms [1].

Source

• [1] Louis D. Silverstein, “Color Display Technology: From Pixels to Perception,” The Reporter, vol. 21, no. 1, pp. 1–12, Feb. 2006.
• [2] Paul Anderson, “Advanced Display Technologies,” JISC Technology & Standards Watch.
• [3] A to Z of Digital Image Processing: Abstract DIP Model - Part III (Science of colour) [Online] http://diwakar-marur.blogspot.in/2014/01/abstract-dip-model-part-iii-science-of.html
• [4] A to Z of Digital Image Processing: Digital Cinema Projection Technologies [Online] http://diwakar-marur.blogspot.in/2013/05/digital-cinema-projection-technologies.html
• [5] ADT Michael Kleper, Advanced Display Technologies, A Research Monograph of the Printing Industry Center at RIT, Rochester, New York, USA, October 2003.
• [6] Eva Siegenthaler, Laura Schmid, Michael Wyss and Pascal Wurtz, “LCD vs. E-ink: An Analysis of the Reading Behavior,” Journal of Eye Movement Research, 5(3):5, pp. 1–7, 2012.

Note

All articles related to science and technology glamourize the technological feats. This 2000 worded article is written in a technology deemphasised and human centered approach. In this approach, technology is seen as a tool to achieve human goals and technology per se is not human goal. I want to acknowledge Mr. Varun Vinod for doing proof correction for this article.

Abstract DIP Model - Part III (Science of colour)

 An abstract model for DIP came across my mind. It contains four sections viz; Acquire, Transfer, Display and Interpret. This post is third part of the series. First and second part discussed Acquire and Transfer blocks. Please refer November 2013 post in my blog for detailed introduction.

A colour picture is mapped into matrix of numbers, compressed and stored as an image file. At the time of display on the computer screen, files are decompressed and numbers are remapped into pixels. In the case of printing, numbers are remapped into dots. Thus display block is connected with remapping of numbers into pixels. To understand the functioning of the block one should know following things; Science of colour, Screen technologies, Printing technologies and Mapping algorithms. In this post only science of colour will be discussed. 

Science of Colour

Light enters into eye via cornea and it is focused by lens and reaches the retina. It lies on the inside of eye wall. It contains two types of photoreceptive cells called cones and rods. Six to seven million cones lie near the central portion of retina called fovea. The maximum absorption occurs at 430, 530 and 560 nm and they can be referred as red, green and blue cones. Approximately 65% of cones are sensitive to red, 33% are sensitive green and remaining 2% is sensitive to blue color. But blue cones are extremely sensitive.  Rods are around 70 million in numbers and they are spread all over retina. The rods are very sensitive to light and low levels of illumination are sufficient to function. Cones require bright light to function. That is why a brightly coloured flower in the daylight appears colourless in the moon light. 

Visible range exists between 400 nm to 700 nm. The entire visible spectrum is divided into 10 nm wide band and represents spectral power. Thus a collection of 31 bands may be used to describe a particular spectral density distribution. Or else simply use red, green and blue primaries to represent a colour. Isaac Newton said, “Indeed rays, properly expressed, are not colored.” In the real world only Spectral Power Distributions (SPDs) exist and colour is perceived by our eyes and brains [1]. In image processing we use simple red, green and blue to describe colour rather than 31 bands SPD.

Any image acquiring device (still or video camera) as well as image display device should have the same spectral response as the human eye. With prevailing technologies, spectral responses of devices can be made as close to human visual system. Radiance means amount of light is emitted from a source. It is objectively measurable quality. Luminance means light intensity perceived by eye. Infra red (IR) radiance can be measured but IR ray luminance will be zero as eyes can’t perceive IR.

CIE MODEL

The science of colorimetry tries to find the relationship between SPD and perceived color. Way back in 1931 International Colour Commission (in French it is Commission Internationale de L’Éclairage   (CIE)) developed a tristimulus model that have X, Y and Z primaries. In this Y corresponds to Luminance of the light. These XYZ tristimulus primaries are developed by colour matching experiment. X, Y and Z form a colour volume. If the luminance component is ignored then it becomes a two-dimensional picture. Representation of colour without the luminance component is created by following way. 

x=X/(X+Y+Z) and y= Y/(X+Y+Z)

A plot between x and y is done and it comes in the shape of shark-fin as in Fig. 1. This is called chromaticity diagram. It is the desired two-dimensional plot without the presence of luminance.

Figure 1. The chromaticity diagram (shark-fin shaped) 

contains colour gamut (triangular shaped) of  a device.

 Camella red is seen in white colour.  Image courtesy: Wikipedia

Gamut:   A triangle inside the chromaticity diagram provides a fair idea about the colour reproduction capability of display or capture device. The gamut is device dependent entity and size varies among devices. Larger gamut is always preferable. At present no device is capable to capture or display all colours shown in chromaticity diagram. For example, Camellia flower shown in Fig. 2, lies outside the gamut of given device [2].  It is shown in the Fig.1 as white spot. As a result, the display device fails to faithfully reproduce the original colour of camellia flower. It will use nearest equivalent colour to represent the flower.

Figure 2. The Camella flower
 Image courtesy: www.techmind.org

RGB MODEL

Later Red, Green and Blue colours are used as primaries and it is called RGB model. This colour space is used for display devices. Any colour can be created in display devices by adding red, green and blue colour in a proportion. This is called Additive Mixing. The normalized values of primaries range from 0 to 1. Thus it forms a perfect colour cube. This colour model is device dependent. It implies a when a RGB value is passed to display devices they need not give guarantee to produce the same colour. Slight variation in colour is expected. Using RGB colour space it is not possible to create all colours. But the colours produced by RGB model are sufficient for practical purposes.

  A variety of devices that is used to capture image and display follow power law equation i.e light intensity is proportional to some power of signal amplitude or pixel value. The exponent component value varies from 1.8 to 2.5.  Pre-multiplying by inverse of exponent is called gamma correction. Appearance of image in a Cathode Ray Tube (CRT) screen will be darker than the original image without the involvement of gamma correction. Introduction of gamma makes RGB colour space as non-linear.  To differentiate linear and non-linear RGB a suffix apostrophe is added with each colour component. Thus R' represents gamma corrected red colour. Many books fails to understand the important difference and use R and R' interchangeably. 

Hewlett-Packard (HP) and Microsoft developed a new colour space, sRGB, which is specifically suited for operating systems and Internet. In sRGB the gamma value is 2.2. It is not a perfect colour space as CIE's XYZ. But it is representative of the majority of the devices on which colour is viewed by average computer user. The new colour system helps to create substantial degree of consistency among the various devices [3].

Figure 3.  (a) Additive colours (Light) (b) Subtractive colours (Pigments)

CMYK MODEL

The RGB colour model that is based on additive mixing of colours is suited for printing purposes. Printing press produces an image by reflective light. This falls into category of colour generation by subtractive process. Refer Fig. 3 for further understanding. The secondary colours that are used in printing are Cyan, Magenta and Yellow. Cyan pigment will absorb red light and reflect all other colours. Likewise magenta will absorb green and yellow will absorb blue. Black colour may be produced by cyan, magenta and yellow.   But it will be muddy-looking black. Carbon black that is used to produce black in printing is cheaper than coloured pigments. These are the two reasons that make four colour model as standard. The colours are cyan, magenta, yellow and black (CMYK) [4].

OTHER MODELS

The RGB and CMYK are the primary colour models used for display and printing. Few other models like YUV, YIQ, CIELAB, CIELUV, HSI, HLS, and HSV color models are used for specific applications. 

YUV MODELS

Television broadcasting carries colour pictures from one place to another and projects the pictures on Television screens. Colour reproduction in screens is performed using additive mixing. Thus obvious choice of colour space should be RGB. But Television and Video systems seldom use the RGB as it is bandwidth inefficient. Next, when colour television broadcast was introduced in way back in 1950s there were lots of B&W TV sets. To cater to monochrome TVs the colour broadcast was made backward compatible. It means old B&W TVs can receive colour TV signals but B&W signals only projected on the screen. Our eye is sensitive to luminance variations than chrominance variations. Thus RGB colours were transformed to YUV colour space. Where Y stand for luminance and U and V contain colour information. Video engineers piggy-backed colour information on the TV composite signal to conserve bandwidth. Thus colour TV signals consumed same bandwidth as monochrome signals (6 to 7 MHz). This was an engineering marvel. The YUV space is used by PAL (Phase Alternation Line, Europe and Asia), NTSC (National Television System Committee, USA), and SECAM (French system) colour TVs [5],[6].

The conversion equations between R'G'B' and YUV is given below 

Y = 0.299R ́ + 0.587G ́ + 0.114B ́

U= – 0.147R ́ – 0.289G ́ + 0.436B ́

V = 0.615R ́ – 0.515G ́ – 0.100B ́

YIQ colour space is a variant of YUV and used in NTSC systems. Here I stand for In-phase and Q stands for quadrature. Digital Video standard ITU-R BT.601 (International Telecommunication Union - Recommendation) uses YCbCr which again a scaled version of YUV. Here Cb and Cr represent chrominance signals. The High Definition Television (HDTV) uses ITU-R BT.709 standard. The primaries used in this colour space closely correspond to the contemporary monitors.

Source

1.  A Guided Tour of Color Space [Online] http://www.poynton.com/papers/Guided_tour/abstract.html
2.  Introduction to colour science [Online] http://www.techmind.org/colour/
3. sRGB: A Standard for Color Management - NEC SpectraView - spectraview.nec.com.au/wpdata/files/40.pdf (PDF, 1.1 MB)
4. Rafael C. Gonzalez and Richard E.Woods, Digital Image Processing, 2nd edition, Pearson Education.
5. Keith Jack, Video Demystified: A Handbook for the Digital Engineer 4th edition, Newnes Publishers
6. Noor A. Ibraheem,  Mokhtar M. Hasan,  Rafiqul Z. Khan,  Pramod K. Mishra, "Understanding Color Models: A Review", ARPN Journal of Science and Technology, vol. 2, no. 3, April 2012, pp.265 -- 275.