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Colour Light and Spectra

Colour Light and Spectra. Visible light is an electromagnetic wave in the 400 nm - 700 nm range. Most light we see is not one wavelength, it's a combination of many wavelengths. The range of frequencies is called a spectrum. Colour Human Retina.

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Colour Light and Spectra

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  1. Colour Light and Spectra • Visible light is an electromagnetic wave in the 400 nm - 700 nm range. • Most light we see is not one wavelength, it's a combination of many wavelengths. • The range of frequencies is called a spectrum.

  2. Colour Human Retina • The eye is like a camera with light sensitive neurons • Each neuron is either a rod or a cone. • Rods are only sensitive to luminance & not sensitive to colour.

  3. Colour Cones and Perception • Cones come in 3 types: red, green and blue. Each responds differently to various frequencies of light. The following figure shows the spectral-response functions of the cones and the luminous-efficiency function of the human eye. • The colour signal to the brain comes from the response of the 3 cones to the spectra being observed.

  4. Colour CIE Chromaticity Diagram • In 1931, the CIE defined three standard primaries (X, Y, Z). The Y primary was intentionally chosen to be identical to the luminous-efficiency function of human eyes. • CIE: Commission Internationale de l’Eclairage (International Commission on Illumination)

  5. Colour RGB Colour Model for CRT Displays • CRT displays have three phosphors (RGB) which produce a combination of wavelengths when excited with electrons

  6. Colour CMY Colour Model • Cyan, Magenta, and Yellow (CMY) are complementary colors of RGB. They can be used as Subtractive Primaries. • CMY model is mostly used in printing devices where the color pigments on the paper absorb certain colors (e.g., no red light reflected from cyan ink).

  7. Colour Conversion between RGB & CMY • White is represented as (1, 1, 1) in RGB and (0, 0, 0) in CMY

  8. Colour YUV Colour Model in Video • Initially, for PAL analog video, it is now also used in CCIR 601 standard for digital video • Y (luminance) is the CIE Y primary Y = 0.299R + 0.587G + 0.114B • Chrominance is defined as the difference between a colour and a reference white at the same luminance. It can be represented by U and V -- the colour differences. U = B - Y V = R - Y • If U = V = 0 then black/white image i.e. No chrominance • In actual PAL implementation: • U = 0.492 (B - Y) V = 0.877 (R - Y) • Eye is most sensitive to Y. In PAL, 5 (or 5.5) MHz is allocated to Y, 1.3 MHz to U and V.

  9. ColourYUV Composition Y U V

  10. ColourYCbCr Colour Model in Video • The YCbCr model is closely related to the YUV, it is a scaled and shifted YUV: • Y = 0.299R + 0.587G + 0.114B + 16 • Cb = -0.148R - 0.291G + 0.439B + 128 • Cr = 0.439R - 0.368G - 0.071B + 128 • The chrominance values in YCbCr are always in the range of 0 to 255 • YCbCr is used in JPEG and MPEG

  11. Colour YIQ Colour Model in Video • YIQ is used in NTSC color TV broadcasting, it is downward compatible with B/W TV where only Y is used. • I is the orange-blue axis, Q is the purple-green axis. I and Q axes are scaled and rotated R - Y and B - Y (by 33 degrees clockwise). • I = 0.877(R - Y) cos 33 - 0.492(B - Y) sin 33 Q = 0.877(R - Y) sin 33 + 0.492(B - Y) cos 33 • Namely, • I = 0.74(R - Y) - 0.27(B - Y) = 0.596R - 0.275G - 0.321B Q = 0.48(R - Y) + 0.41(B - Y) = 0.212R - 0.523G + 0.311B • The YIQ transform: • Eye is most sensitive to Y, next to I, next to Q. In NTSC broadcast TV, 4.2 MHz is allocated to Y, 1.5 MHz to I, 0.55 MHz to Q.

  12. Colour Types of Colour Video Signal • Component video -- each primary is sent as a separate video signal • The primaries can either be RGB or a luminance-chrominance transformation of them (e.g., YIQ, YUV). • Best color reproduction • Requires more bandwidth and good synchronization of the three components • Composite analogue video -- colour (chrominance) and luminance signals are mixed into a single carrier wave. Some interference between the two signals is inevitable. • Composite digital video -- colour (chrominance) and luminance bits are mixed into a single stream. • S-Video (Separated video, e.g., in S-VHS) -- a compromise between component analog video and the composite video. It uses two lines, one for luminance and another for composite chrominance signal.

  13. Composite Analogue VideoSystem Diagram

  14. Composite Analogue VideoInterlacing

  15. Composite Analogue Video:Line & Field Syncs of even and odd fields and active Image area

  16. Composite Analogue VideoCarrier Burst (PAL)

  17. Composite Analogue VideoLuminance & Colour Video Signal Spectra • The luminance signal is located at nFL where n=0,1,2,3… and FL is the line frequency. • The chrominance signal is inserted between the gaps of the luminance signal at nFL/2 where n is odd integer.

  18. Composite Analogue VideoCombined Luminance & Chrominance Signals in Spectra • The bandwidth of the luminance signal is 5.5MHz. • The bandwidth of the chrominance signal is 2.0MHz. • The two bandwidths coincide.

  19. Composite Analogue Video: Phase Alternation Line (PAL)Sequential Colour avec Memoire (SECAM) • 625 scan lines per frame, 25 frames per second (40 msec/frame) • Aspect ratio 4:3 • Interlaced, each frame is divided into 2 fields, 312.5 lines/field • Uses YUV (YCbCr) color model

  20. Composite Analogue Video: North American Telecom Standards Committee (NTSC) • 525 scan lines per frame, 30 frames per second • Aspect ratio 4:3 • Interlaced, each frame is divided into 2 fields, 262.5 lines/field • 20 lines reserved for control information at the beginning of each field • So a maximum of 485 lines of visible data • Laserdisc and S-VHS have actual resolution of ~420 lines • Ordinary TV -- ~320 lines • Each line takes 63.5 microseconds to scan. Horizontal retrace takes 10 microseconds so the active line time is 53.5 microseconds. • uses YIQ color model

  21. Composite Analogue VideoNTSC, SECAM, PAL • Phase Shift Keying is used to carry the two colour signals on 2 phases (0º and 90º) with the same carrier frequency. • For NTSC, (R-Y) is sent on one phase and (B-Y) on the other phase. • For PAL, (R-Y) is sent on one phase and (B-Y) on the other phase. The phases are alternated from one line to another to compensate for phase errors. • For SECAM, (R-Y) is sent on one line and (B-Y) on the other line and both colours used for each line.

  22. Composite Digital VideoAdvantages • Advantages: • Direct random access --> good for nonlinear video editing • No problem for repeated recording • No need for blanking and sync pulse • Almost all digital video uses component video

  23. Composite Digital Video Colour Subsampling • 4:4:4 --> Colour signal is not subsampled e.g. (Cr0, Cb0, Y0)(Cr1, Cb1, Y1)(Cr2, Cb2, Y2)(Cr3, Cb3, Y3)(Cr4, Cb4, Y4) ... • 4:2:2 --> Horizontally subsampled colour signals by a factor of 2. Each pixel is two bytes, e.g., (Cb0, Y0)(Cr0, Y1)(Cb2, Y2)(Cr2, Y3)(Cb4, Y4) ... • 4:1:1 --> Horizontally subsampled by a factor of 4 • 4:2:0 --> Subsampled in both the horizontal and vertical axes by a factor of 2 between pixels. • Note on notation: • 4:2:2 for every four luminance pixels in a row there are 2 red and 2 blue) • 4:2:0 for every four luminance pixels in a row there are 2 red/blue and 0 blue/red)

  24. Composite Digital Video Colour Subsampling

  25. Composite Digital Video: CCIR-601Consultative Committee for International Radio

  26. Composite Digital Video:Advanced Digital Television Standard - HDTV (High Definition TV). • The aspect ratio for HDTV is 16:9 as opposed to 4:3 in NTSC, PAL, and SECAM. (A 33% increase in horizontal dimension.) • In the picture rate column, the "I" means interlaced scan, and the "P" means progressive (non-interlaced) scan. • Both NTSC rates and integer rates are supported (i.e., 60.00, 59.94, 30.00, 29.97, 24.00, and 23.98).

  27. Image FormatsPixels and Images • Pixels -- picture elements in digital images • Image Resolution -- number of pixels in a digital image (Higher resolution always yields better quality.) • Bit-Map -- a representation for the graphic/image data in the same manner as they are stored in video memory.

  28. Image FormatsMonochrome Images • Each pixel is stored as a single bit (0 or 1) • A 640 x 480 monochrome image requires 37.5 KB of storage. • Dithering is often used for displaying monochrome images

  29. Image FormatsGreyscale Images • Each pixel is usually stored as a byte (value between 0 to 255) • A 640 x 480 grayscale image requires over 300 KB of storage.

  30. Image Formats24-bit Colour Images • Each pixel is represented by three bytes (e.g., RGB) • Supports 256 x 256 x 256 possible combined colors (16,777,216) • A 640 x 480 24-bit color image would require 921.6 KB of storage • Many 24-bit color images are stored as 32-bit images, the extra byte of data for each pixel is used to store an alpha value representing special effect information

  31. Image Formats8-bit Colour Images • One byte for each pixel • Supports 256 out of the millions colors possible, acceptable color quality • Requires Color Look-Up Tables (LUTs) • A 640 x 480 8-bit color image requires 307.2 KB of storage (the same as 8-bit grayscale)

  32. Image Format StandardGIF(GIF87a, GIF89a) • Graphics Interchange Format (GIF) devised by the UNISYS Corp. and Compuserve, initially for transmitting graphical images over phone lines via modems • Uses the Lempel-Ziv Welch algorithm (a form of Huffman Coding), modified slightly for image scan line packets (line grouping of pixels) • Limited to only 8-bit (256) color images, suitable for images with few distinctive colors (e.g., graphics drawing) • Supports interlacing • GIF89a supports simple animation(Graphics Control Extension has control over delay time, transparent index, etc. Software such as Coral Draw will allow access and editing of GIF images.)

  33. Image Format StandardTIFF • Tagged Image File Format (TIFF), stores many different types of images (e.g., monochrome, grayscale, 8-bit & 24-bit RGB, etc.) --> tagged • Developed by the Aldus Corp. in the 1980's and later supported by Microsoft • TIFF is a lossless format (when not utilizing the new JPEG tag which allows for JPEG compression)

  34. Image Format StandardJPEG • A standard for photographic image compression created by the Joint Photographics Experts Group • Takes advantage of limitations in the human vision system to achieve high rates of compression • Lossy compression which allows user to set the desired level of quality/compression

  35. Image Format StandardPostscript/Encapsulated • A typesetting language which includes text as well as vector/structured graphics and bit-mapped images • Used in several popular graphics programs (Illustrator, FreeHand) • Does not provide compression, files are often large

  36. Video Format StandardDigital Video Disk (DVD) • DVD • Uses MPEG-2 operating at “Main Profile at Main Level” MP@ML and uses CCIR601 with 4:2:0 chrominance sub-sampling. • Supports data rate of 15Mbit/second

  37. Video Format StandardDigital Video (DV) • DV is intended for consumer and semi-professional use. There are two versions DVCAM and DVPRO. • DVPRO uses MJPEG with CCIR601 and 4:2:2 chrominance sub-sampling. • DVCAM uses MJPEG with CCIR601 and 4:1:1 chrominance sub-sampling. • Motion JPEG (MJPEG) is JPEG applied to each frame of video

  38. System Dependant Image Format:Microsoft Windows:BMP • A system standard graphics file format for Microsoft Windows • Used in PC Paintbrush and other programs • capable of storing 24-bit bitmap images

  39. System Dependant Image Format Macintosh: PAINT and PICT • PAINT was originally used in MacPaint program, initially only for 1-bit monochrome images. • PICT-JPEG format is used in MacDraw (a vector based drawing program) for storing structured graphics

  40. System Dependant Image Format X-windows: XBM • Primary graphics format for the X Window • Supports 24-bit color bitmap • Many public domain graphic editors, e.g., xv • Used in X Windows for storing icons, pixmaps, backdrops, etc.

  41. System Dependant Image Format QuickTime • It comprises of several components: • Photo Compressor: based around JPEG standards and is compatible with most JPEG decoders. • Animation Compressor: based around run-length encoding. • Video Compressor: Proprietary Apple standard based around spatial and temporal redundancy. • Graphics Compressor: Proprietary Apple standard. • Compact Video Compressor: Proprietary Apple standard that compresses video more than Video Compressor.

  42. System Dependant Image Format Windows Media • It does not describe a compression technology but the framework for supporting different compression algorithms. • It identifies the compression algorithm from the format/header of the file which contains the compressed data

  43. System Dependant Image Format RealMedia • RealMedia is a standard for streaming audio and video on the Internet. • RealVideo is a proprietary standard • RealAudio is a proprietary standard

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