Chapter 6 Color Multimedia Systems. Key Points Colour is a subjective sensation produced in the brain. The measurable physical quantity that best models.

Chapter 6Chapter 6ColorColor

Multimedia SystemsMultimedia Systems

Key PointsKey PointsColour is a subjective sensation produced Colour is a subjective sensation produced in the brain. in the brain. The measurable physical quantity that besThe measurable physical quantity that best models colour is a t models colour is a spectral power distrispectral power distribution (SPD)bution (SPD). . In the In the RGB colour modelRGB colour model, a colour is repr, a colour is represented by three values, giving the proportesented by three values, giving the proportions of red (R), green (G) and blue (B) lighions of red (R), green (G) and blue (B) light which make up light of the desired colour. t which make up light of the desired colour.

Key PointsKey Points

The The RGB colour gamutRGB colour gamut is a proper subse is a proper subset of the visible colours. t of the visible colours.

The The colour depthcolour depth of an image is number of an image is number of bits used to hold a colour value. of bits used to hold a colour value.

Indexed colourIndexed colour: The stored 8-bit value is : The stored 8-bit value is used as an offset into a used as an offset into a colour lookup tacolour lookup table (CLUT)ble (CLUT) or or palettepalette containing up to 25 containing up to 256 24-bit colour values. 6 24-bit colour values.

Key PointsKey PointsDitheringDithering helps avoid the helps avoid the posterizationposterization that ca that can occur when indexed colour is used, by using pn occur when indexed colour is used, by using patterns of dots to simulate a wider range of colouatterns of dots to simulate a wider range of colours using optical mixing. rs using optical mixing.

The The CMYKCMYK model is important for printing, as it model is important for printing, as it models the way colour is produced by mixing ink models the way colour is produced by mixing ink or paint. or paint.

Other colour models include Other colour models include HSVHSV, , YUVYUV and its v and its variants, and the CIE device-independent spaces ariants, and the CIE device-independent spaces Lab and Luv. Lab and Luv.

Key PointsKey PointsA 24-bit image can be thought of as being A 24-bit image can be thought of as being made up of three 8-bit made up of three 8-bit channelschannels, one eac, one each for red, green and blue in RGB colour.h for red, green and blue in RGB colour.Colour correction etc. can be performed by Colour correction etc. can be performed by using techniques from Chapter 5 on individusing techniques from Chapter 5 on individual channels. ual channels. Colour managementColour management software uses software uses devicdevice profilese profiles to help maintain consistent colo to help maintain consistent colour. ur.

Color & ScienceColor & ScienceColor is a subjective sensation produced in the Color is a subjective sensation produced in the brain.brain.

Color & ScienceColor & ScienceLight is a form of electromagnetic radiation, we Light is a form of electromagnetic radiation, we can measure its wavelength.can measure its wavelength. http://micro.magnet.fsu.edu/primer/lightandcolor/indehttp://micro.magnet.fsu.edu/primer/lightandcolor/inde

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Microwave OvensMicrowave Ovens

The wavelength is tuned to frequencies The wavelength is tuned to frequencies that are readily absorbed by water that are readily absorbed by water molecules in food causing them to absorb molecules in food causing them to absorb energy and release heat as they vibrate.energy and release heat as they vibrate.

Containers holding the food in microwave Containers holding the food in microwave ovens usually contain very little water and ovens usually contain very little water and tend to remain cool. tend to remain cool.

Color & ScienceColor & ScienceVisible light (Visible light ( 可見光可見光 ): 400nm~700nm ): 400nm~700nm

Color & ScienceColor & ScienceSpectral power distribution (SPD)Spectral power distribution (SPD)

Color & ScienceColor & ScienceReceptor CellsReceptor Cells Rods: night-vision and cannot distinguish colorRods: night-vision and cannot distinguish color Cones: in three sorts which respond to different Cones: in three sorts which respond to different

wavelengthswavelengths

Cone ResponseCone Response

Tristimulus TheoryTristimulus Theory

Any colors can be specified by just three Any colors can be specified by just three values, giving weights of three values, giving weights of three components.components.

Each type of cone responds to R, G, BEach type of cone responds to R, G, B

Primary ColorsPrimary ColorsAdditive mixingAdditive mixing TV, monitorsTV, monitors

Subtractive mixingSubtractive mixing Paint absorbs lightPaint absorbs light

Color Temperature Color Temperature The color temperature model is based on the The color temperature model is based on the relationship between the temperature of a relationship between the temperature of a theoretical standardized material, called a theoretical standardized material, called a black black body radiatorbody radiator, and the energy distribution of its , and the energy distribution of its emitted light as the radiator is brought to emitted light as the radiator is brought to increasingly higher temperatures, measured in increasingly higher temperatures, measured in Kelvin (K).Kelvin (K).Although this radiator does not exist, many Although this radiator does not exist, many metals behave very similar to a black body so metals behave very similar to a black body so we can take a metal pot as an example for this we can take a metal pot as an example for this discussion.discussion.

Color TemperatureColor Temperature

RGB ColorsRGB ColorsRGB color modelRGB color model

Not possible to represent all visible color as a Not possible to represent all visible color as a combination of fixed R, G, B.combination of fixed R, G, B.

Other Color SpacesOther Color Spaces

Color DepthColor DepthColor depthColor depth 24, 30, 36, 48 bits24, 30, 36, 48 bits Grey scale image: R=G=BGrey scale image: R=G=B PNG format: up to 48 bitsPNG format: up to 48 bits Scanner: 48 bitsScanner: 48 bits

Accurate approximations when reduced to a lower resolution Accurate approximations when reduced to a lower resolution for displayfor display

Millions of colors, true color: 24 bitsMillions of colors, true color: 24 bitsThousands of colors, hi-color: 16 bitsThousands of colors, hi-color: 16 bits256 colors: 8 bits256 colors: 8 bits

Grey-level: 256 = 8 bitsGrey-level: 256 = 8 bits

Indexed ColorIndexed ColorDirect colorDirect colorIndexed colorIndexed color

Palette of 256 colorsPalette of 256 colors Color lookup table (CLUT)Color lookup table (CLUT)

Logical colors => physical colorsLogical colors => physical colors 24 bits color => indexed color24 bits color => indexed color

1/3 of data1/3 of data PNG, BMP, TGA, TIFF: palettePNG, BMP, TGA, TIFF: palette

GIF: only 256 colors GIF: only 256 colors JPEG: 24 bit colorJPEG: 24 bit color

Missing color: the nearest colorMissing color: the nearest color Visible artifactsVisible artifacts Posterization: gradations replace by sharp boundariesPosterization: gradations replace by sharp boundaries

Web-safe PaletteWeb-safe Palette 216 colors, Displayed on any platform216 colors, Displayed on any platform

PosterizationPosterizationArea of a single color are replaced by a pattern Area of a single color are replaced by a pattern of dots of several colorsof dots of several colors Pink = red + whitePink = red + white

Dithering: high resolutionDithering: high resolution

Halftoning: greyscale imagesHalftoning: greyscale images N by N pixels = NN by N pixels = N22+1 grey levels+1 grey levels

Other Color ModelsOther Color ModelsCMYKCMYK C = G+B = W-RC = G+B = W-R

M = R+B = W-GM = R+B = W-GY = R+G = W-BY = R+G = W-B

Complementary colorComplementary color

Art and designArt and design

The dyes do not supply color to light reflected The dyes do not supply color to light reflected off surface, but to light that penetrates through off surface, but to light that penetrates through them and gets reflected or scattered back from them and gets reflected or scattered back from beneath it.beneath it.

Mixing Colored InksMixing Colored Inks

Cyan ink Absorption of red light

Cyan lightWhite light

Cyan ink

Absorption of green light

Blue lightWhite light

Magenta

Absorption of red light

Not possible to manufacture inks which Not possible to manufacture inks which absorb only light of precisely the absorb only light of precisely the complementary color.complementary color.Some unwanted colors are absorbed.Some unwanted colors are absorbed.

RGB gamut > CMY gamutRGB gamut > CMY gamut C+M+Y != perfect blackC+M+Y != perfect black

With black: CMYKWith black: CMYK

Hue, saturation, brightnessHue, saturation, brightness As white is mixed in, saturation decreases.As white is mixed in, saturation decreases.

Color wheelColor wheel

HSVHSV

Color PickersColor Pickers

Color DifferencesColor Differences

Brightness=(R+G+B)/3Brightness=(R+G+B)/3

LuminanceLuminanceY=0.2125R + 0.7154G + 0.0721BY=0.2125R + 0.7154G + 0.0721BGreen contributes far moreGreen contributes far more

Color differencesColor differences U=B-Y and V=R-YU=B-Y and V=R-Y YUVYUV YCYCBBCCRR

Device-independent Color SpacesDevice-independent Color Spaces

RGB, CMYK: device-dependentRGB, CMYK: device-dependentCIE XYZ: device-independentCIE XYZ: device-independentNot perceptually uniformNot perceptually uniformthe same change in one of vales produced the same change in one of vales produced the same change in appearancethe same change in appearanceR: (1 to 11) != (101 to 111)R: (1 to 11) != (101 to 111)Perceptually uniformPerceptually uniform CIE L*a*b*: as in CMYK for prepress industryCIE L*a*b*: as in CMYK for prepress industry CIE L*u*v*: as in RGB for monitors and CIE L*u*v*: as in RGB for monitors and

scannersscanners

Channels and Color CorrectionChannels and Color Correction

Color images = R + B + C channelsColor images = R + B + C channels

Each channel can be manipulated Each channel can be manipulated separately.separately.

Color correctionColor correction Compensate for the deficiencies of scanners Compensate for the deficiencies of scanners

and other input devicesand other input devices Time-consumingTime-consuming

Wizards, assistantsWizards, assistants

Color CorrectionColor CorrectionColor balance + hue & saturation adjustmentColor balance + hue & saturation adjustment Color balanceColor balance

3 sliders: C+R, M+G, Y+B3 sliders: C+R, M+G, Y+B

Replace a specified color with a different cReplace a specified color with a different colorolorCMYK: printingCMYK: printing YC YCBBCCRR: video: video

Alpha channelAlpha channeldifferent from color channelsdifferent from color channels Blue screen: alpha = blue channelBlue screen: alpha = blue channel

Mask out backgroundMask out background

JPEG: process each channel separatelyJPEG: process each channel separately YCYCBBCCRR: downsample color difference: downsample color difference

ChromakeyChromakey

Consistent ColorConsistent ColorColor profilesColor profiles

White=(255,255,255)White=(255,255,255)specified in a device-specified in a device-independent color spaceindependent color space

GammaGamma

Gamma CorrectionGamma Correction

Sample Input Graph of Input

Gamma Corrected Input Graph of Correction L' = L ^ (1/2.5)

Monitor Output Graph of Output

kNI

Color ProfileColor ProfileEPS, TIFF JFIF, EPS, TIFF JFIF, PNG: color profilePNG: color profile At least R, G, B, white At least R, G, B, white

point, gammapoint, gamma Fig. 6.13Fig. 6.13

ICCICC

International Color ConsortiumInternational Color Consortium Apple’s ColorSyncApple’s ColorSync Kodak Precision Color Management SystemKodak Precision Color Management System TIFF, EPS: complete ICC profilesTIFF, EPS: complete ICC profiles

sRGB: standard RGBsRGB: standard RGB ITU-R BT.709 standardITU-R BT.709 standard Gamma=2.2 and white point= 6500 KGamma=2.2 and white point= 6500 K