Pengenalan Warna
Outline
•Warna dan panjang gelombang
•Warna yang dapat ditangkap matamanusia
•Pencocokkan warna
•CIE (diagram kromatisCIE, color gamut)
•Ruang warna
Visible Spectrum
•We perceive electromagnetic energy having wavelengths in the range 400-700 nm as visible light.
Color Perception
•Different spectra can result in a perceptually identical sensations called metamers
•Color perception results from the simultaneous stimulation of 3 cone types (trichromat)
•Our perception of color is also affected by surround effects and adaptation
•Experiment:
–Subject views a colored surface through a hole in a sheet, so that the color looks like a film in space
–Investigator controls for nearby colors, and state of mind
Trichromacy
•The receptor performance implies that coloursdo not have a unique energy distribution.•Colors which are a distribution over all wavelengths can be matched by mixing three (R G B)
•Color Matching :
–Given any colourlight source, regardless of the distribution of wavelengths that it contains, we can try to match it with a mixture of three light sources
X = r R + g G + b B
where R, G and B are pure light sources and r, g and b their intensities
•Subtractive matching
–Not all colourscan be matched with a given set of light sources
–We can add light to the colourwe are trying to match:
X + r R = g G + b B
with this technique all colourscan be matched.
The CIE Diagram
•The CIE (Commission Internationaled’Eclairage) diagram was devised as a standard normalisedrepresentation of color.
•Given three light sources we can mix them to match any given color, providing we allow ourselves subtractive matching.
•Suppose we normalisethe ranges found to [0..1] to avoid the negative signs.
•Normalisedcolors
–Having normalisedthe range over which the matching is done we can now normalisethe colourssuch that :
r + g + b = 1
thus,x = r/(r+g+b)
y = g/(r+g+b)
z = b/(r+g+b) = 1 -x –y
The CIE Diagram
•Convex Shape
–Notice that the pure colours(coherent λ) are round the edge of the CIE diagram.
–The shape must be convex, since any blend (interpolation) of pure coloursshould create a color in the visible region.
–The line joining purple and red has no pure equivalent. The colourscan only be created by blending.
•Intensities
–Since the coloursare all normalisedthere is no representation of intensity.
–By changing the intensity perceptually different colourscan be seen.
•White Point
–When the three colourcomponents are equal, the colouris white: x = 0.33 , y = 0.33
•Saturation
–Pure coloursare called fully saturated.
–These correspond to the coloursaround the edge of the horseshoe.
–Saturation of a arbitrary point is the ratio of its distance to the white point over the distance of the white point to the edge.
Color Gamuts
•The chromaticity diagram can be used to compare the "gamuts" of various possible output devices (i.e., monitors and printers).
Color Monitor
•Color monitors are based on adding three the output of three different light emitting phosphors.
Color Printer
•When printing color we use a subtractive representation.
•Inks absorb wavelengths from the incident light, hence they subtract components to create the color.
•The subtractive primaries are
–Magenta (purple)
–Cyan (light Blue)
–Yellow
•Additive vsSubtractive Colourrepresentation
–The subtractive representation is capable of representing far more of the colourspace than the additive.
Color Space –HSI/HSV
•For interactive image manipulation it is preferable to use the HSI representation
•HSI has three values per color:
–Hue -corresponds notionally to pure color.
–Saturation -The proportion of pure colour
–Intensity -the brightness
•Hexconesubset of cylindrical (polar) coordinate system
•Conversion between RGB and HSI
I = ( r + g + b )/3
( Sometimes, I = max(r,g,b))
S = ( max(r,g,b) -min(r,g,b) ) / max(r,g,b)
Hue(which is an angle between 0 and 360o) is best described procedurally
•Calculating Hue :
–if (r=g=b)
Hue is undefined, the colouris black, white or grey.
–if (r>b) and (g>b)
Hue = 120*(g-b)/((r-b)+(g-b))
–if (g>r) and (b>r)
Hue = 120 + 120*(b-r)/((g-r)+(b-r))
–if (r>g) and (b>g)
Hue = 240 +120*(r-g)/((r-g)+(b-g))
Alpha Channels
•Colourrepresentations in computer systems sometimes use four components -r g b α.
•The fourth is simply an attenuation of the intensity which:
–allows greater flexibility in representing colours.
–avoids truncation errors at low intensity
–allows convenient masking certain parts of an image.
Outline
•Warna dan panjang gelombang
•Warna yang dapat ditangkap matamanusia
•Pencocokkan warna
•CIE (diagram kromatisCIE, color gamut)
•Ruang warna
Visible Spectrum
•We perceive electromagnetic energy having wavelengths in the range 400-700 nm as visible light.
Color Perception
•Different spectra can result in a perceptually identical sensations called metamers
•Color perception results from the simultaneous stimulation of 3 cone types (trichromat)
•Our perception of color is also affected by surround effects and adaptation
•Experiment:
–Subject views a colored surface through a hole in a sheet, so that the color looks like a film in space
–Investigator controls for nearby colors, and state of mind
Trichromacy
•The receptor performance implies that coloursdo not have a unique energy distribution.•Colors which are a distribution over all wavelengths can be matched by mixing three (R G B)
•Color Matching :
–Given any colourlight source, regardless of the distribution of wavelengths that it contains, we can try to match it with a mixture of three light sources
X = r R + g G + b B
where R, G and B are pure light sources and r, g and b their intensities
•Subtractive matching
–Not all colourscan be matched with a given set of light sources
–We can add light to the colourwe are trying to match:
X + r R = g G + b B
with this technique all colourscan be matched.
The CIE Diagram
•The CIE (Commission Internationaled’Eclairage) diagram was devised as a standard normalisedrepresentation of color.
•Given three light sources we can mix them to match any given color, providing we allow ourselves subtractive matching.
•Suppose we normalisethe ranges found to [0..1] to avoid the negative signs.
•Normalisedcolors
–Having normalisedthe range over which the matching is done we can now normalisethe colourssuch that :
r + g + b = 1
thus,x = r/(r+g+b)
y = g/(r+g+b)
z = b/(r+g+b) = 1 -x –y
The CIE Diagram
•Convex Shape
–Notice that the pure colours(coherent λ) are round the edge of the CIE diagram.
–The shape must be convex, since any blend (interpolation) of pure coloursshould create a color in the visible region.
–The line joining purple and red has no pure equivalent. The colourscan only be created by blending.
•Intensities
–Since the coloursare all normalisedthere is no representation of intensity.
–By changing the intensity perceptually different colourscan be seen.
•White Point
–When the three colourcomponents are equal, the colouris white: x = 0.33 , y = 0.33
•Saturation
–Pure coloursare called fully saturated.
–These correspond to the coloursaround the edge of the horseshoe.
–Saturation of a arbitrary point is the ratio of its distance to the white point over the distance of the white point to the edge.
Color Gamuts
•The chromaticity diagram can be used to compare the "gamuts" of various possible output devices (i.e., monitors and printers).
Color Monitor
•Color monitors are based on adding three the output of three different light emitting phosphors.
Color Printer
•When printing color we use a subtractive representation.
•Inks absorb wavelengths from the incident light, hence they subtract components to create the color.
•The subtractive primaries are
–Magenta (purple)
–Cyan (light Blue)
–Yellow
•Additive vsSubtractive Colourrepresentation
–The subtractive representation is capable of representing far more of the colourspace than the additive.
Color Space –HSI/HSV
•For interactive image manipulation it is preferable to use the HSI representation
•HSI has three values per color:
–Hue -corresponds notionally to pure color.
–Saturation -The proportion of pure colour
–Intensity -the brightness
•Hexconesubset of cylindrical (polar) coordinate system
•Conversion between RGB and HSI
I = ( r + g + b )/3
( Sometimes, I = max(r,g,b))
S = ( max(r,g,b) -min(r,g,b) ) / max(r,g,b)
Hue(which is an angle between 0 and 360o) is best described procedurally
•Calculating Hue :
–if (r=g=b)
Hue is undefined, the colouris black, white or grey.
–if (r>b) and (g>b)
Hue = 120*(g-b)/((r-b)+(g-b))
–if (g>r) and (b>r)
Hue = 120 + 120*(b-r)/((g-r)+(b-r))
–if (r>g) and (b>g)
Hue = 240 +120*(r-g)/((r-g)+(b-g))
Alpha Channels
•Colourrepresentations in computer systems sometimes use four components -r g b α.
•The fourth is simply an attenuation of the intensity which:
–allows greater flexibility in representing colours.
–avoids truncation errors at low intensity
–allows convenient masking certain parts of an image.
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