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Slide 2 : Color Topics

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Color Topics in Computer Graphics

NB. : in the lecture on Diplays devices, we insisted on the particulate aspect of light (photon emission). But light is dual and today we will discuss about the waving side of light.

If you want to see all the colors, use a rainbow or a prism.

• Color Perception

• Color Spaces

• Gamma

• Alpha

Hearn & Baker Ch 15

 

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Slide 3 : Elements of Color

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Elements of Color

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Slide 4 : Visible Spectrum

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Visible Spectrum

We perceive electromagnetic energy having wavelengths in the range 400-700 nm as visible light.

What we perceive as "light", or different colours, is a narrow frequency band within the electromagnetic spectrum.

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Slide 5 : The Eye

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The Eye

The photosensitive part of the eye is called the retina. The retina is largely composed of two types of cells, called rodsand cones. Only the cones are responsible for color perception.

 

 

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Slide 6 : The Fovea

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The Fovea

	Cones are most densely packed within a region of the eye called the fovea. There are three types of cones, referred to as S, M, and L. They are roughly equivalent to blue, green, and red sensors, respectively. Their peak sensitivities are located at approximately 430nm, 560nm, and 610nm for the "average" observer.

 

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Slide 7 : The Fovea

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The Fovea

	Colorblindness results from a deficiency of one or more cone type. Electron Micrograph of Human Visual Receptors

 

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Slide 8 : Color Perception

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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

• Ex :
  

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Slide 9 : Color Matching

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Color Matching

In order to define the perceptual 3D space in a "standard" way, a set of experiments can (and have been) carried by having observers try and match color of a given wavelength, lambda, by mixing three other pure wavelengths, such as R=700nm, G=546nm, and B=436nm in the following example.

Note that the phosphours of color TVs and other CRTs do not emit pure red, green, or blue light of a single wavelength, as is the case for this experiment.

The scheme above can tell us what mix of R,G,Bis needed to reproduce the perceptual equivalent of any wavelength. A problem exists, however, because sometimes the red light needs to be added to the target before a match can be achieved. This is shown on the graph by having its intensity, R, take on a negative value.

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Slide 10 : CIE Color Space

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CIE Color Space

In order to achieve a representation which uses only positive mixing coefficients, the CIE ("Commission Internationale d'Eclairage") defined three new hypothetical light sources, x, y, and z, which yield positive matching curves:

If we are given a spectrum and wish to find the corresponding X, Y, and Z quantities, we can do so by integrating the product of the spectral power and each of the three matching curves over all wavelengths. The weights X,Y,Z form the three-dimensional CIE XYZ space, as shown below.

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Slide 11 : CIE Chromaticity Diagram

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CIE Chromaticity Diagram

Often it is convenient to work in a 2D color space. This is commonly done by projecting the 3D color space onto the plane X+Y+Z=1, yielding a CIE chromaticity diagram. The projection is defined as: Giving one of the following chromaticity diagram.

 

 

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Slide 12 : A few definitions

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A few definitions...

	Spectrophotometer A device to measure the spectral energy distribution. It can therefore also provide the CIE xyz tristimulus values. Illuminant C A standard for white light that approximates sunlight. It is defined by a color temperature of 6774 K. Complementary colors Colors which can be mixed together to yield white light. For example, colors on segment CD are complementary to the colors on segment CB. Dominant wavelength The spectral color which can be mixed with white light in order to reproduce the desired color. color B in the above figure is the dominant wavelength for color A. Non-spectral colors Colors not having a dominant wavelength. For example, color E in the above figure. Perceptually uniform color space A color space in which the distance between two colors is always proportional to the perceived distance. The CIE XYZ color space and the CIE chromaticity diagram are not perceptually uniform, as the following figure illustrates. The CIE LUV color space is designed with perceptual uniformity in mind.

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Slide 13 : Color Gamuts

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Color Gamuts

The chromaticity diagram can be used to compare the "gamuts" of various possible output devices (i.e., monitors and printers).

Note that a color printer cannot reproduce all the colors visible on a color monitor.

 

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Slide 14 : The RGB Color Cube

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The RGB Color Cube

The additive color model used for computer graphics is represented by the RGB color cube, where R, G, and B represent the colors produced by red, green and blue phosphours, respectively.

The color cube sits within the CIE XYZ color space as follows.

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Slide 15 : Color Printing

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Color Printing

Green paper is green because it reflects green and absorbs other wavelengths. The following table summarizes the properties of the four primary types of printing ink.

dye color	absorbs	reflects
cyan	red	blue and green
magenta	green	blue and red
yellow	blue	red and green
black	all	none

To produce blue, one would mix cyan and magenta inks, as they both reflect blue while each absorbing one of green and red. Unfortunately, inks also interact in non-linear ways. This makes the process of converting a given monitor color to an equivalent printer color a challenging problem.

Black ink is used to ensure that a high quality black can always be printed, and is often referred to as to K. Printers thus use a CMYK color model.

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Slide 16 : Color Conversion

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Color Conversion

To convert from one color gamut to another is a simple procedure. Each phosphour color can be represented by a combination of the CIE XYZ primaries, yielding the following transformation from RGB to CIE XYZ:

The transformation yields the color on monitor 2 which is equivalent to a given color on monitor 1. Conversion to-and-from printer gamuts is difficult. A first approximation is as follows:

C = 1 - R
M = 1 - G
Y = 1 - B

The fourth color, K, can be used to replace equal amounts of CMY:

K = min(C,M,Y) C' = C - K
M' = M - K
Y' = Y - K

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Slide 17 : RGB-Based Color Spaces

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RGB-Based Color Spaces

The RGB space is a three-dimensional color space whose components are the red, green, and blue intensities that make up a given color. For example, scanners read the amounts of red, green, and blue light that are reflected from or transmitted through an image and then convert those amounts into digital values. Information displayed on a color monitor begins with digital values that are converted to analog signals for display on the monitor. The analog signals are transmitted to the phosphors on the face of the monitor, causing them to glow at various intensities of red, green, and blue (the combination of which makes up the required hue, saturation, and brightness of the desired colors).

RGB-based color spaces are the most commonly used color spaces in computer graphics, primarily because they are directly supported by most color displays and scanners. RGB color spaces are device dependent and additive. The groups of color spaces within the RGB base family include

• RGB spaces
• HSV and HLS spaces

RGB Spaces

Anycolor expressed in RGB space is some mixture of three primary colors: red, green, and blue. Most RGB-based color spaces can be visualized as a cube, as in the following figure, with corners of black, the three primaries (red, green, and blue), the three secondaries (cyan, magenta, and yellow), and white.

RGB color space (Red corner is hidden from view)

sRGB Color Space

The sRGB color space is based on the ITU-R BT.709 standard. It specifies a gamma of 2.2 and a white point of 6500 degrees K. You can read more about sRGB space at the International Color Consortium site at . This space gives a complimentary solution to the current strategies of color management systems, by offering an alternate, device-independent color definition that is easier to handle for device manufacturers and the consumer market. sRGB color space can be used if no other RGB profile is specified or available.

igh end users with good quality reproduction devices may find that the sRGB space, which limits colors to the sRGB gamut, is too restrictive for their required color quality.

HSV and HLS Color Spaces

HSV space and HLS space are transformations of RGB space that can describe colors in terms more natural to an artist. The name HSV stands for hue, saturation, and value . ( HSB space , or hue , saturation , and brightness , is synonymous with HSV space.) HLS stands for hue, lightness, and saturation. The two spaces can be thought of as being single and double cones, as shown in next figure .

The components in HLS space are analogous, but not completely identical, to the components in HSV space:

• The hue component in both color spaces is an angular measurement, analogous to position around a color wheel. A hue value of 0 indicates the color red; the color green is at a value corresponding to 120°, and the color blue is at a value corresponding to 240°. Horizontal planes through the cones in next figure are hexagons; the primaries and secondaries (red, yellow, green, cyan, blue, and magenta) occur at the vertices of the hexagons.
• The saturation component in both color spaces describes color intensity. A saturation value of 0 (in the middle of a hexagon) means that the color is "colorless" (gray); a saturation value at the maximum (at the outer edge of a hexagon) means that the color is at maximum "colorfulness" for that hue angle and brightness.

HSV (or HSB) color space and HLS color space

• The value component in HSV describes the brightness. In both color spaces, a value of 0 represents the absence of light, or black. In HSV space, a maximum value means that the color is at its brightest. In HLS space, a maximum value for lightness means that the color is white, regardless of the current values of the hue and saturation components.

Slide 18 : HSV, HLS and HSI

HSV, HLS and HS

• HSV (Hue, Saturation and Value),

• HLS (Hue, Luminance and Saturation) and

• HSI (Hue, Saturation and Intensity)

are three variants of what is effectively the same colour space.

They all are effectively the RGB space twisted so that the neutral diagonal becomes the lightness axis, the saturation the distance from the central lightness axis and the hue the position around the center. The only difference between these models is the measurement of saturation, or the strength of the colour.

In HSV, created in 1978 by Alvey Ray Smith, saturation had a maximum on 0 when lightness was zero and 1 when lightness was 1. This is somewhat odd and anti-intuitive when the strength of the colour of white is considered. HLS is much more intuitive. The saturation peaks at a lightness of 0.5 and has a maximum of zero at black and white. The HSI colour space is in fact identical to HLS under a different name.

Figure: The RGB derived colour spaces (a) HSV, (b) HLS and (c) the most commonly used HSV space.

The most commonly used one, confusingly also called HSV, is a combination and variation on HSV and HLS, appearing to be a simple cylinder. This was done to have a uniform saturation limit. There are the same anomalies at white and black with saturation as with the original HSV. Whenever HSV is referred to in this report it will be to this colour space.

HSV is almost identical to RGB, slightly warped for symmetry's and simplicity's sake. It has made no attempts whatsoever towards perceptual uniformity but has achieved more intuitive dimensions.

Computer Monitor

To explain gamma correction we will begin with where you are looking - your computer monitor.

Almost every computer monitor, from whatever manufacturer, has one thing in common. They all have a intensity to voltage response curve which is roughly a 2.5 power function. Don't be afraid, this just means that if you send your computer monitor a message that a certain pixel should have intensity equal to x, it will actually display a pixel which has intensity equal to x ^ 2.5 Because the range of voltages sent to the monitor is between 0 and 1, this means that the intensity value displayed will be less than what you wanted it to be. (0.5 ^ 2.5 = 0.177 for example) Monitors, then, are said to have a gamma of 2.5

 Sample Input to Monitor  Graph of Input
  Output from Monitor      Graph of Output  L = V ^ 2.5

Note: The grayscale images will not look very good on 8-bit color computers.Also the gamma correction for these images is 1.0 so they were actually designedto be viewed on a system such as a Sun or a PC with no hardware correction. Theymay appear brighter on other systems. The important thing here is therelative difference that you see.

Gamma correction of the input

To correct this annoying little problem, the input signal to the monitor (the voltage) must be "gamma corrected".

The solution, fortunately, is a simple one. Since we know the relationship between the voltage sent to the monitor and the intensity which it produces, we can correct the signal before it gets to the monitor. The signal is adjusted so that it is essentially the complement of the curve shown above. There are other considerations as well when one speaks of a "correct" image. These include the ambient light in a room where the computer is, the brightness and contrast settings on the monitor, and finally personal taste.

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

If gamma correction is done properly for the computer system, then the outputshould accurately reflect the image input.

Note that the task of gamma correction is accomplished by raising the input value to the 1/2.5 power. This is referred to as a gamma correction of 2.5. because we are correcting the input for a monitor whose gamma is 2.5

Real World Application

As we mentioned above, most monitors work in about the same way with respect to gamma correction. Most computers, or more specifically, most computer systems, do not work in exactly the same way, however.

By computer systems we mean everything from the software that is running (like Netscape) to the graphics cards installed, to the standard hardware on the motherboard. Different computers do different things and many "systems" have different configurations of all of the above things.

Macintoshes, for example, have partial gamma correction built-in to their hardware. Silicon Graphics computers also have built-in gamma correction, but it is different from the Macintosh. Suns and PCs have no standard built-in gamma correction but some graphics cards installed in these computers may provide this functionality.

System Gamma

Macintosh

The Macintosh has built-in gamma correction of 1.4. This means that after the software sends the signal to the framebuffer, there are internal hardware corrections which will further process the signal, specifically by gamma correcting it another 1.4 - That is, the signal is raised to the 1/1.4. Therefore, to get full correction, the software itself should first adjust the signal by raising it to the 1/1.8 power. (2.5/1.4 = 1.8) Thus the system gamma on a Macintosh is 1.8.

Note some graphics cards in Macintoshes may have their own software to change the standard gamma and Adobe Photoshop 3.0 is now released with a gamma control panel from Knoll software which allows the user to change the system gamma of their Macintosh. The 1.8 standard is still accepted as the universal Mac System Gamma, but users should be aware that a Mac can be set differently. The Knoll software control panel for the Mac rewrites the look up table, (LUT) with a value of g/2.5 where g is the gamma the user selects. Thus selecting 1.8 will rewrite the LUT with 1.8/2.5 = 1/1.4 - the default setting. (The values in the LUT are 1/1.4 and this is called a 1.4 correction)

The SGI is similar to the Macintosh but instead of a hardware correction of 1.4 the SGI applies gamma correction of 1.7. Thus the system gamma for an SGI is 2.5/1.7 or roughly 1.5. Sometimes you may see that an SGI has a system gamma of 1.4. This calculation is made on the assumption that monitors have a response curve closer to a 2.4 power function.

SGI's also come with a utility to rewrite the internal hardware correction. These values are stored in a look up table, (LUT) and can be altered. The default is 1/1.7 as mentioned above. (The values in the table are 1/1.7, so we call this a 1.7 correction) Sometimes the value in the LUT may be referred to as the SGI system gamma. This is not the definition used on other platforms. Unlike the Mac gamma control panel, the SGI gamma utility will rewrite the LUT with the actual value set by the user.

Suns and PCs have no standard hardware correction (although certain graphics cards for these platforms may) and therefore their system gammas are roughly 2.5.

Common graphics software such as Adobe Photoshop allows the user to set the gamma correction value they want. (In Photoshop it is found in Monitor Setup under Preferences under the File Menu.)

Extract from : CGSD - Gamma Correction Explained :

Slide 21 : CGSD - Gamma Correction Explained