Color

1. Color CSC361/661 -- Digital Media Spring 2002 Burg/Wong

2. Color • Light is electromagnetic energy in the 400- to 700 nanometer wavelength part of the spectrum, perceived as the colors ranging through violet, indigo, blue, green, yellow, orange, and red. • The colors that we see in the world around us are generally not pure colors consisting of a single wavelength. Rather, color sensation results from the dominant wavelength of the light reflecting off or emanating from an object.

3. Color Terminology • Hue – color • Monochromatic color – a color that is created from only one wavelength. (Most colors that we experience are NOT monochromatic. They result from a combination of wavelengths. The dominant wavelength gives us our color sensation.) • Chrominance – color information • Luminance – lightness or brightness information • Additive color systems – based on adding colored light (as in computer monitors). A combination of all colors gives white. • Subtractive color systems – based on adding pigments (as in printing). A combination of all colors gives black.

4. Human Perception of Color • To humans, color sensation is a matter of subjective perception resulting from the effect of light on the cones of the eyes. • There are three types of cones, each one with a particular sensitivity to red, green, or blue light. • This decomposition of light into three color components is called the tristimulus theory of color and is the basis for the RGB color model.

5. Response of Cones in the Eyes • This graphs shows the results of experiments in which the fraction of light absorbed by each type of cone is measured for each color in the visible spectrum. The green cone absorbs the most light. (Note that this is essentially the same graph as Figure 2.1 of the handout given in class, except that the units are different, eliminating the negative values.)

6. The Eye’s Sensitivity to Different Colors • This graph shows the eye’s overall response as the dominant wavelength (i.e., the hue) is varied across the visible spectrum. (Luminance is kept constant.) The eye’s sensitivity peaks at around 550 nm, the wavelength of yellow-green light. Note that this graph is the sum of the three graphs in the previous slide.

7. Testing Color Response for RGB Model • To create the RGB color model, it is necessary to determine how much of each color component is needed to create all the dominant wavelengths in the visible spectrum. • This is done by projecting “pure” colors onto one screen, mixing amounts of R, G, and B on a neighboring screen, and asking a large number of people to say when the colors match. • Matching color C is expressed by C = R*R + G*G + B*B

8. RGB Color Matching • This graph shows the amounts of red, green, and blue light needed by an average observer to match color samples as they vary across the spectrum. (Luminance is kept constant.) A negative value means it is not possible to match the original color with RGB as primaries, so some R, G, or B has to be added to the original color sample.

9. RGB Color on a Computer • The RGB color model works well for computers because it matches the technology of monitors. • On a color monitor, color is produced by exciting three adjacent dots made of red, green, and blue phosphors. Because the dots are so small, they are blended into one color by the eye. Note that the color is not blended by putting one color of light over another – it is blended by the eye.

10. RGB Color on a Computer • Not all colors perceivable by humans can be shown on a computer screen with the RGB model. • The same RGB values will not necessarily result in the same colors on two different monitors because monitors are not calibrated to a single standard. • RGB colors are not pure, saturated colors. This is because the kind of light emitted by an excited phosphor is not of a single wavelength, but has a spectral power distribution over a band of frequencies.

11. RGB Color on a Computer • RGB is perceptually non-linear. This means that equal differences in the RGB values do not correspond to equal differences in the perceived color. Low RGB values produce small changes in color on the screen (as you move from one low value to the next). Large RGB values produce very perceivable differences as you move from one high RGB value to the next.

12. CIE Color • The graph on slide 13 shows that not all visible colors can be represented with RGB as primaries. (You can’t add positive amounts of RGB to get all the colors. In some cases, you have to “take some color away” from the original sample being matched.) • The Commission Internationale de l’Eclairage (CIE) decided that we need a standard color model that is based on primaries which, when mixed together, produce all the visible colors.

13. CIE Standardization • Another motivation for the CIE model is that the RGB and CMYK models are device dependent. That is different monitors use different R, G, and B colors of phosphors. Different printers use different CMYK colors of ink. CIE standardization gives us a way to map between systems.

14. CIE Color Primaries • The CIE color primaries X, Y, and Z replace R, G, and B. X, Y, and Z are “artificial primaries,” not visible colors like R, G, and B. • These primaries can be combined in various proportions to produce all the colors the human eye can see.

15. CIE Color Matching • This graph shows the amounts of X, Y, and Z light needed by an average observer to match color samples as they vary across the spectrum. (Luminance is kept constant.) Notice that no negative values are needed.

16. CIE Color Space • The graph on the previous slide shows how X, Y, and Z can be combined to create any visible color, where all the colors in the spectrum are considered at the same luminance. • This graph shows the entire CIE color space, where not only the color but the luminance varies. • In the CIE color model,a color C is given by • C = X*X + Y*Y + Z*Z

17. CIE Color Model on the X+Y+Z = 1 Plane • If we want to consider each component as a percentage of the total amount of light, we can “normalize” the values: Note: X + Y + Z is the total amount of light energy. Also note that x + y + z = 1

18. CIE Color Space • This graph shows the amounts of X, Y, and Z needed for all colors in the visible spectrum. The X + Y + Z = 1 plane is shown as the triangle embedded in the graph. • The x, y, and z computed on the last slide lie on the X+Y+Z=1 plane. It is convenient to consider this plane only, which effectively reduces our consideration to constant luminance.

19. CIE Chromaticity Diagram • Picture the portion of the CIE color space that intersects the X+Y+Z=1 plane. • Now picture projecting that part of the X+Y+Z=1 plane down onto the X, Y plane. • This is how the CIE Chromaticity Diagram is created (next slide).

20. CIE Chromaticity Diagram • This graph represents the hue and saturation of all colors in the visible spectrum. This is “chromaticity” information. • All perceivable colors with the same chromaticity but differet luminances map into the same point in this graph. • The 100 percent spectrally pure colors are on the curved perimeter of the graph. The dot in the center represents the chromaticity of daylight (white light).

21. Dominant Wavelength on CIE Color Diagram • To determine what the dominant wavelength of a color A is from the diagram, draw a line between C (white) and the closest perimeter. The dominant wavelength is at B. • The degree of saturation is given by the proportion of segment AB to segment CB. The closer A is to the perimeter, the more saturated the color.

22. Color Gamuts Represented on CIE Diagram • All colors on the line IJ can be created by additively mixing colors I and J; all colors in the triangle IJK can be created by mixing colors I, J, and K.

23. RGB Color Gamut in Terms of CIE Diagram • We can see how much of the visible spectrum is displayable on a computer monitor by looking at the gamut within the RGB diagram, represented by the triangle. • Phosphors on a computer monitor have these approximate values: red green blue • x ~.61 ~.25 ~.15 • y ~.34 ~.62 ~.063

24. CMYK model • CMYK is primarily a printing color model. • Cyan, magenta, and yellow are called the subtractive primaries. • In practice, cyan, magenta, and yellow don’t produce all the colors needed for printing. Blacks come out muddy. So a pure black is added in. That’s the K.

25. CMYK Model • Cyan, magenta, yellow, and black • Cyan is white light with red taken out. C = G + B = W - R • Magenta is white light with green taken out. M = R + B = W - G • Yellow is white light with blue taken out. Y = R + G = W - B

26. CMYK vs. RGB • The colors printable with the CMYK model do not overlap exactly with the colors displayable on an RGB monitor, as represented by their respective gamuts within the CIE diagram.

27. Hue, Saturation, and Lightness • One way to represent color is by dividing it into its hue, saturation, and lightness components. • Hue (or color) is determined by the dominant wavelength. • Saturation is a matter of how much white light is added in. The less white light, the more saturated the color. • Lightness is how much black is in the color. • Hue and saturation are elements of chrominance. Lightness is a matter of luminance.

28. HSV • HSV stands for hue, saturation, and value (where value represents lightness or brightness). • Some people find the HSV model more intuitive than RGB. It is easier to think of colors in terms of their hue, tint, and shade rather than as combinations of red, green, and blue.

29. YUV Color Model • YUV is a general term that refers to any color model that has one luminance component (Y) and two chrominance (i.e., color) components (U and V). (You’ll also see references to the YIQ model, which is the same thing.) • Y’CBCR is a specific instance of a YUV model.

30. YUV Color Model • YUV is a color model appropriate to color TV because it makes it possible to send the color information separate from the luminance information, so that signals for black and white vs. color TV are easily separated. • YUV is also a good representation for compression, because some of the chrominance information can be thrown out without loss of quality in the picture (since the human eye is less sensitive to chrominance than luminance).