CS 376 Introduction to Computer Graphics 01 / 26 / 2007 Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS Spring 2007 Today’s Topics chromaticity diagram –color gamut, complementary colors, dominant wavelength RGB, YIQ CMY, CMYK, HSV Introduction to Open GL

Homework READING: –For openGL: 2.8, 2.9 first four sections of chapter 3 Michael Eckmann - Skidmore College - CS Spring 2007

Color source: Wikipedia

Michael Eckmann - Skidmore College - CS Spring 2007 Color Last time we saw how the Chromaticity diagram could be used to find color gamuts of 2 or more colors and how to find complementary colors. Also, recall that –All perceivable colors with the same chromaticity but different brightness map to the same point. –The spectrally pure colors are on the outer curve.

Michael Eckmann - Skidmore College - CS Spring 2007 Color

Michael Eckmann - Skidmore College - CS Spring 2007 Color The chromaticity diagram is also useful for determining purity and hue for a color –purity (saturation): ratio of distance of line from color (eg. C 1 ) to C to the distance of line from C to spectral color C s on the outer curve. –hue (dominant wavelength): draw a line from C through the color C 1 to the spectral color curve (the intersection of this line with the outer curve is the hue) why? because the color C 1 can be represented by some combination of white ( C ) and the spectral color C s on the outer curve. –if the dominant wavelength lands on the “purple line” then...

Michael Eckmann - Skidmore College - CS Spring 2007 Color For the colors (called non-spectral colors) in the chromaticity diagram that lie such that when drawing a line starting at C through the color will intersect the “purple” line –the dominant wavelength is the complement of some wavelength draw a line from the color C 2 through C (white) to the outer spectral curve C sp the intersection is the wavelength that is subtracted from the spectral distribution of the color –purity (saturation): is still the closer to white, the less pure the further from white, the more pure

Michael Eckmann - Skidmore College - CS Spring 2007 Color Is the Chromaticity diagram useful for determining the brightness of a color?

Michael Eckmann - Skidmore College - CS Spring 2007 Color Is the Chromaticity diagram useful for determining the brightness of a color? –No, the brightness (light energy) was normalized to 1 –recall, Color = xX + yY + zZ –x' = x / (x+y+z) –y' = y / (x+y+z) –z' = z / (x+y+z) –Since x' + y' + z' = 1, any color can be represented by just two of the x', y' and z'. –By normalizing based on the total light energy (brightness) we are only now dealing with hue and purity (collectively known as Chromaticity)

Michael Eckmann - Skidmore College - CS Spring 2007 RGB Color Model The color gamut for RGB colors appears on the next slide. Color = rR + gG + bB r, g and b range from 0.0 to 1.0 notice: white's (r,g,b) = (1,1,1) black's (r,g,b) = (0,0,0) complementary colors are those that add up to (1,1,1) can anyone tell me 2?

Michael Eckmann - Skidmore College - CS Spring 2007 RGB Color Model

Michael Eckmann - Skidmore College - CS Spring 2007 YIQ Color Model Graphics monitors use RGB (separate signals for R, G and B) NTSC television monitors use a composite signal called the YIQ model. Y is luminance (brightness) I & Q together incorporate the hue and purity (collectively chromaticity) and some luminance RGB can be transformed into YIQ and vice versa which means, given RGB values, you can compute the equivalent color in YIQ and vice versa. See section 12.5 of textbook for transforms. One nice feature of YIQ is that since Y is the luminance, if that's all that is captured by the television then you end up with “black & white” television (grey-scale) Also since the human eye responds more to luminance than color (recall the number of rods vs. cones) more bandwidth is set aside for the Y value than I & Q.

Michael Eckmann - Skidmore College - CS Spring 2007 Additive vs. Subtractive Both RGB and YIQ color models discussed thus far are additive models. That is, colors are produced by adding light sources. When we view a monitor or television we are looking at the light sources themselves. How about CIE --- is it additive? Since hard copy devices print colors, the colors we see are reflected from the paper (when white light hits the paper). A subtractive process is required to print something that will reflect a certain color of light on paper. We are not viewing the light source this time, but instead what color light is reflected off the paper when white light is shown on it.

Michael Eckmann - Skidmore College - CS Spring 2007 Additive (left), Subtractive (right) source: wikipedia

Michael Eckmann - Skidmore College - CS Spring 2007 CMY & CMYK Color Models C = Cyan, M = Magenta, Y = Yellow white's (c,m,y) = (0,0,0) black's (c,m,y) = (1,1,1) cyan absorbs red (and reflects green and blue) magenta absorbs green yellow absorbs blue cyan & magenta ink combined will reflect blue light (b/c red and green are absorbed) cyan & yellow ink combined will reflect green light magenta & yellow ink combined will reflect red light

Michael Eckmann - Skidmore College - CS Spring 2007 CMY & CMYK Color Models Transformation from RGB to CMY is simple. [C, M, Y] T = [1, 1, 1] T - [R, G, B] T Transformation from CMY to RGB is: [R, G, B] T = [1, 1, 1] T - [C, M, Y] T CMYK C = Cyan, M = Magenta, Y = Yellow, K = Black instead of using cyan, magenta and yellow ink all combined to make black, typically a fourth ink, black is used to produce black and grey levels

Michael Eckmann - Skidmore College - CS Spring 2007 Recap on Color Models RGB color model is an additive model. Amounts of red, green and blue are combined by adding light sources of these colors to produce new colors. CMY and CMYK are subtractive models. Amounts of cyan, magenta and yellow are combined by adding pigments of these colors to produce new colors that reflect certain wavelengths of light. –it is subtractive because adding pigments reduce what light is reflected, when all 3 primaries (C M and Y) are put together then the result is no light is reflected (black).

Michael Eckmann - Skidmore College - CS Spring 2007 HSV Color Model

Michael Eckmann - Skidmore College - CS Spring 2007 HSV Color Model Hue is an angle between 0 and 360 degrees Saturation (purity) is one axis and Value (luminance) is another axis You can picture this as a circular cone if you prefer or even as a cylinder because S is considered a ratio from the V axis (0) to max 1.

Michael Eckmann - Skidmore College - CS Spring 2007 HSV Color Model HSV color model is more intuitive/useful to people/artists Starting with the pure hue: Vary the saturation (add white) to get different tints Vary the value (add black) to get different shades

What's most important of Color theory for us in this course? Know the terminology – dominant wavelength, hue, brightness, luminance, purity, saturation, chromaticity, color gamut, idea of primary colors, etc. Know the relationship between wavelength and frequency Know what the CIE chromaticity digram is useful for and how –determining: color gamuts, complementary colors, purity –comparison of: color gamuts, purity Know how additive (e.g. RGB, YIQ) and subtractive color models (e.g. CMY) work to produce new colors Know about more intuitive models like HSV and why they're useful Know that different color models can be transformed into each other by a transform matrix. Michael Eckmann - Skidmore College - CS Spring 2007