Chapter II, Digital Color Theory: Lesson II Creating / Displaying Digital Color http://www.kodak.com/country/US/en/digital/dlc/book3/chapter2/digColorM2_1.shtml

                             For thousands of years, people have used natural dyes and pigments to produce color

                             Artists have produced masterful mixtures of color. Of course, each painting was a unique one of a kind.

                             The printing press allowed reproduction of color. Block printing in multiple colors dates back to the 15th century.

                             Color photography was the breakthrough that allowed direct reproduction of the colors in nature. The first color photograph was taken by James Clerk Maxwell to illustrate the principle of additive color. Three separate black-and-white film exposures were made and projected using filters

                             Today's color film integrates three micro-thin layers of colorants on a clear base. The colorants are cyan, magenta, and yellow dyes.

                             Color photography captures gradations of color and tone that are smooth and natural to the eye. It's called a "continuous tone" medium.

                             Development of the halftone screen in the 1880's was the breakthrough that allowed process color printing of color images. The screen creates small halftone dots, which can simulate subtle gradations of color and tone.

                             In effect, this process mimics the first color photograph. Filters separate color into black-and-white images. These separated images are recombined on a press with dots of color ink.

                             Today, four process color inks are normally used: cyan, magenta, yellow, and black. Black was once called the "key" and is represented by a "K."

                             Television was another notable advance in color reproduction. Red, green, and blue phosphors create the colors we see on a TV monitor. Video began as an analog technology.

                             Computer monitors display colors recorded digitally. Digital imaging is not revolutionizing color reproduction. The purpose of most digital imaging is to produce a full color print.

                             Colorants in desktop digital printers vary. Some combinations of cyan, magenta, yellow, and black dyes, inks or toners can be used. Understanding how color is produced can help you create high-quality digital images.

                             Additive and subtractive color mixing are the two primary methods for reproducing a range of colors.

                             The additive system combines light to produce a range of colors. Red, green, and blue are the primary additive colors. Equal amounts of all three produce white light.

                             When two equal amounts of primary additive colors are mixed, complementary colors are created.

Example 1                                                                                                                     Where blue and green combine, cyan is created. Red is not used to create cyan. So red and cyan are defined as complementary colors.

Example 2                                                                                                                     Where red and green light are mixed, yellow is created. Since red and green appear yellow, yellow can be defined as the absence of blue. The complement of blue is yellow.

Example 3                                                                                                                     Equal amounts of red and blue create magenta. The complement of magenta is green.

                             Television is the most familiar application of additive color mixing. A close look at the screen image shows clusters of red, green and blue dots or stripes, which are phosphors.

                             These phosphors emit colored light when struck by electrons. Red, green and blue light is emitted in mixtures that simulate a wide range of colors.

                             Filters offer another method for controlling the color of light. They play an important role in digital scanners and cameras. Filters control the colors recorded by passing some colors and blocking others.

                             For example, if we shine white light through a green filter, only the green light passes. Red and blue are absorbed. Remember that combining red and blue light produces magenta. So you can think of the green filter as absorbing magenta. A filter passes its own color and absorbs or blocks its complement.

                             Red filters pass red light and absorb cyan. Cyan is a combination of green and blue.

                             Blue filters pass blue light and absorb yellow light. A blue filter subtracts red and green from white light. Filters produce color by subtracting wavelengths of light.

                             Unlike additive color mixing, the subtractive system works by taking color away from white light. When all color has been removed from light, what's left is black. The subtractive system uses colored pigments and dyes that filter light. Its primary colors are cyan, magenta and yellow.

                             There is a relationship between primary additive and subtractive colors. You can see this by placing the colors in a triangle. Additive primaries are placed at points around the triangle. Subtractive primaries are placed between the two additive primaries that combine to produce them.

                             Subtractive colors subtract the color across from them, their complement, from white light.

                             Here you see the effect magenta paint has on white light, which is composed of equal amounts of red, green and blue. Green light is subtracted by the magenta paint. Only red and blue are reflected. Red and blue combine to form magenta, which is the color you see.

                             When cyan is mixed with magenta paint, the cyan subtracts its complement red from the remaining light. That leaves only blue ... which is what you see.

                             If the third subtractive primary yellow is added to the mix, all light is blocked. Combining equal amounts of cyan, magenta and yellow subtracts all light and produces black.

                             A full range of intermediate colors is produced by controlling the amount of each primary in the subtractive color mixture.

                             The three-color process is at the heart of color reproduction on any type of paper.

                             Whether the colorants are dyes, inks, or toners, they act as filters. They block complementary colors and reflect their own color from the white surface of the paper.

                             The three color process produces a wide range of color hues by varying the strength of subtractive primaries that are overprinted. Let's consider why red, green and blue primaries are not used in the three-layer process.

                             Let's begin with the red layer. It will pass its own color red. It will filter out green and blue, which create its complement, cyan. Now suppose you need to make yellow from red, green and blue layers. To make yellow, you need to combine green and red. When you overprint green on top of red, you find that all color is blocked. The green layer blocks its complement magenta which includes red. There is no color left to be reflected from the paper

                             Additive colors cannot be used because they block two of the primary additive colors. Subtractive colors block only one. This is why layers of cyan, magenta and yellow are used in many color systems, including photography and printing.

                             This chart compares the color gamut of additive and subtractive systems. They are quite different. Each can create some colors the other system can't produce. This makes precise matching difficult.

                             In general, additive systems such as computer monitors can create more light colors than a subtractive system, which conversely, can create more dark colors. Each excels at producing its own primaries.

                             Successful color reproduction is the art and science of mixing and matching colors to the client's satisfaction.

Digital systems capture, display, process and print color images.                              Digital systems capture, display, process and print color images.

                             At each stage, color is represented in digit code as a series of ones and zeros.

Each imaging device must convert that numeric code into a color image.                              Each imaging device must convert that numeric code into a color image.

                             The pixel is the basic building block of all digital images. The term stands for "picture element." It is the smallest unit of an image

                             A pixel can be compared to a stitch in a needle point image. Each represents the smallest unit of the image. When you see the entire image, the stitches blend together to form a recognizable picture. The needle point picture is a uniform grid of stitches. Every stitch is located very precisely within the grid. It has the same properties as a pixel.                             

                             A single stitch displays only one color, and all stitches are the same shape and size. Your computer image is also made up of individual "stitches" or pixels. Pixels are very small. On a monitor, there may be 72 pixels per inch. Laser printers produce 300 or more per inch. Like a stitch in a needle point image, every pixel resides within a uniform grid, called a "bit map," that cannot be varied.

                             A digital device must be capable of representing three properties of a pixel: its color, location, and size.

                             Pixels cannot overlap, so their size is determined by the resolution of the grid or "bit map."

The                              The resolution of output devices is typically much higher than the 72 pixels per inch that you see on your computer monitor, so pixels are smaller.

                             Location of a pixel is assigned a numeric value, based on the horizontal and vertical pixel-count on the grid.

                             Any location can be specified by assigning an X and Y value to the horizontal and vertical axes.

                             In this example, an X value of 320 and a Y value of 240 will position a pixel in the middle of the screen.

                             Now, let's see how color values are assigned to pixels. Color must be converted to digital code: zeros and ones. A single bit is one electrical impulse. It can be on or off. White or black.

                             When two bits are used, the computer can count to four. It can identify four discrete colors or tones. Adding an additional bit raises the possible number of colors by a power of two. Many computers use 8 bits to represent color values. This means that a single pixel can represent 28 or 256 individual colors.

                             Most digital imaging applications use 24-bit color. Three channels of 8-bit color are intermixed. Since each channel has 256 values, the total is 2563 which is 16.7 million color values.

                             The monitor uses additive color mixing to produce color. It emits red, green and blue light in varying proportions.

                             The missions of colors mentioned earlier are produced by adjusting values of red, green and blue primaries.

                             Let's consider how magenta is generated. It's a combination of red and blue. Since there is no green present, the computer assigns a zero value to green. Red and blue are given one of 256 possible values, depending on the saturation and lightness of the color.

Computer monitors use a cathode ray tube or CRT to generate color.                              Computer monitors use a cathode ray tube or CRT to generate color.

                             The inside of the monitor screen is covered with phosphor dots or stripes, as we saw earlier. Dots are arranged in groups of three, called "triads."

                             Three electron guns fire a beam of electrons through a shadow mask to excite the three phosphors in each triad. The glowing of the phosphor dots in the triad is controlled by the beam current. Because the three color phosphors are clustered very close together, they are viewed as one pixel. When a scanned picture is displayed, the color of each pixel is determined by the color values in the scanned image file.

                             In all image editing and creation software, color palettes provide a way to choose colors. The most common type is the system palette. If the system uses 8-bit color, the palette will include 256 colors

                                                          Another method for specifying color is the Pantone Matching System (PMS). Many color software packages allow you to select a Pantone color from an on-line library. It is best to refer to a Pantone swatch book to see how a Pantone color will look when printed.

                             Most software also allows you to create a customized color palette, using a color picker. The picker allows you to specify color by values of Hue, Saturation and Brightness .. or values of Red, Green and Blue.

                             The RGB color space defines color based on values of red, green and blue within the color.

                             Some people prefer to work with Hue, Saturation and Brightness values. This enables you to change the brightness or lightness of an image, without affecting its hue.

                             CMYK is the color space used for printed reproduction. People familiar with the meaning of halftone dot values often choose this option. Color management software is available to optimize color values as they move through the reproduction process. Each imaging component produces a unique range of colors. Software profiles are available to produce the highest-quality results as images move from one stage to the next.