About color

Introduction

Explain what color is in the computer environment, and how it is handled by the computer is a very difficult task, but I will try to make a “easy-to-understand” resume, so you (probably) will enhance your 2D/3D projects in many ways, as long as I know, this knowledge will improve also the tools you use, so make a bookmark now.

Computers only work with ceros and ones, so all the information must be coded this way. When you store a text all the letters are encoded using this method, and then decoded in the reverse direction so you, human, can read the text. Well, with color is the same. The way it works is very important, and you must understand the bit-level work in order to take full advantage of you image processing system, web image editing tools, composition system, editing system and of course, your rendering engine. Some maths (again), supose this byte (a group of 8 bits), we can “address” 256 tones of grey, starting always from tone 0 to 255, that is, 256 tones which in fact is 2^8 (power), and this is the tone of red (for example) we addressed…


bits

when working with black and white images only (greys) one channel is used, from black to white. If we work in RGB mode, we need three channels, that is how monitors work. If you plan to print your work you must convert your RGB image to CMYK mode, which is the way printers work.So you could ask how many colors can we reproduced on a standar screen, well, with the RGB 8 bit model we can arrive to 256 x 256 x 256, that is just 16.777.216 of tones!!! Problably you think this is too much, but not, it isn´t. Later I will discuss this. Here I must give some new concepts about color encoding, are you ready?

COLOR MODELS
The models are the mathematical way we choose to represent and handle the color space.

rgb model
A large percentage of the visible spectrum can be represented by mixing three basic components of colored light in various proportions and intensities. These components are known as the primary colors: red, green, and blue (RGB). When the three primary colors overlap, they create the secondary colors: cyan, magenta, and yellow (CMY).Since the primary colors combine to create white, they are also called additive colors, so all the light is reflected back to the eye. Additive colors are used for lighting, video, film recorders, and monitors.

cmyk model
while the RGB model depends on a light source to create color, the CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, a portion of the spectrum is absorbed. Color that is not absorbed is reflected back to your eye. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to absorb all color and produce black; for this reason they are also called subtractive colors. Because all printing inks contain some impurities, these three inks actually produce a muddy brown and must be combined with black (K) ink to produce a true black. (The letter K is used to avoid confusion, because B might also stand for blue.) Combining these inks to reproduce color is called four-color process printing. The additive and subtractive colors are complementary colors. Each pair of subtractive colors creates an additive color.

hsb model
The HSB model is based on the human perception of color. In the HSB model, all colors are described in terms of three fundamental characteristics:Hue is the wavelength of light reflected from or transmitted through an object. More commonly, hue is identified by the name of the color such as red, orange, or green. Hue is measured as a location on the standard color wheel and is expressed as a degree between 0º and 360º.Saturation, sometimes called chroma, is the strength or purity of the color. Saturation represents the amount of gray in proportion to the hue and is measured as a percentage from 0% (gray) to 100% (fully saturated). Brightness is the lightness or darkness of the color, as percentage from 0% (black) to 100% (white) relative to the color.

lab model
The Lab color model is based on the original color model proposed by the Commission Internationale d’Eclairage (CIE) as an international standard for color measurement. This model was refined and named CIE Lab. The Lab model addresses the problem of the variability of color reproduction that results from the use of different monitors or different printing devices. Lab color is designed to be device independent; that is, it creates consistent color regardless of the specific device, such as the monitor, printer, or computer, that you use to create or output the image. Lab color consists of a luminance, or lightness component (L) and two chromatic components: the a component, which ranges from green to red, and the b component, which ranges from blue to yellow.

COLOR MODES

grayscale
Grayscale mode uses up to 256 shades of gray to represent an image. Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white and 100% is equal to black). Images produced using black-and-white or grayscale scanners are typically displayed in Grayscale mode. You can convert both Bitmap-mode and color images to grayscale. Grayscale mode lets you convert a color image to a high-quality black-and-white image.

indexed
An indexed-color image is based on a palette of maximum 256 colors. When you convert an image to indexed color, Photoshop builds a color lookup table, which stores and indexes the colors in the image. If color in the original image does not appear in the table, the program matches the color to the closest color in the color table or simulates the color using the available colors. Indexed color mode is useful when you want to limit the palette of colors used in an image, for example, when you want to use the image in a multimedia animation application or on a Web page. Using an indexed color table lets you reduce the file size of an image while maintaining the visual quality that you need.
The computer scheme adds some “extra” combination, we call them modes, they are a way to work, not a model itself.

multichannel
As you might guess from its name, a multichannel image is one that contains multiple channels, each having 256 levels of gray. Multichannel images are used for specialized printing purposes, such as printing a grayscale image with spot color or converting a duotone for printing in Scitex CT format.
You can convert any image composed of more than one channel to a multichannel image. When you convert to a multichannel image, the original channels are assigned numbers. When you convert a color image to multichannel, the individual color channels are converted to grayscale information (remember that these was always a grayscale information channel) that reflects the color values of the pixels in each channel. If you delete a channel from an RGB, a CMYK, or a Lab image, the image is automatically converted to multichannel mode.

COLOR GAMUT
The gamut of a color system is the range of colors that can be displayed or printed. The spectrum of colors that can be viewed by the human eye is wider than any method of reproducing color.

rt_gamut

Among the color models used in your software, Lab has the largest gamut and encompasses all the colors in the RGB and CMYK gamuts. The RGB gamut contains the subset of these colors that can be viewed on the computer or television monitor (which emits red, green, and blue light). Some colors, such as pure cyan or pure yellow, can’t be displayed accurately on a monitor. The smallest gamut is that of the CMYK model, which consists of colors that can be printed using process-color inks. When colors that cannot be printed are displayed on the screen, they are referred to as out-of-gamut colors (that is, they are outside the CMYK gamut).
In addition to determining the number of colors that can be displayed in an image, color modes affect the number of channels and the file size of an image. In general, increasing the number of colors or channels in an image also increases the file size.

RECOMENDATIONS
You can perform all tonal and color corrections in either RGB or CMYK mode. If your image is intended for video display, you may never convert it to CMYK mode. Conversely, if you began with a CMYK scan, you won’t perform any corrections in RGB mode. If you are working with an RGB image that you intend to separate, Adobe recommends that you perform most of your color corrections in RGB mode, and then use CMYK mode for fine-tuning as necessary. If you are concerned with precise CMYK values, however, or if you want to adjust the CMYK plates directly, you may still prefer to perform your color corrections in CMYK mode.
Keep in mind the following when deciding whether to perform color corrections in RGB or CMYK mode:
Working in RGB mode requires significantly less memory and, therefore, improves performance.
Performing corrections in RGB mode ensures device-independence: that is, the corrections you make to the image are preserved regardless of the monitor, computer, or output device you use. If any of these devices change, you simply need to change the appropriate Monitor Setup and Printing Inks Setup options, and then reconvert the RGB image to CMYK. Note that converting multiple times back and forth between RGB and CMYK modes is not recommended because color values are rounded in the conversion process.

JARGON
Probably you have been already heard of things like pixel depth, alpha channel and may be interesting to do a tiny resume of what we all refer to, so we all speak the same language.

Bitmap images
Bitmap images are made up of one bit of color (black or white) per pixel, and require the least amount of disk space.

Grayscale images
Grayscale images are made up of 8 bits of information per pixel and use 256 shades of gray to simulate gradations in color.

Indexed image
Indexed color images are single-channel images (8 bits per pixel) that use a color lookup table containing 256 colors. For editing you should convert temporarily to RGB mode.

RGB Image
RGB images use three colors to reproduce up to 16.7 million colors on-screen it it is 8 bits per channel.

Pixel depth
Also called bit resolution or color depth, measures how much color information is available for each pixel in an image. Greater pixel depth (more bits of information per pixel) means more available colors and more accurate color representation in the image.

Alpha channel
Is an added channel in the selected mode, normally RGB, that stores transparency information. This channel determines if the current pixel is more or less opaque like this, 0% (black) means transparent and 100% (white) is totally opaque.

FINAL WORDS

At the begining I said that 8 bit/channel encoding is not enough, well, probably is enough for you at press, or TV, but if you want to make any kind of correction for cinema you should work with 10 bit/channel or 12 bit/channel. Very few machines can work with so many information and the storage capacities increase in a very important way, making the most amazing machine feel slow, but you must understand that your client never will work with you if you show him a ton of artifacts on the screen.The degradation of the image because the process of manipulating these material is the other big problem, keep it simple.

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