RGB color model

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The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. The very idea for the model itself and the abbreviation "RGB" come from the three primary colors in additive light models.

Note that the RGB color model itself does not define what exactly is meant by "red", "green" and "blue", so that the same RGB values can describe noticeably different colors on different devices employing this color model. While they share a common color model, their actual color spaces can vary considerably.

Contents

Biological basis of primary colors

Primary colors are related to biological rather than physical concepts, based on the physiological response of the human eye to light. The human eye contains photoreceptor cells called cones which normally respond most to yellowish-green (long wavelength or L), green (medium or M), and bluish-violet (short or S) light (peak wavelengths of 564 nm, 534 nm, and 420 nm respectively). The difference in the signals received from the three kinds allows the brain to perceive a wide gamut of different colors.

Now suppose that light in the yellow range of wavelengths (approximately 577 nm to 597 nm) enters the eye and strikes the retina. Light of these wavelengths would activate both the medium and long wavelength cones of the retina. Upon striking the retina, the physiological occurs: electrical messages are sent by both kinds of cones to the brain. Once received by the brain, the psychological occurs: the brain recognizes that the light has activated both kinds of cones and interprets this (at some higher level) to label the object as yellow. In this sense, the yellow appearance of objects is simply the result of yellow light from the object entering our eye and stimulating the relevant kinds of cones simultaneously.

To generate optimal color ranges for species other than humans, other primary colors would have to be used. For species with four different color receptors, such as many birds, one would use four primary colors; for species with just two kinds of receptors, such as most mammals, one would use two primaries.

RGB and displays

One common application of the RGB color model is the display of colors on a cathode ray tube, liquid crystal display, or plasma display, such as a television or a computer's monitor. Each pixel on the screen can be represented in the computer's memory as independent values for red, green and blue. These values are converted into intensities and sent to the display. By using the appropriate combination of red, green and blue light intensities, the screen can reproduce many of the colors between its black level and white point. Typical display hardware used for computer monitors in 2003 uses a total of 24 bits of information for each pixel (commonly known as bits per pixel or bpp). This corresponds to 8 bits each for red, green, and blue, giving a range of 256 possible values, or intensities, for each color. With this system, approximately 16.7 million discrete colors can be reproduced, although the human eye can distinguish between only around 10 million discrete colors (this number varies from person to person depending upon the condition of the eye and the age of the person).

Video electronics

Analog video standards

Composite video - S-Video - SCART - RGB - Component video

RGB is a type of component video signal used in the video electronics industry. It consists of three signals - red, green and blue - carried on three separate cables. Extra cables are sometimes needed to carry synchronising signals. RGB signal formats are often based on modified versions of the RS-170 and RS-343 standards for monochrome video. This type of video signal is widely used in Europe since it is the best quality signal that can be carried on the standard SCART connector. Outside Europe, RGB is not very popular as a video signal format – S-Video takes that spot in most non-European regions. However, almost all computer monitors around the world use RGB.

Non-linearity

The intensity of the color output on computer display devices is normally not proportional to the R, G, and B values. That is, even though a value of 127 is very close to halfway between zero and 255, the light intensity of a computer display device when displaying (127, 127, 127) is normally only 18% of that when displaying (255, 255, 255), instead of at 50%. See gamma correction for more background on this issue.

Professional color calibration

Proper reproduction of colors in professional environments requires extensive color calibration of all the devices involved in the production process. This results in several transparent conversions between device-dependent color spaces during a typical production cycle in order to ensure color consistency throughout the process. Along with the creative processing, all such interventions on digital images inherently damage it by reducing its gamut. Therefore the denser the gamut of the original digitized image, the more processing it can support without visible degradation. Professional devices and software tools allow for 48 bpp images to be manipulated (16 bits per channel) in order to increase the density of the gamut.

Representations

24-bit representation

Color depth

8-bit color
15/16 bit: Highcolour
24/32 bit: Truecolour
Web-safe color

Related

RGB color model
Palette

Main article: Truecolor

When written, RGB values in 24 bpp, also known as Truecolor, are commonly specified using three integers between 0 and 255, each representing red, green, and blue intensities, in that order. For example:

The above definition uses a convention known as full-range RGB. This convention is so often used that some people have come to view it as universal. This is a problem because most color science discusses color values in the range 0.0 to about 1.0, rather than 0 to 255. If in doubt, it is best to describe the units in which a color is specified.

Full-range RGB can only represent fifteen shades of a given hue. This tends to undermine the representation of shades of any solid color, especially pure gray; such gradients tend to be noticeably quantized. This effect is most prevalent in photographic images with dark shadows or that depict outer space or nighttime scenes. For this reason, 16-bit-per-channel modes (see below) are sometimes favored for editing such images, especially when the images are destined for reproduction within a wider color space.

Typically, RGB for digital video is not full range. Instead, video RGB uses a convention with scaling and offsets such that (16, 16, 16) is black, (235, 235, 235) is white, etc. For example, these scalings and offsets are used for the digital RGB definition in CCIR 601.

Memory space

The amount of memory, in bytes, that a 24-bit image occupies in its raw state, can be found by multiplying the number of pixels in the image by 3. A 640 × 480  24-bit RGB color image will have 640 × 480 = 307,200 pixels. Thus, the memory space required is 307,200 × 3 = 921,600 bytes = 900 kilobytes.

16-bit mode

Main article: Highcolour

There is also a 16 bpp mode (sometimes called HiColor), in which there are either 5 bits per color, called 555 mode, or an extra bit for green (because the eye can distinguish more shades of green than of other colors), called 565 mode.

32-bit mode

The so-called 32 bpp mode is almost always identical in precision to the 24 bpp mode, there are still only eight bits per component, the eight extra bits are simply not used at all (except possibly as an alpha channel). The reason for the existence of 32bpp modes is the higher speed at which most modern hardware can access data that is aligned to byte addresses evenly divisible by a power of two, compared to data not so aligned.

48-bit mode (sometimes also called 16-bit mode)

"16-bit mode" can also refer to 16 bit per component, resulting in 48 bpp. This modes makes it possible to represent 65536 tones of each color component instead of 256. This is primarily used in professional image editing, like Adobe Photoshop for maintaining greater precision when a sequence of more than one image filtering algorithms is used on the image. With only 8 bit per component, rounding errors tend to accumulate with each filtering algorithm that is employed, distorting the end result.

RGBA

With the need for compositing images came a variant of RGB which includes an extra 8 bit channel for transparency, thus resulting in a 32 bpp format. The transparency channel is commonly known as the alpha channel, so the format is named RGBA. Please note that since it doesn't change anything in the RGB model, RGBA is not a distinct color model, it's only a file format which integrates transparency information along with the color information in the same file.

Colors in web design

Main article: Web colors

Colors used in web design are commonly specified using RGB; see web colors for an explanation of how colors are used in HTML and related languages. Initially, the limited color depth of most monitors led to a limited color palette of 216 RGB colors - defined by the Netscape Color Cube. However, with the predominance of 24-bit displays, the use of the full 16.7 million colors of the HTML RGB color code no longer poses problems for most viewers.

In short, the web safe color palette consists of the 216 combinations of red, green and blue where each color can take one of six values (in hexadecimal): #00, #33, #66, #99, #CC, or #FF. Clearly, 63 = 216.

The RGB color model for HTML was formally adopted as an Internet standard in HTML 3.2, however it had been in use for some time before that.

History of RGB color model

The use of the RGB color model as the standard for presentation of color on the Internet has its roots in the 1953 RCA color-TV standards and in Edwin Land's use of an RGB standard in the Land / Polaroid camera.

See also

External links

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