RGB color model
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.
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, green, and blue light (wavelengths of 564nm, 534nm, and 420nm respectively). The color yellow, for example, is perceived when the yellow-green receptor is stimulated slightly more than the green receptor, and the color red is perceived when the yellow-green receptor is stimulated significantly more than the green receptor.
Although the peak responsitivities of the cones do not occur at the red, green and blue wavelengths, those three colors are described as primary because they can be used relatively independently to stimulate the three kinds of cones.
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 & cathode ray tube
One common application of the RGB color model is the display of colors on a cathode ray tube or liquid crystal display such as a television picture tube 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 cathode ray tube or LCD 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.
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.
- (0, 0, 0) is black
- (255, 255, 255) is white
- (255, 0, 0) is red
- (0, 255, 0) is green
- (0, 0, 255) is blue
- (255, 255, 0) is yellow
- (0, 255, 255) is cyan
- (255, 0, 255) is magenta
There is also a 16 bpp mode, in which there are either 5 bits per color, called 555 mode, or an extra bit for green (because the eye can see more shades of green than of other colors), called 565 mode. The 24 bpp mode is typically called Truecolor, while the 16 bpp mode is called HiColor .
16-bit mode can also refer til 16 bit/component, resulting in 48 bpp. This modes makes it possible to represent 65535 tones of each color component instead of 255. This is primarily used in professional image editing, like Adobe Photoshop for maintaining larger precision.
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.
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.
Colors in web design
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.
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.
- Additive color
- Color model
- CMYK color model
- List of colors
- HSV color space
- HLS color space
- Colour banding
- Bits per pixel
- RGB Hex Color Chart - Chart of web friendly colors for web developers
- Color Charts
- RGB-Color Mixer Java-Plugin required
- Learning by Simulations RGB Color Model