Color is usually described by three relatively independent attributes. The combination of the three independent variables naturally forms a space coordinate, which is the color space. The color can be described from different angles with different attributes in three groups, resulting in different color spaces. However, the color object to be described is objective. Different color spaces only measure the same object from different angles.
Color space can be divided into two categories according to the basic structure: base color space and color, bright separation color space. The former is typically RGB, including cmy, CMYK, CIE xyz, etc. The latter includes ycc/YUV, lab, and a batch of "Color Space ". Cie xyz is the benchmark for defining all color spaces. It is a base color space and also a color space separated by light and color. The child type "color-like color space" in the color and bright separation color space divides the color into a table bright attribute and two table color attributes. One table color attribute is the color phase, different variables can be used to define two properties other than the color phase, while the concept of the color phase remains unchanged. Therefore, a group of people can use the color phase property together, the highlighted attribute and the table color attribute constitute a color space. They are a family of color spaces and are collectively referred to
HSB color space.
The RGB color space is a big classification. Specifically, the RGB space also contains a variety of spaces. sRGB is the standard RGB space jointly developed by HP and Microsoft, in addition to Adobe RGB, apple RGB, colormatch RGB, etc. They express RGB in different ways, so that they have different color widths and gamma values (see the next article ), I will describe these differences in RGB in another article.
For example, the lab mode is described as follows:
The RGB mode is a color adding mode for the luminous screen, and the CMYK mode is a color-reflective printing mode. The lab model does not depend on light or paint. It is a theoretical color model determined by CIE that includes all colors visible to the human eye. The lab mode makes up for the shortcomings of the RGB and CMYK color modes.
The lab mode consists of three channels, but not R, G, and B. One of its channels is brightness, that is, L. The other two are color channels, represented by a and B. Channel A includes colors from dark green (base brightness value) to gray (center brightness value) to bright pink (High Brightness Value); Channel B is from bright blue (bottom brightness value) to Gray (moderate brightness) to yellow (High Brightness ). Therefore, this mixture of colors will produce bright colors.
The lab mode has the most colors defined, is independent of light and equipment, and the processing speed is as fast as the RGB mode, much faster than the CMYK mode. Therefore, you can use the lab mode in image editing with confidence. In addition, the color is not lost or replaced when the lab mode is switched to CMYK mode. Therefore, the best way to avoid color loss is to apply the lab mode to edit the image and then convert it to the CMYK mode to print the output.
When you convert the RGB mode to the CMYK mode, Photoshop automatically converts the RGB mode to the lab mode and then to the CMYK mode.
In terms of the color range, the first is the lab mode, the second is the RGB mode, and the third is the CMYK mode.
The advantage of the lab mode is that it makes up for the shortcomings of the previous two color modes. RGB has too many transitional colors between blue and green, while there are too few transitional colors between green and red. CMYK mode loses more colors when editing and processing images, the lab mode is compensated in these aspects.
The lab mode is similar to the RGB mode. The color mixing produces brighter colors. Only the value of the brightness channel affects the color variation. The lab mode can be seen as the RGB mode of two channels plus the brightness channel mode.
The lab mode is device-independent. You can use this mode to edit and process any image (including a gray image), which is as fast as the RGB mode and several times faster than the CMYK mode. The lab mode ensures that there is no loss of color in the CMYK range during color mode conversion.
If you convert an RGB image to the CMYK mode, add an intermediate step in the operation step, that is, convert it to the lab mode first. In the formatting process of non-color newspapers, it is often used to convert images into grayscale images using the lab mode.
The following is a brief introduction to various color spaces:
- HSV Color Space
The model of HSV (hue, saturation, value) color space corresponds to a conical subset in the cylindrical coordinate system, and the top surface of the cone corresponds to V = 1. it contains the r = 1, G = 1, B = 1 in the RGB model, and the color is bright. Color H is given by the rotation angle around the V axis. The red color corresponds to the angle 0 °, the green color corresponds to the angle 120 °, and the blue color corresponds to the angle 240 °. In the HSV color model, each color is 180 ° different from its complementary color. The saturation s value ranges from 0 to 1, so the radius of the top surface of the cone is 1. The color field represented by the HSV color model is a subset of the CIE color chart. In this model, the saturation is, and the purity is generally less. V = 0, H, and s are not defined at the vertex (that is, the origin point) of the cone,
Black. S = 0, V = 1, h at the center of the top surface of the cone, Which is white. From this point to the origin, it indicates that the brightness is dimmed, that is, it has different gray levels. Values of S = 0 and H for these points are not defined. It can be said that the V-axis pairs in the HSV model should be the primary diagonal lines in the RGB color space. The color v = 1, S = 1 on the circumference of the top surface of the cone. This color is solid color. The HSV model corresponds to the painter's color scheme. The painter uses the method of changing the color concentration and color depth to obtain different colors from a solid color. In a solid color, add white to change the color concentration, and add black to change the color depth, at the same time, add a different proportion of white, black to get a variety of different colors.
- CMYK color space
CMYK (cyan, magenta, yellow) color space is used in the printing industry, the printing industry through the green (C), product (M), yellow (y) different Sites of primary color ink are stacked to show a variety of colors and order adjustment, which is the cmy color space of the primary color. In actual printing, the blue (C), product (M), yellow (Y), Black (BK) four-color printing is generally used, and the black version is adjusted to the dark tone in the middle of the printing. When the three primary colors of red, green, and blue are mixed, they are white, but black is generated when the three primary colors of blue, purple, and yellow are mixed. Since the actual ink does not produce pure color, black is included in the separate color, and this model is called CMYK. CMYK color space is related to the device or the printing process, the process method,
Ink characteristics, paper characteristics, and so on, different conditions have different printing results. Therefore, the CMYK color space is called the table color space related to the device. In addition, CMYK has multiple values. That is to say, for the same color with the same absolute color, the CMYK digital combination can be used to represent and print the same printing process. This feature brings a lot of trouble to color management, as well as a lot of flexibility to control. In the printing process, it is necessary to go through a color separation process. The so-called color separation is to convert the RGB color used in the computer into the CMYK color used for printing. There are two complicated problems in the conversion process. One is that the two color spaces are not exactly the same in the display color range.
The color gamut is large, while CMYK is small. Therefore, the color gamut must be compressed. The other two colors are related to specific devices, and the color itself is not absolute. Therefore, a device-independent color space is required for conversion, that is, the above XYZ or LAB color space can be used for conversion.
- HSL Color Space
HSL (hue, saturation, lightness) color space, which is represented by the color of the user's desktop graphics program. It uses a hexagonal cone to represent its own color model.
- HSB Color Space
HSB (hue, saturation, brightness) color space, which is represented by the color of the user's desktop graphics program and uses a hexagonal cone to represent his own color model.
- HSI color space
The HSI color space is used to describe the color from the human's visual system, using hue, saturation or chroma, and brightness (intensity or brightness. The HSI color space can be described using a conical space model. The cone model used to describe his color space is quite complex, but it can clearly show the variation of the color tone, brightness, and color saturation. Generally, the color and saturation are called the color, which is used to indicate the color category and depth. Human Vision is much more sensitive to brightness than to brightness. To facilitate color processing and recognition, human vision systems often use HSI color space, it is more human visual than RGB color space. A large number of algorithms can be used in the HSI color space in image processing and computer vision.
They can be processed separately and are independent of each other. Therefore, the workload of image analysis and processing can be greatly simplified in the HSI color space. The HSI color space and the RGB color space are different representations of the same physical quantity, so there is a conversion relationship between them.
- Ycc Color Space
The color space invented by Kodak, because photocd requires a compression mode when storing images, photocd uses the ycc color space, and the ycc space uses the brightness as its main component, it has two separate color channels and uses ycc color space to save images, saving storage space.
- XYZ Color Space
The International Lighting Commission (CIE) conducted a large number of visual measurements and statistics for normal people. In 1931, it established the "Standard Color observer", which laid the quantitative basis for the modern CIE standard color measurement. Because "Standard Color observer" is used to calibrate spectral colors with negative stimulus values, the calculation is inconvenient and difficult to understand. Therefore, in 1931, based on the RGB system, A new color system is built with three hypothetical primary colors x, y, and z. It matches the tristimulus value of the isoenergy spectrum, and is named "cie1931 standard color observer's tristimulus value", referred to as "cie1931 standard color observer ". This system is called "cie1931 standard color system" or "2 ° field of view XYZ color system ". CIEXYZ color space can be converted to yxy color space with a slight change, where Y takes the value of Y in the three stimulus values,
Brightness. x and y indicate the color. Definition: In color management, it is very important to select a color space irrelevant to the device. The color space irrelevant to the device is determined by the International Lighting Commission (CIE, this includes two standards: CIEXYZ and cielab. They contain all the colors that human eyes can tell. In addition, the establishment of the cieyxy color measuring system creates conditions for the determination of colors. However, in this space, the distance between two different colors does not correctly reflect the difference in people's color perception, that is, in the cieyxy color building diagram, the color tolerance varies in different positions and directions, which is the non-uniformity of the yxy color space. The existence of this defect makes it impossible to intuitively evaluate the color in the yxy and XYZ spaces.
- Lab color space
The LAB color space is a color model developed by CIE (International Lighting Commission. Any color in nature can be expressed in the lab space, and its color space is larger than the RGB space. In addition, this mode is used to describe human visual sensing in a digital way and has nothing to do with the device. Therefore, it makes up for the shortcomings that RGB and CMYK modes must depend on the Color Characteristics of devices. The color space of the lab is larger than that of the RGB and CMYK modes. This means that the color information that RGB and CMYK can describe can be reflected in the lab space. The LAB color space uses the coordinate lab, where l brightness; positive numbers of a represent red, negative ends represent green; positive numbers of B represent yellow, negative ends represent blue (A, B) with L = 127f (y)-16,
A = 500 [F (x/0.982)-f (y)], B = 200 [f (y)-f (z/1.183)]; where: F (x) = 7.787x + 0.138, x <0.008856; f (x) = (x) 1/3, x> 0.008856
- YUV color space
In Modern Color TV systems, a three-pipe color camera or a Color CCD (DOT coupler) camera is usually used to obtain the color image signal after color separation and amplification correction respectively, after the matrix conversion circuit, the Brightness Signal y and two chromatic aberration Signals R-y and B-Y are obtained. Finally, the sender encodes the brightness and chromatic aberration Signals respectively and sends them out through the same channel. This is our commonly used YUV color space. The importance of using YUV color space is that its brightness signal y is separated from the color signal u and v. If only the Y Signal component is used, and the U and V components are not used, the black and white gray images are displayed. The color TV uses the YUV space to solve the compatibility problem between the color TV and the black and white TV with the Brightness Signal y, so that the black and white TV can also receive the color signal. According to the NTSC standard
When the brightness is represented by Y, its relationship with red, green, and blue colors can be described by the following equation: Y = 0.3r + 0.59G + 0.11b, which is a commonly used brightness formula. Chromatic Aberration U, V is made of B-y, R-Y compressed according to different proportions. If you want to convert the YUV space into an RGB space, you only need to perform inverse operations. Similar to the YUV color space, there is also the LAB color space. It also uses brightness and chromatic aberration to describe the color components, where L is the brightness, A and B are the color components respectively.
