Color mode not only determines the number of colors that can be displayed in the image, but also affects the number of channels and file size of the image. The channel mentioned here is also an important concept in Photoshop. Each Photoshop image has one or more channels, and each channel stores the information of color elements in the image. The default number of color channels in an image depends on its color mode. For example, a CMYK image has at least four channels to distinguish information representing cyan, magenta, yellow and black. In addition to these default color channels, other channels called Alpha channels can be added to the image, so that the selection can be saved and edited as a mask, and spot color channels can be added. Sometimes there are as many as 24 channels in an image. By default, there is still a channel in bitmap mode, with gray two-tone and indexed color images. RGB and Lab images have three channels; CMYK images have four channels.
I. HSB mode
The HSB mode is defined based on the observation of color by human eyes. In this mode, all colors are described by hue or hue, saturation and brightness.
1. hue (h)
Tone is the physical and psychological characteristics of color related to the main wavelength of color. It is known from experiments that visible light with different wavelengths has different colors. Many wavelengths of light can be mixed in different proportions to form various colors, but as long as the wavelength composition is certain, then the color is certain. Achromatic colors (black, white and gray) have no hue attribute; All colors (red, orange, yellow, green, cyan, blue, purple, etc. ) is an attribute that represents the appearance of a color. It's all hue, and sometimes hue is also called hue.
2. Saturation
Saturation refers to the intensity or purity of a color, indicating the proportion of gray components in a hue, expressed by 0%- 100% (pure color).
3. brightness (b)
Brightness is the relative brightness of a color, usually measured by 0% (black)-100% (white).
Second, RGB mode.
RGB mode is based on the mixing principle of three primary colors of light in nature, and three primary colors of red (R), green (G) and blue (B) are assigned to each color scale according to the brightness values from 0 (black) to 255 (white), so as to specify their colors. When the primary colors with different brightness are mixed together, 256*256*256 colors will be produced, which is about 654.38+06.7 million. For example, bright red may have an R value of 246, a G value of 20 and a B value of 50. When the brightness values of the three primary colors are equal, gray is generated; When all three brightness values are 255, pure white is produced; And when all brightness values are 0, pure black is generated. When three colors are mixed, the generated color is generally higher than the brightness value of the original color, so the method of generating color in RGB mode is also called color light additive method.
Third, CMYK mode
CMYK color mode is a printing mode. Among them, four letters refer to cyan, magenta, yellow and black respectively, representing four colors of ink in printing. There is no difference between CMYK mode and RGB mode in essence, but the principle of color generation is different. In RGB mode, color is produced by mixing light emitted by light sources, while in CMYK mode, color is produced by light shining on paper with different proportions of C, M, Y and K inks, and part of the spectrum is absorbed and reflected to human eyes. Because when C, M, Y and K are mixed, with the increase of C, M, Y and K, less and less light will be reflected to human eyes, and the brightness of light will be lower and lower. All methods of generating colors in CMYK mode are also called color light subtraction.
Fourth, the laboratory model.
The prototype of Lab mode is the standard of color measurement formulated by CIE Association in 193 1, which is redefined in 1976 and named CIELab. This mode solves the color support difference caused by the difference between the display and the printing device, that is, it is independent of the device.
Lab color is represented by brightness component L and two color components A and B. The value range of L is 0- 100, component A represents the change of spectrum from green to red, component B represents the change of spectrum from blue to yellow, and the value range of A and B is-120- 120.
Lab mode contains the widest range of colors, which can include all colors in RGB and CMYK modes. CMYK mode contains the least number of colors, and some colors of machetes on the screen cannot be realized in printed matter.
Verb (abbreviation for verb) Other color patterns
In addition to the basic RGB mode, CMYK mode and Lab mode, Photoshop also supports (or processes) other color modes, including bitmap mode, grayscale mode, two-tone mode, indexed color mode and multi-channel mode. These color patterns have their special uses. For example, the image in gray mode has only gray value and no color information; Although the index color mode can use colors, compared with RGB mode and CMYK mode, there are really few colors that can be used. Here are some color patterns.
1. bitmap mode
Bitmap mode uses two colors (black and white) to represent pixels in an image. Images in bitmap mode are also called black-and-white images. Because its depth is 1, it is also called a one-bit image. Because the bitmap mode only uses black and white to represent the pixels of the image, many details will be lost when the image is converted into bitmap mode, so Photoshop provides several algorithms to simulate the missing details in the image.
When the width, height and resolution are the same, the image size of bitmap mode is the smallest, which is about 1/7 of gray mode and 1/22 of RGB mode.
2. Gray mode
Gray mode can use up to 256 gray levels to represent the image, which makes the transition of the image smoother and more delicate. The brightness value of each pixel of the gray image is between 0 (black) and 255 (white). Gray value can also be expressed as a percentage of black ink coverage (0% equals white, 100% equals black). Images generated by black folding or grayscale scanners are usually displayed in grayscale.
3. Dual-tone mode
Two-tone mode uses 2-4 kinds of color inks to create an image composed of two tones (2 colors), three tones (3 colors) and four tones (4 colors). In the process of converting a gray image into a two-tone mode, the tone can be edited to produce special effects. The main purpose of using the two-tone mode is to express as many color levels as possible with as few colors as possible, which is very important to reduce the printing cost, because each additional tone needs more cost when printing.
4. indexed color model.
Indexed color mode is a common image mode in Internet and animation. When a color image is converted into an image of indexed color, it contains nearly 256 colors. The indexed color image contains a color table. If the colors in the original image cannot be represented by 256 colors, Photoshop will select the closest color from the available colors to simulate these colors, which can reduce the size of the image file. Used to store the colors in the image and establish a color index for these colors. The color table can be defined during the conversion process or modified after the image is indexed.
5. Multi-channel mode
Multi-channel mode is very useful for images with special printing requirements. For example, if an image only uses one or two or two or three colors, using multi-channel mode can reduce the printing cost and ensure the correct output of image colors.
6.8-bit/16-bit channel mode
In grayscale RGB or CMYK mode, you can use 16 bit channel instead of the default 8-bit channel. By default, an 8-bit channel contains 256 color levels. If it is increased to 16 bit, the number of color levels of each channel is 65536, so that more color details can be obtained. Photoshop can recognize and input the image of 16-bit channel, but this image has many limitations and cannot use all filters. In addition, the image of the 16 bit channel cannot be printed.
Six, color mode conversion
In order to output images correctly in different situations, it is sometimes necessary to convert images from one mode to another. Photoshop converts the required color mode by executing the commands in the Image/Mode submenu. This color mode conversion sometimes permanently changes the color values in the image. For example, when an RGB mode image is converted into a CMYK mode image, the RGB color values outside the CMYK color gamut are adjusted outside the CMYK color gamut, thereby narrowing the color range.
Because some colors will lose some color information after conversion, it is best to save a backup file for them before conversion, so as to restore the image when necessary.
1. Converts a color image to grayscale mode.
When a color image is converted to gray mode, Photoshop will lose all the color information in the original image, but only keep the gray level of pixels.
Gray mode can be used as an intermediate mode for conversion between bitmap mode and color mode.
2. Convert images in other modes into bitmap mode.
Converting an image into bitmap mode will reduce the color of the image to two, which greatly simplifies the color information in the image and reduces the file size. To convert an image into bitmap mode, it must be converted into grayscale mode first. This will remove the hue and saturation information of pixels and only keep the brightness value. However, since only a few editing options are available for bitmap mode images, it is best to edit the images in grayscale mode and then convert them.
Bitmap mode images edited in gray mode may be different after switching back to bitmap mode. For example, pixels that are black in bitmap mode may be grayed out after editing in grayscale mode. If the pixel is bright enough, it will turn white when switching back to bitmap mode.
3. Convert other modes into index mode.
When converting a color image into an index color, many colors in the image will be deleted, leaving only 256 colors, that is, the number of standard colors supported by many multimedia animation applications and web pages. Only images in grayscale mode and RGB mode can be converted to indexed color mode. Because the gray pattern itself is composed of 256 gray levels, there is no obvious difference in color or image size after conversion into index color. However, after the RGB mode image is converted to the index color mode, the size of the image will be significantly reduced and the visual quality of the image will be damaged.
4. Convert RGB mode image into CMYK mode.
If the image in RGB mode is converted to CMYK mode, the colors in the image will be separated and the color gamut will be limited. Therefore, if the image is in RGB mode, it is best to edit it in RGB mode first, and then convert it into a CMYK image.
5. Use the laboratory mode for mode conversion.
Among the color modes that Photoshop can use, Lab mode has the widest color gamut, including all colors in RGB and CMYK color gamut. Therefore, no color loss will be caused when using the Lab mode for conversion. Photoshop uses Lab mode as the internal conversion mode to complete the conversion between different color modes. For example, when converting an image in RGB mode to CMYK mode, the computer will first convert the RGB mode to Lab mode, and then convert the image in Lab mode to CMYK mode.
6. Convert other modes into multi-channel mode.
Multi-channel mode can be obtained by changing the color mode and deleting the color channel of the original image.
Converting CMYK images into multi-channel mode can create spot colors of cyan, magenta, yellow and black (spot colors are special pre-mixed inks used to replace or supplement the printing of four-color inks; Spot color channel is a special color channel, which can add preview spot color to images. ).
Converting RGB images into multichannel mode can create images composed of cyan, magenta and yellow spot colors.
Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to multichannel mode. The original channel is converted into a spot color channel.