Several image modes in PHOTOSHOP
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Bitmap mode
Bitmaps, ie black and white images, are binarized images, non-continuously tuned images, and are generally used for text, lines, and so on. In PHOTOSHOP, to convert a continuous tone image to a bitmap, you can use the following methods:
1. 50% threshold method, that is, the original tone is demarcated at 50% gray level, 50% is 黒, and 50% is white.
2. Pattern dithering, which uses geometric representations to represent the tone according to the gradation of the image.
3. Diffusion dithering, which uses some irregular small dots to represent the gray level of an image.
4. Halftone screen method, this method is similar to using PHOTOSHOP software to screen the image, and can set the frequency and angle of the screen. The screen shape can have round, diamond, ellipse, straight line, square, A few choices of crosshairs.
5. Custom pattern method, this method can fill the tone of the image in the image with the geometric pattern, text, graphics and so on.
The above four methods of 2, 3, 4, and 5 can express the light and dark changes of the original image.
Gray mode
The grayscale mode can reflect the light and dark levels of the image. The image in grayscale mode is an 8-bit image that can represent 256 grayscale colors.
Duotone mode
Duotone mode simulates the effect of ink overprinting when printing in two or more specific colors. The method of use is shown in the figure below:
You can select two colors, three colors, four colors, and then select the color in the squares marked with ink 1, 2, 3, 4, the selected color can be either the printing color or the spot color, click the box next to it. You can also change the tone of the image by adjusting the curve. In the embossed color below, you can see the color change from light to dark after several inks are overprinted. Click on the embossed color to see the effect of 1+2, 2+3, 1+2+3 and other color overprinting.
Index color mode
The index color mode uses 8-bit binary to describe the image, ie there are typically only 256 colors in the image. Usually the image we print is a 24-bit true color image that can reproduce 16.7 million colors, so the index color mode can only be used as a special effect, but not as a general image mode for printing.
RGB mode
The three colors of R (red), G (green), and B (blue) are the three primary colors of color light. Light is the source of all colors. Different wavelengths of light have different wavelengths, and the wavelength range of color light that can be seen by the human eye is generally between 380 and 760 nm. The three primary colors of the color light are added in equal amounts and appear white, so we call it the additive color method. Devices such as monitors, scanners, and digital cameras use RGB color mode for image color representation. In this mode, the color of each pixel in the image is described by three channels R, G, and B. Each channel is 8 bits. This mode is 24 bits and can express 224 (ie 16777216). ) a variety of colors. Many scanners now scan images in CMYK mode, but the scanner scans through white light, which can be converted to CMYK by color separation after collecting reflection or transmission information. In PHOTOSHOP, the range of variation of R, G, and B is set to 0 to 255.
CMYK mode
This mode is a printing mode that we are all familiar with and more common. C (cyan), M (magenta), and Y (yellow) are the three primary colors of the ink, and the colors in nature can be obtained by overprinting the different ratios of the three inks. The color of the printed matter we see is because the ink absorbs the complementary light in the three primary colors of the colored light, and reflects the other colors in the three primary colors of the colored light, creating a color feeling on our optic nerve. The more ink, the darker the color. Since the coloring of the ink is from the composite white light, one or a certain color light is subtracted, and another color light is obtained to form a color. Therefore, we call the ink coloring method a subtractive coloring method. What needs to be specifically mentioned here is K (black). Theoretically, the C, M, and Y colors should be black, but for some reasons, such as ink purity, transparency, carrier properties, etc., the purity of black cannot be obtained, so K (black) is added to the printing mode. . CMYK mode is similar to RGB mode, each channel is 8 Bit, a total of 32 Bits. In PHOTOSHOP, C, M, Y, and K vary from 0 to 100%. The larger the value, the darker the color.
Lab mode
Since RGB and CMYK are in different gamut spaces and different in principle, the range of colors expressed is different, which causes the color we see on the display to differ from the color of the printed matter. To this end, CIE (International Commission on Illumination) has developed the Lab color mode. It is a device-independent color mode, a color system that is independent of various input, display and output devices, and is unique at any time, place, and device. Lab's gamut space is larger than RGB and CMYK modes, so it can contain RGB and CMYK gamut spaces, which are widely used in color management systems. In PHOTOSHOP, the usual mode conversion (such as conversion between RGB and CMYK modes) uses Lab mode. L: brightness of the table, value range: 0 to 100; a: variation component of the red to green range, value range: -120 to +120; b: variation component of the blue to yellow range, value range: -120~ +120. The figure below shows the relationship between Lab gamut space and RGB and CMYK gamut spaces.
In addition to the above modes, it is also necessary to mention the HSB mode. It is also a more common color system, which uses color attributes to describe colors. H (Hue) means hue, such as red, blue, green, etc. It is the reaction of the human eye to different wavelengths of light, and is also the essence of color. Its range of variation: 0-360°, 0° and 360° coincide with red. S (Saturation) is the saturation component, which is the ash component contained in the color. It varies from 0 to 100%, and has the highest saturation at 100%. B (Brightness) indicates brightness, which is the brightness and darkness of the color. The range of variation: 0 to 100%, the brightest at 100%.