(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
Bitmap mode
Bitmaps are black-and-white images, which are binary images and discontinuous tone images. They are generally used for text and lines. 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, above 50% is?, And below 50% is white.
2. Pattern dithering method, that is, the use of geometric figures to express the tone according to the tone change of the image.
3. Diffusion colorimetric method, that is, using some irregular small dots to express the gray level of the 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 be round, diamond, ellipse, straight line, square, Several options for crosshairs.
5. Custom pattern method, this method can use the geometric pattern, text, graphics, etc. selected by yourself to fill the image to express the tone of the image.
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 changes of the light and dark levels of the image. The grayscale image is an 8-bit image that can express 256 grayscale colors.
Duotone mode
Two-tone mode is to simulate the effect of ink overprinting during printing with two or several specific colors. See the picture below for usage:
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You can choose two-color, three-color, and four-color, and then select the color in the boxes marked with inks 1, 2, 3, and 4. The selected color can be either a printing color or a spot color. Click the box next to it. You can also change the tone of the image by adjusting the curve. In the embossing colors below, you can see the color change of several inks from light to dark after overprinting. Click the embossing color to see the effect of 1 + 2, 2 + 3, 1 + 2 + 3 and other colors after overprinting.
Index color mode
The index color mode uses 8 bits to describe the image, that is, there are generally only 256 colors in the image. Usually the images we print are 24-bit true color images, which can reproduce 16.7 million colors, so the index color mode can only be used as a special effect, not as a general image mode for printing.
RGB mode
The three colors R (red), G (green), and B (blue) are the three primary colors of color light. Light is the source of all colors. The wavelength of different colored lights is different, and the wavelength range of the colored light that the human eye can see is generally between 380 and 760 nm. The three primary colors of color light add up in equal amounts to appear white, so we call it the additive color method. The display, scanner, digital camera and other devices use RGB color mode to express the image color. In this mode, the color of each pixel in the image is described through the three channels R, G, and B, where each channel is 8 Bit, this mode is 24 Bit, which can express 224 (that is, 16777216 ) Colors. Many scanners can now scan images into CMYK mode, but the scanner scans through white light, which can be converted to CMYK after color separation after collecting reflection or transmission information. In PHOTOSHOP, the variation range of R, G, B is set at 0-255.
CMYK mode
This mode is a printing mode that we are all familiar with and commonly used. C (cyan), M (magenta), and Y (yellow) are the three primary colors of ink. The colors in nature can be obtained by overprinting these three inks in different proportions. The color of the printed matter we see is that the ink absorbs the complementary color light in the three primary colors of the color light, and reflects the other colors in the three primary colors of the color light, which produces a color sense on our optic nerve. The more ink, the darker the color. Since the color of the ink is subtracted from the composite white light by subtracting one or a few colors of light to obtain another color of light, the color of the ink is called subtractive color. What needs special mention here is K (black). Theoretically, the C, M and Y three-color superposition should be black, but due to some reasons, such as ink purity, transparency, carrier properties, etc., the purity 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 Bit. In PHOTOSHOP, the range of C, M, Y, K is 0 ~ 100%, the greater the value, the darker the color.
Lab mode
Because RGB and CMYK are in different color gamut spaces and different principles, the range of colors expressed is also different, which causes the colors we see on the display to be different from the colors of the printed matter. To this end, CIE (International Commission on Lighting) has developed the Lab color model. It is a color mode that has nothing to do with the device, a color system that is independent of various input and display output devices, and it is unique at any time, place, and device. Lab's color gamut space is larger than RGB and CMYK modes, so it can contain RGB and CMYK color gamut spaces, and it is widely used in color management systems. In PHOTOSHOP, the commonly used mode conversion (such as the conversion between RGB and CMYK mode) will use Lab mode. L: table brightness, value range: 0 ～ 100; a: table red to green range change component, value range: -120 ～ + 120; b: table blue to yellow range change component, value range: -120 ～ +120. The following figure shows the relationship between Lab color gamut space and RGB, CMYK color gamut space.
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In addition to the above modes, it is also necessary to mention the HSB mode. It is also a relatively common color system. It uses color attributes to describe colors. H (Hue) represents the hue, such as red, blue, green, etc. It is the human eye's response to light waves of different wavelengths, and is also the essential content of color. Its variation range: 0 ~ 360 °, 0 ° and 360 ° coincidence represents red ; S (Saturation) means saturation, which is the gray component contained in the color, and its variation range: 0 ~ 100%, the saturation is highest at 100%; B (Brightness) means brightness, which is the lightness and darkness of the color, it The range of change: 0 ～ 100%, the brightest at 100%.
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