The application of channels in image processing II. Several special channels II. As can be seen from the previous section, the earliest channel concept evolved from the shutter in the traditional photography process to represent the special image of the selection range. After that, the computer image processing technology has developed rapidly, and the concept of channel has been greatly expanded, which covers many fields such as vector drawing, 3D modeling, material, rendering, etc., and is no longer limited to the original meaning of "selection range" in graphic design. These various "channels" have different names, uses and calculation methods, but they are essentially similar to the original channel concept: they all exist in a single hue gray-scale image attached to other images
1. primary color channel, alpha channel and spot color channel
in the previous chapters, we have carefully understood the channel, that is, the digitization process of monochrome images. So, how does the computer use these numbers to represent color images? First, let's understand the concept of primary color and the mixing principle of addition and subtraction
in the art class of primary school, we learned the concept of red, yellow and blue. The red, yellow and blue here should be magenta, yellow and cyan. By mixing the three colors in different proportions, other colors can be obtained; The maximum mixing of these three colors will make all visible light of all wavelengths within its range be absorbed and show black. We call these three metacolors "three primary colors of light source", and we call this method of mixing in which the brightness of the color decreases continuously during the mixing process subtractive mixing
generally, in printing, this subtractive mixing principle is applied: on white paper, through the mixing of three primary colors of the light source ink, various colors and their combined images are obtained. However, in practice, the black obtained by mixing is of high cost and poor quality, so a low-cost black ink (black) is usually artificially added to be printed together with the product, yellow and green. Therefore, this printing process is also called "four-color printing", and its color system is called "CMYK color system"
corresponding to the subtractive mixing principle in printing is the additive mixing principle followed by the display element. The three colors red, green and blue (what to do if the electronic universal testing machine breaks down blue) are called "three primary colors of the object". The mixing of the three colors of light can produce any other color, and the maximum mixing will produce the color with the highest brightness - white. We know that most of the display devices around us (such as CRT cathode ray tube, LCD panel, etc.) apply the additive mixing principle. Therefore, when these devices are not started, the darker the background color and the lower the brightness, the better the imaging effect. The display color system is also called RGB color system
the laws of nature are so simple and beautiful, and the ever-changing colors are only an organic combination of three simple primary colors. Therefore, any color image can be regarded as the superposition of three different primary color images as a whole. Since any monochromatic gray-scale image can be regarded as a channel, we can use 3 ~ 4 channels to record a color photo. Each channel records the distribution information of a corresponding primary color on the color image, so we call it "primary color channel". Pictures used for display purposes (such as station color pages) can be decomposed into three primary color channels R, G and B, while pictures to be output (such as posters, magazine covers, wrapping paper, etc.) are decomposed into three primary color channels C, m and Y and one K channel
since a single pixel of each channel requires 8 bits of storage space, in the three color channel, each pixel is mixed by three monochrome pixels, which also requires 8 bits × 3=24 bits for storage. In this way, when the amount of data becomes three times of the original, the number of colors that can be expressed becomes 224 ≈ 1.6 × 107 kinds. We usually call the color gamut composed of 16million colors "24bit true color"
increases the nonlinear error of the system. Alpha channel is specially designed to save the selected area. When generating an image file, it is not necessary to generate an alpha channel. Usually, it is artificially generated by people in the process of image processing, and the information of the selected region is read from it. Therefore, when outputting master making, the alpha channel will be deleted because it is irrelevant to the final generated image. But sometimes, for example, when the 3D software finally renders the output, an alpha channel will be generated for post synthesis in the plane processing software
in addition to the Photoshop file format PSD, both GIF and TIFF files can save the alpha channel. Gif files can also use alpha channel for image background removal. Therefore, we can use this feature of GIF file to make arbitrary shape graphics
3. spot color channel
in order to make your printing work different, you often need to do some special processing. For example, fluorescent ink or luminous ink is added, and the colorless system (such as bronzing) of overprint printing is used. These special color inks (we call them "spot color") cannot be mixed with three primary color inks. At this time, spot color channel and spot color printing are used
in the image processing software, there is a complete list of spot color inks. We only need to select the spot color ink, and the corresponding spot color channel will be generated. However, in processing, the spot color channel is just opposite to the primary color channel. Black represents selection (i.e. inkjet ink), and white represents non selection (no inkjet ink). This clean transparent plastic lunch box sometimes needs special attention
spot color printing can make the work more textural and shocking in visual effect. However, since most spot colors can not show the effect on the display, the production process also contains considerable experience