Talk about the input and output of pictures

When the designer completes a manuscript, he then hands the file to the output center or printing house, and outputs the file to different materials, such as color draft, film, or printing plate. The input resolution of the picture will directly affect the quality of the output. To do well, first of all, you need to know the quality of each output device and the principle of resolution.

Pixel & Bit depth

Pixels are the basic elements that constitute a bitmap image. The pixel shape is square. You only need to open a picture file in some graphics software such as PhotoShop and zoom in the picture until you see a lot of small squares in the image. This is the pixel.

Each pixel contains the value of color, in bits (bit), the amount of bit information possessed by this value (1, 4, 8 bits or more) is called bit depth (bit depth) ), The greater the bit depth value, that is, the more data data, the more detailed and realistic the image, and the richer the color or gray scale.

Bit is a binary value, it is the smallest unit of digital information, the content of each bit is '1' or '0'. In a one-bit image, each pixel contains a bit of information, which can show one of the two states of 0 or 1, indicating that the pixel is 'white' or 'black'. Two-bit images can show one of four possible values, with 00, 01, 10, and 11 representing the four shades of 'black', 'dark gray', 'light gray', and 'white', respectively status. Pixels up to 8-bit images can exhibit 256 shades. As the amount of information contained in a pixel is larger, the image becomes more detailed. Generally speaking, black and white grayscale images have 256 tone changes and colors have 24 bits (2 to the 24th power, which is equal to 167,000 colors) to achieve photo-realistic color richness.

For a 24-bit RGB (8 bits per color channel component) color image, the red (R) component has 256 levels of change, the green (G) component has 256 levels of change, and the blue (B) component has 256 levels of change As a result of all the integration, the color value of each pixel in the image has a total of 16,777,216 possible values, so that it can show more than 16 million tones or colors. The CMYK color image is 32 bits, and each plate color occupies 8 bits of information. The four colors are: Cyan, Magenta, Yellow, and Black.

Image resolution

Resolution is arguably the most confusing term in the field of digital imaging. It can be summarized into the following three categories, namely the number of pixels for image input (ppi), the number of dots for printout (dpi) and the number of dots for printing dots (lpi ).

Line unit of printed dots: lpi (1ines perinch)

This is every British? Plus: ⑼? Sullen guan ?? timid ?? ⒕? Quemou 硎 do R words to take this burden? Make-up ㄖ reveal? 5-line printing, and color newspapers are printed in 100 ~ 120 lines. Most color magazines, books, leaflets and brochures will be printed with 133 ~ 175 screens, some high-quality magazines and books will use 200 lines or higher, but it should be noted that the higher the number of screens, the higher the print quality. Because different papers have different printability or surface smoothness, for example, printing on newsprint with 150 lines is like writing on a straw paper with a pen, which will only result in a significant increase in halftone dots. The result will be a significant loss of fine parts, so it should be understood that the optimal number of network lines for each printing business. The following are some reference settings.

Unit of output device: dpi (dots per inch)

That is "Every British? Plus" 關 ?? rdquo ;, is the most commonly used resolution representation method, regardless of the resolution of the image or the output quality of the output device, we generally use dpi as the quantity. In fact, dpi The original meaning was to describe the resolution unit of the output device. Because all output devices are marked with small dots to make images, so the dpi becomes the amount of the output device. For example: the film output machine has a resolution of 2540dpi, It means that it can wash the larvae at 540 print points per inch. The following are the common output machine resolution and working settings.

Image input pixel unit: ppi (pixels per inch)

That is, every British? Pygmy? Ju? Pi is mainly to describe the resolution of input devices, such as scanners, digital cameras, etc., because various input devices eventually turn images into pixels and store them on the computer, so ppi also becomes The unit of measure for image resolution is now.

Scanning resolution

In the process of using the scanner, it is important to determine the scanning resolution, that is, how many ppi to scan the image, so before scanning you must understand the final output size of the image and the output quality of the output device. If the scan resolution is too low, there will be too few pixels in the image, making the image look rough. Conversely, if the resolution of the image is too high: the image file will be too large, which affects efficiency during work and output.

Some color publishers mistakenly believe that the more data stored in the image, the better the image quality, so they scan all the images at the highest resolution of their scanners, resulting in huge documents that save, place, and separate colors Slow. Therefore, to understand the scanning resolution, it is necessary to understand the characteristics of various output devices. The following is the input resolution calculation method for the output device.

Formula: Input resolution = number of dots × quality factor × magnification

(ppi = lPi × QF × RF)

To achieve the best results, experts suggest that the quality factor is 2, which means that if the scanned image is output at the same size, its scanning resolution should be twice the number of net lines it ultimately outputs. So if the number of screen lines used for printing is 150 lpi, then each photo copied at the same size should be scanned at a resolution of 300 ppi. In fact, the quality factor is not necessarily 2, reducing this factor to 1.8 or even 1.6, the decrease in output quality is not easy to notice. If huge documents make your system run slowly, then you can consider using a quality factor lower than 2 when deciding the resolution of the computer scan. For example, the quality factor is reduced from 2.0 to 1.6, the document size can be reduced by 36%, and the processing speed is increased accordingly. The following table shows the scanning resolution required by different printing screens and different quality factors.

Output resolution & graylevel

Formula for calculating gray level: gray level = (dpi / lpi) 2

On a 300dpi laser printer, if the dots of 60 network lines are output, each dot has 5 × 5 (300/60 = 5) print dots to form, then each dot will have 25 levels of gray (5 × 5 = 25), so if you want to get all the gray levels at the same time and maintain a sufficiently high number of network lines, then you need to output on a higher resolution device.

For example, at 2400dpi output, the printing screen line is set to 150 lines, which means that 16 × 16 (2400/150 = 16) printing dots are used to form a grain of dots, and each grain of dots can produce 256 gray levels (16 × 16 = 256 ), This combination is enough to meet the needs of the computer's 256 gray levels.

But if 1200dpi and 150 lines are used for output, it means that 8 × 8 (1200/150 = 8) printing dots are used to form a grain of dots, and each grain of dots can only produce 64-level grayscale changes (8 × 8 = 64) , Not enough to handle full-tone pictures. Therefore, when outputting the film, the number of network points and the number of network cables must be matched to obtain the best results. The following chart shows the best combination of the number of network points and the number of network lines.

In summary, the output point directly affects the output prime number and speed. Therefore, users must pay attention to how to match the input and output points to obtain the best results. The following table shows the gray levels that can be produced by different combinations of printed screens and different output points. Tune.

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