Digital images are usually obtained by scanning (sometimes also obtained by digital cameras). The factors that affect digital images are various. In general, the correctness of digital images is the basis for guaranteeing the quality of digital images. Scanning is the most commonly used digitization process, such as the digitization of graphic arts, slides, or photographic prints. Therefore, the scanning quality and the performance of the final output device are the most important factors affecting the image quality. This article mainly analyzes the factors affecting digital image quality from the aspects of resolution, pixel depth, image color model, and storage format of image files.

First, the resolution

The resolution of an image refers to the number of pixels per unit length of an image, which is generally expressed in ppi (pixels per inch), which is the number of pixels per inch. The resolution of the image is actually the scanning resolution spi (samples per inch). We cannot confuse it with dpi (dots per inch). Dpi is used to measure the output resolution of a laser printer or imagesetter. It indicates how many dots per inch. For example, an image imagesetter with an output resolution of 2450 dpi produces more than 6 million dots per square inch (2450 x 2450 = 6002500). The standard 300 dpi laser printer produces 90,000 dots per square inch. The more points the image contains, the higher the image resolution and the better the print quality. Another unusual measure of the imagesetter is the number of pixels per millimeter, rels x, where x is the number of pixels per millimeter. For example, reis 4 is 4 plxels per millimeter, which is approximately 102 ppi (or spi).

There is also a concept that is the resolution of the printing press. The resolution of the printing press is expressed in line per inch (LPI), that is, how many lines per inch, which is usually also referred to as the number of lines of the screen, halftone screen, Screen number or screen frequency. The scanning resolution spi is directly related to the screen frequency. When a digital image is output to a printer or an imagesetter, similar to traditional printing, it will be decomposed into dots. Output devices generate dots by converting them to a set of smaller on or off states, which are pixels. If the output device is an image imagesetter, it can be output to film and paper. When printing, the pixels are grouped into a series of cells from which dots are formed. The dots are formed by turning on or off the pixels in the control unit and determining the gray scale.

Pixels are the many tiny square image elements that make up an image. The pixel value of the digital image is a value given by the computer when the original image is digitized, and it represents the average luminance information of a small square of the original document, or the average reflection density information of the small square. For a scanned image, pixels contain each sampled information such as color, grayscale, black or white. The size of the pixel depends on the scanning resolution. For example, 150 spi means that the scanner samples every 1 inch of 1/150; 72 spi means that every 1 inch of 1/72 is sampled. The higher the scanning resolution, the more details of the image are obtained.

Scan resolution

When an image is scanned at a very low resolution, the pixels obtained are larger, the details of the image are less, the color information is less expressed, and the quality of the image is significantly reduced. On the other hand, if the scanning resolution is too high, it will not necessarily achieve the desired effect. When the scan resolution is too high, the scanned image file will be unnecessarily large, and it takes a long time to process the RIP. The printer can only generate images with a limited number of lines per inch, so it does not necessarily improve the quality of the final output. Even if the scanned image is sent to the Web, the result is the same. Because, most users use 72ppi resolution to view images on the monitor. In order to get the best scan, it is recommended to consider using the following empirical formula:

Color Image Scanning For color or grayscale images, the appropriate scan resolution is related to the screen frequency you want. Regarding the screen frequency, you can get it on your printer or ask a printing professional. In general, newspapers are printed at a screen rate of 85 lpi. Most lithographic magazines use 133 lpi or 150 lpi. Some art books printed on coated paper use 200 lpi. Knowing the screen frequency, the sweep frequency can be calculated using the following formula:

a) For screen frequencies of 133 lpi or higher:

Scan resolution = screen frequency × 2 × original image scaling

b) For screen frequencies less than 133 lpi:

Scan resolution = screen frequency × 1.5 × original image scaling

For example, if you want to scan a 3×5 image, the reproduction size is 18/5×6 (inches) (120% of the original image). If you use 85 lpi screen frequency, you can use 153 spi (85×1.5×1.2= Scan resolution scan of 153).

Scanning black and white line art Black and white images such as line art, logos, and text are often referred to as bitmap images. This term is used because it requires only 1 bit per pixel for black and white images. In color images and grayscale images, color and gray gradients can hide the borders and integrate the image into its background. In a black-and-white image, the strong contrast between black and white leads the eye's attention to the outline. Therefore, the scanning requirements for black and white line art are different from those for color images. To obtain the best resolution, the scanning should be as close as possible to the final output resolution. Otherwise, images printed with low scan resolution may appear "jaggies."


The following formula can be used to scan black and white line art products:

Scan resolution = output resolution × original image scaling

The resolution of printers and imagesetters is measured in dots per inch (dpi), but no matter how high your output device resolution is, 600 spi is very good for many line art works. The best resolution is not more than 1200 spi (even if the output resolution can be very high). The difference in quality of scanned images exceeding this number is difficult to distinguish with the naked eye, and excessively high scan lines only make the image Increases the output of the image slower.

In printing, to obtain high-quality digital images, not only the resolution of the image cannot be less than 1.5 times the screen frequency, but also the quality of the image depends on the paper used. Printing at maximum resolution and screen frequency is not always feasible. Not all printers support the highest screen output, and most paper is not suitable for high screen frequency printing. For example, when printing in a newspaper at a high screen frequency, it absorbs dots and causes too much ink to spread, resulting in very blurry output quality. Therefore, paper is the deciding factor in how much screen frequency is used. (to be continued)