Tone and color images - Informatics

Tone and color of the image

The tone characteristic of the original is important for selecting image processing methods by software. Shadow (shadows), midtones (midtones) and brightly lit (Highlight) images are distinguished by tone. The color characteristic of the image determines the scanning mode or type of scanner. The following are distinguished by color:

• black and white or line art, which includes contour drawings, text, engravings, logos and other originals. For their processing, black-and-white scanners working in bilevel mode are used, perceiving a black or white color. They provide for setting the threshold of sensitivity, allowing you to extend the range of input originals from too light to excessively dark. Dashed graphics, which can be not only black and white, but also single-color, require accurate transmission of image contours, therefore scanning is performed with the maximum resolution;

• Halftone images, which include black and white photographs, slides, drawings, appear in different shades of gray. For such images, the notion gradation of the scale of brightness, is introduced characterizing the number of shades of gray. There are scanner models capable of creating 16, 64 and 256 gradations of brightness. In cheap scanners that are unable to reproduce the true brightness scale, the following is used to emulate halftone images:

is a halftoning technique that creates a halftone image from black dots of different sizes (halftone dots), depending on the brightness of the image itself. In scanners working with fixed-size points, halftones are reproduced using groups of points of the same size for each image point,

is a Dithering technique that creates a halftone image by grouping several points of the input image into 2 x 2 (4 pixels), 3 x 3 (9 pixels), 4 x 4 (16 pixels), etc. cells. The ratio of the number of black dots to white determines the gradation of gray. To form a raster image structure, a rectangular shape of the cell is used, which explains the name of this technology. Rasterization is used in devices capable of generating small dots with high accuracy.

In gray-scale scanners, it is advisable to have a gray number-level control, as it is not always necessary to scan with the highest possible transmission depth of the hues. This adjustment will reduce the number of bits for digital representation of a pixel, save disk space and shorten the printing time of such an image;

• color images, which include color slides, photographs and drawings, consisting of a variety of shades of different tonality. When scanning a color image, the original is illuminated through an RGB color filter. Differences in the modifications of color scanners are determined:

- the number of passes when reading information. For example, ScanJet series scanners use one pass, Microtek scanners - three,

- the number of light sources used: Epson and Sharp models use three sources instead of one (for each color separately).

By the degree transparency of the original , they can be divided into two large groups:

• Reflective or opaque originals - all kinds of photographs, drawings, pages of books and magazines, booklets, etc., read in reflected light;

• Transparent originals (color and black-and-white slides and negatives) that transmit light. To work with transparent originals, you need a specialized scanner or a model of a traditional scanner, one of which options is the additional ability to process transparent originals. Specialized scanners for processing slides and transparencies are characterized by high optical resolution and the ability to increase small originals to a size, for example, a journal or poster page.

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