The principle of tone reproduction in continuous-tone images | Digital Image Processing

The principle of tone reproduction in continuous-tone images | Digital Image Processing

Table of content.

The principle of tone reproduction in continuous-tone images

 

Concept

The principle of halftone reproduction in continuous tone images is to convert the original document's continuous levels of light and dark (tones) into a discrete dot structure. By utilizing the difference in dot coverage rates, it simulates the human eye's perception of brightness, thereby achieving the reproduction of tones on printed materials.

 

Principle

1. The use of halftone as a tonal carrier:


The light and dark levels of continuous-tone originals cannot be directly replicated through traditional printing methods such as lithography, letterpress, or screen printing. Instead, the image must be divided into discrete halftone dots using halftone technology, which simulates the brightness differences of the original by varying the area of the dots.

2. The necessity of halftone technology: 


When using an unhalftoned continuous-tone negative to make a print, intermediate tones are lost due to insufficient exposure, causing the photoresist layer to be washed away. By generating a halftone negative with a plate camera or electronic color separation equipment, dividing the image into regular or random halftone dots, and then transferring it to the printing plate, the transfer of layers can be precisely controlled.

 

Continuous tone: There is a natural and continuous change in gradation from highlights to shadows.

Highlight: The bright areas of the original manuscript (10% screen coverage)

Light tone (low coverage): A range of tones with high brightness, where the ink coverage area is small, and there is more reflected light, resulting in lighter colors (10%~30% screen coverage)

 Midtone: A range of tones with brightness between that of the light and dark tones (40%~60% screen coverage)

 Shadow (high coverage): A range of tones with low brightness, where the ink coverage area is large, and there is less reflected light, resulting in darker colors (70%~90% screen coverage)

 

Implementation Process

1. Original Processing: Take a continuous tone positive film and generate a halftone negative through screen printing technology, dividing the image into discrete structures. 

2. Plate Transfer Printing: Transfer the halftone structure to the printing plate (such as PS plates or screen plates), controlling exposure and development parameters to preserve midtones. 

3. Printing Reproduction: Reproduce the light and dark levels of the original on paper by varying the ink coverage rate (changes in dot size or density).

 

Applications and Limitations

1. Applicability:

Gravure printing: Expresses gradation through changes in ink layer thickness;

 

Planographic/Relief/Screen Printing: Depends on the dot coverage rate and requires precise control of screen printing parameters.

 

2. Limitations:

Tone compression: Due to limitations such as paper whiteness and ink density, the original document's tonal range needs to be compressed, sacrificing some levels.


Dot enlargement: Mechanical deformation caused by pressure and fountain solution during the printing process requires compensation through parameter calibration.

 

Key Influencing Factors

 

1. Printing Materials: The smoothness of the paper and the color rendering efficiency of the ink directly affect the performance of halftones. 

2. Process Parameters: The number of screen lines, angle, and resolution need to be adjusted according to the observation distance (for example, low line numbers are used for distant advertising posters, while high line numbers are used for catalogs).

3. Equipment Precision: The resolution of electronic color separation machines and laser typesetting machines determines the clarity of the halftone edges.

 

 

Digital Image Processing


Image


 An image is an entity obtained by observing the objective world through various observation systems in different forms and means. It can act directly or indirectly on the human eye and subsequently generate visual perception.


Images and Information


 About 75% of the information humans obtain from the external (objective) world comes from the visual system.

Image Representation


 A 2-D array: f(x, y)

  • x, y: the coordinates of a point in the 2-D spatial XY plane

  • f: the value representing the property F of the image at position (x, y)

  • f, x, and y can be any real numbers

  • Property F: can correspond to different physical quantities

    • In grayscale images, it is represented by gray level

Image Elements


 An image is a collection of many image elements.

  • 2-D image: pixel (picture element), commonly referred to as a pixel

  • 3-D image: voxel (volume element), commonly referred to as a voxel

f(x, y) → f(x, y, z), f(x, y, t)


Image Illustration

Digital Image Representation

Definition of Digital Image Processing


 From image processing to computer vision is a continuous and unified continuum:

  • Image processing (image → image)

  • Image analysis (image → data)

  • Image understanding (image → interpretation)

Digital image processing is defined here as processing in which both the input and the output are images.

 

 

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