Half tonning Technique:

1. Newspaper, photographs simulate a grey-scale image that can printed using only black ink.

2. A newspaper picture is, in fact, made up of a pattern of tiny black dots of varying size.

3. The human visual system has a tendency to average brightness over small areas, so the black dots and their white background merge and are perceived as an intermediate shade of grey.

4. The process of generating a binary pattern of black and white dots from an image is termed half toning.

5. In traditional newspaper and magazine production, this process is carried out photographically by projection of a transparency through a 'halftone screen' onto film.

6. The screen is a glass plate with a grid etched into it.

7. Different screens can be used to control the size and shape of the dots in the half toned image.

8. In computer graphics, half toning reproductions are approximated using rectangular pixel region say 2 x 2 pixels or 3 x 3 pixels.

9. These regions are called as “Halftone Patters” or “Pixel Patterns”.

10. 2 x 2 pixel patterns for creating five intensity levels are shown in figure 43.

Dithering technique:

1. Another technique for digital half toning is dithering.

2. It is the technique for approximating halftones without reducing resolution, as pixel grid patterns do.

3. Dithering can be accomplished by Thresholding the image against a dither matrix.

4. To obtain n2 intensity levels, it is necessary to setup an n x n dither matrix Dn whose elements are distinct positive integers in the range of 0 to n2 – 1.

5. Matrix for 4 intensity level and 9 intensity level is shown below.

1. The elements of a dither matrix are thresholds.

2. The matrix is laid like a tile over the entire image and each pixel value is compared with the corresponding threshold from the matrix.

3. The pixel becomes white if its value exceeds the threshold or black otherwise.

4. This approach produces an output image with the same dimensions as the input image, but with less detail visible.

5. High order dither matrices can be obtained from lower order matrices with the recurrence relation.

Algorithm to halftone an image using a dither matrix

Halftoning Technique:-

The process of generating a binary pattern of black and white dots from an image is termed halftoning. In traditional newspaper and magazine production, this process is carried out photographically by projection of a transparency through a 'halftone screen' onto film. The pictures produced by halftoning process are called halftones. The screen is a glass plate with a grid etched into it. Different screens can be used to control the size and shape of the dots in the halftoned image.

In computer graphics, halftone reproductions are approximated using rectangular pixel regions, say 2x 2 pixels or 3x 3 pixels. These regions arc called halftone patterns or pixel patterns. figure (e) shown the halftone patterns to create number of intensity levels.

Dithering Techniques:-

Dithering refers to techniques for approximating halftones without reducing resolution, as pixel grid patterns do. The term dithering is also applied to halftone approximation methods using pixel grids, and sometimes it is used to refer to colour halftone approximations only.Dithering is the attempt by a computer program to approximate a color from a mixture of other colors when the required color is not available. For example, dithering occurs when a color is specified for a Web page that a browser on a particular operating system can't support. The browser will then attempt to replace the requested color with an approximation composed of two or more other colors it can produce. The result may or may not be acceptable to the graphic designer. It may also appear somewhat grainy since it's composed of different pixel intensities rather than a single intensity over the colored space.

Random values added to pixel intensities to break up contours are often referred as dither noise. Numbers of methods are used to generate intensity variations. Ordered dither methods generate intensity variations with a one-to-one mapping of points in a scene to the display pixels. To obtain n2 intensity levels, it is necessary to set up an n x n dither matrix whose elements are discrete positive integers in the range of 0 to n-1. For e.g. it is possible to generate four intensity levels with

The matrix elements for D2 and D3 are in the same order as the pixel mask for setting up 2 x 2 and 3 x 3 pixel grids respectively. For bi-level system we have to determine display intensity values by comparing input intensities to the matrix elements. Each input intensity is first scaled to the range 0 ≤ I ≤ n2. If the intensity I is to be applied to screen position (x, y), we have to calculate row and column numbers for the either matrix as

If I > D1(i, j) the pixel at position (x, y) is turned on; otherwise the pixel is not turned on. Typically, the number of intensity levels is taken to be a multiple of 2. High order dither matrices can be obtained from lower order matrices with the recurrence relation.