Mumbai university > Comp > SEM 4 > Computer Graphics

Marks: 10M

Year: Dec 2015

Half toning

1. Many displays and hardcopy devices are bi-level

2. They can only produce two intensity levels.

3. In such displays or hardcopy devices we can create an apparent increase in the number of available intensity value.

4. When we view a very small area from a sufficient large viewing distance, our eyes average fine details within the small area and record only the overall intensity of the area.

5. The phenomenon of apparent increase in the number of available intensities by considering combine intensity of multiple pixels is known as half toning.

6. The half toning is commonly used in printing black and white photographs in newspapers, magazines and books.

7. The pictures produced by half toning process are called halftones.

8. In computer graphics, halftone reproductions are approximated using rectangular pixel regions, say 22 pixels or 33 pixels.

9. These regions are called halftone patterns or pixel patterns.

10. Figure shows the halftone pattern 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 method using pixel grid, and something it is used to refer to color halftone approximations only.

• Random values added to pixel intensities to break up contours are often referred as dither noise.

• Number of methods is 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 pixel.

• To obtain $n^2$ intensity levels, it is necessary to set up an n*n dither matrix $D_n$ whose elements are distinct positive integers in the range of 0 to $n^2-1$.

• For example it is possible to generate four intensity levels with

• The matrix element for D_2 and D_3 are in the same order as the pixel mask for setting up 22 and 33 pixel grid 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 < $n^2$.

• If the intensity I is to be applied to screen position (x, y), we have to calculate numbers for the either matrix as

$$ i = (x mod n) +1 \ \ j = (y mod n) +1$$

• If $ I \gt D_n(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.

• Another method for mapping a picture with mn points to a display area with mn pixels is error diffusion.

• Here, the error between an input intensity value and the displayed pixel intensity level at a given position is dispersed, or diffused to pixel position to the right and below the current pixel position.