Desktop Publishing
Bit mapped Images
A bit-mapped image is representation, made of rows and columns of dots, of a graphics image in computer memory. The value of each dot (whether it is filled in or not) is stored in one or more bits of data. For simple monochrome images, one bit is sufficient to represent each dot, but for colours and shades of grey, each dot requires more than one bit of data. The more bits used to represent a dot, the more colours and shades of grey that can be represented.
The density of the dots, known as the resolution, determines how sharply the image is represented. This is often expressed in dots per inch (dpi ) or simply by the number of rows and columns, such as 640 by 480.
To display a bit-mapped image on a monitor or to print it on a printer, the computer translates the bit map into pixels (for display screens) or ink dots (for printers). Optical scanners and fax machines work by transforming text or pictures on paper into bit maps.
Bit-mapped graphics are often referred to as raster graphics. The other method for representing images is known as vector graphics or object-oriented graphics. With vector graphics, images are represented as mathematical formulas that define all the shapes in the image. Vector graphics are more flexible than bit-mapped graphics because they look the same even when you scale them to different sizes. In contrast, bit-mapped graphics become ragged when you shrink or enlarge them.
Pixel
Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected.
The number of bits used to represent each pixel determines how many colours or shades of grey can be displayed. For example, in 8-bit colour mode, the colour monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colours or shades of grey.
On colour monitors, each pixel is actually composed of three dots — a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make colour pixels appear fuzzy.
The quality of a display system largely depends on its resolution, how many pixels it can display, and how many bits are used to represent each pixel. VGA systems display 640 by 480, or about 300,000 pixels. In contrast, SVGA systems display 1,024 by 768, or nearly 800,000 pixels. True Colour systems use 24 bits per pixel, allowing them to display more than 16 million different colours.
Vector graphics
Same as object-oriented graphics, refers to software and hardware that use geometrical formulas to represent images. The other method for representing graphical images is through bit maps, in which the image is composed of a pattern of dots. This is sometimes called raster graphics. Programs that enable you to create and manipulate vector graphics are called draw programs, whereas programs that manipulated bit-mapped images are called paint programs.
A vectored (wireframe) image and as it looks in view mode (Coreldraw)
Vector-oriented images are more flexible than bit maps because they can be resized and stretched. In addition, images stored as vectors look better on devices (monitors and printers) with higher resolution, whereas bit-mapped images always appear the same regardless of a device's resolution. Another advantage of vector graphics is that representations of images often require less memory than bit-mapped images do.
Almost all sophisticated graphics systems, including CAD systems and animation software, use vector graphics. In addition, many printers (PostScript printers, for example) use vector graphics. Fonts represented as vectors are called vector fonts, scalable fonts, object-oriented fonts, and outline fonts.
Note that most output devices, including dot-matrix printers, laser printers, and display monitors, are raster devices (not however plotters). This means that all objects, even vector objects, must be translated into bit maps before being output. The difference between vector graphics and raster graphics, therefore, is that vector graphics are not translated into bit maps until the last possible moment, after all sizes and resolutions have been specified. PostScript printers, for example, have a raster image processor (RIP) that performs the translation within the printer. In their vector form, therefore, graphics representations can potentially be output on any device, with any resolution, and at any size.
Aliasing In computer graphics is the process by which smooth curves and other lines become jagged because the resolution of the graphics device or file is not high enough to represent a smooth curve. Smoothing and antialiasing techniques can reduce the effect of aliasing.
Aliased and Anti-aliased images.
Halftone
In printing, a continuous tone image, such as a photograph, that has been converted into a
black-and-white image. Halftones are created through a process called dithering, in which the density and pattern of
black and white dots are varied to simulate different shades of grey.
In conventional printing, halftones are created by photographing an image through a screen. The screen frequency, measured in lines per inch, determines how many dots are used to make each spot of grey. In theory, the higher the screen frequency (the more lines per inch), the more accurate the halftone will be. However, actual screen frequencies are limited by the technology because higher screen frequencies create smaller, more tightly packed dots. If you are printing on a low resolution device, therefore, you may get better results with a lower screen frequency.
Normal and halftone colour images.
Grey scaling
The use of many shades of grey to represent an image. Continuous-tone images, such as black-and-white photographs, use an almost unlimited number of shades of grey. Conventional computer hardware and software, however, can only represent a limited number of shades of grey (typically 16 or 256). Gray-scaling is the process of converting a continuous-tone image to an image that a computer can manipulate.
While grey scaling is an improvement over monochrome, it requires larger amounts of memory because each dot is represented by from 4 to 8 bits. At a resolution of 300 dpi, you would need more than 8 megabytes to represent a single 8½ by 11-inch page using 256 shades of grey. This can be reduced considerably through data compression techniques, but grey scaling still requires a great deal of memory.
16 and 256 shades of grey
Many optical scanners are capable of grey scaling, using from 16 to 256 different shades of grey. However, grey scaling is only useful if you have an output device — monitor or printer — that is capable of displaying all the shades. Most colour monitors are capable of grey scaling, but the images are generally not as good as on dedicated grey-scaling monitors.
Note that grey scaling is different from dithering. Dithering simulates shades of grey by altering the density and pattern of black and white dots. In grey scaling, each individual dot can have a different shade of grey.
Continuous tone
Refers to images that have a virtually unlimited range of colour or shades of greys.
Photographs and television images, for example, are continuous-tone images. In contrast, computer
hardware and software is digital, which means that they can represent only a limited number
of colours and gray levels. Converting a black-and-white continuous-tone image into a
computer image is known as grey scaling.
Continuous-tone printers can print each dot at many different shades of lightness and darkness. Though this isn't true continuous-tone because the level of shades is limited, there are enough shades (256 or more) so that the difference between one shade and the next is imperceptible to the human eye.
Colour
Colour Separations
Full colour images are printed, both using four colours of ink. Cyan Magenta and Yellow inks are used to create the range of colours: close juxtaposition of small dots of two or more of these colours of ink will give the eye the impression of the colour formed by mixing these pigments. Some mingling of inks may also take place on the printed page. In a printing press, the paper is passed through successive rollers each of which takes its image from a plate bearing ink of a particular colour. The colours Cyan Magenta and Yellow, mixed in different proportions, will in theory yield a full spectrum of colours and range of shades. However in practice it is difficult to obtain good dark shades of grey by mixing typical coloured inks; a Black ink is therefore used on a fourth printing plate. Black is represented by K in the initials CMYK.
Subtractive, or CMYK colour |
Addative, or RGB colours |
To determine the intensity of each colour, equating to its proportion in the mixture of colours, screening is used to create dots of different size or density. The extent to which the white paper background is exposed or obscured by the use of smaller or larger dots will determine the lightness or darkness of an area.
The following pictures illustrate the breakdown of an image into four colour separations each of which has a screen applied. The colour separations are re-assembled in the final picture to yield an approximation to the original image. The appearance of this final picture is similar to the appearance of a typical colour printed product (such as a picture in a magazine) when viewed under a microscope.
The original image
The colour separations
Within the CYMK model we see that all colours, and shades of those colours are composed of dots printed onto the paper. Blue for example is produced by adding Cyan, and magenta dots. Lighter shades of blue are produced by printing fewer dots, allowing the white of the paper to show through, darker shades by printing an increasing number of black dots.
This of course applies to other colours
|
Blue |
Green (Cyan + Yellow + Black (K)) |
Red (Yellow + Magenta) |
Colour schemes
© Allen. C. Roffey 18:22 05/09/2005
To the right you see the Munsell colour wheel, showing the relationships between both harmonious, and complementary colours, however there are other relationships which we will take a look at.
Here we see monochromatic colour schemes for Red, Green, Blue and Yellow, as you can see only the shades (tints) of the colour change. This is achieved by adding white.
Analagous colours are next to each other on the colour wheel.
