Raster s& Vecto rs Raster
Raster images are capable of displaying a myriad of colours in a single image and allow for colour editing beyond that of a vector image. They can display finer nuances in light and shading at the right resolution. Raster images cannot be made larger without sacrificing quality. Raster images are often quite large files.
Raster (photographs) and vectors (illustrations) are the mainstay image format of printed material today. A raster image does not contain sufficient information to produce a clear image when enlarged, as pixelation begins to occur. Whereas a vector image is made up of paths, each with a mathematical formula that tells the path how it is shaped and what colour it is bordered with or filled by.
Vector
Vector images are scalable, so that the same image can be designed once and resized infinitely for any size application – from business card to billboard to logo. Vectors display at the highest resolution allowed by the output device vector images are relatively lightweight. Vector images cannot display the natural qualities of photographs.
Chann els & Plates
Understanding the principle of how the four-colour process builds an image allows the designer to treat each colour pass separately, and in doing so obtain better colour adjustment and/or graphic interventions.
It is neither practical or economical to mix up ink and print every individual colour to be found in a piece of artwork or colour photograph, so another method has to be used. The secondary colours Cyan, Magenta and Yellow, and Key (also known as black) are the colours used by printers to reproduce full-colour work. These four colours make up the four-colour process system, which you may have heard before, called CMYK. These colour make up the four plates which a typical Offset Litho printer would use. You can make up colour of most art work by silk-screen printing if you turn your image into four layers (made up of CMYK) and exposing these onto four seperate screens. However it can be quite difficult to allign each of these screen up perfectly each time.
Print Order Logically, only a colour that has already been printed can be ‘overprinted’ with another colour. In standard printing magenta follows cyan, therefore can overprint yellow, magenta or cyan. When using varnish you also need to consider whether the special finish will overprint (as the example opposite shows), or knockout.
Ink Trapping Ink trapping is the overlapping of areas of coloured text or shapes to compensate for mis-registration on the printing press. Ink trapping is required because the halftone dots that make up the printed images overlap (because they are different sizes and at different screen angles); therefore, the colours must also be overlapped to prevent the appearance of white gaps where they are supposed to meet.
The order in which each of the CMYK process colours prints has an impact on the resultant printed design. Understanding this concept require more than simply knowing the order that with which to print the plates or screens in. Knowledge of the print order is fundamental for understanding overprinting tecniques.
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Over Printin Printers do not like to have flat colours overlapping, because it slows dow the rate of drying of the inks and introduces the possibilty of smudging. A colourwill change shade, too, when printed over another one . Overprint An overprint describes the printing of one colour on top of another. A surprint is where a percentage of tone of a colour is used; and a reverse out is where the white (or colour) of the page or substrate is used and the printed colour forms that background or base colour. If overprinting technques are used creatively you can echeve some interesting effects.
By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. Overprinting prevents knockouts and makes the topmost overlapping printing ink appear transparent in relation to the underlying ink.
Tones
Half_
A series of screens containing halftone dots are used to replicate the continuous photographic tones in the print process. Once printed, these dots gives the illusion of full-colour image. The dots, which can be formed from various shapes, can be manipulated in terms of their size, spacing and screen angle, as the examples below illustrate.
Half tone is a continuous tone image converted to line by turning it into a pattern of dots, either digitally, by aser, or by photographic it through it through screen. Although, you will probably know a haltone image digitally.
Screen Angles Each of the four process colours has standard screen angles (black 45 degrees, magenta 75 degrees, yellow 90 degrees and cyan 105 degrees). The use of different angles prevents screen interference and the development of moire patterns, and results in clear colours and the ability to reproduce a four colour images. When printing in two colours, for example black and any other process or special colour, each will be sent at different angles. The black will be screened at 45 degrees, as this angles is the least obvious to the human eye and black is the strongest colour, and the second colour will be screened at 75 degrees. this principle follows all the way to the four colour printing, and indeed, if a fifth colour is added, it will be given an alternative screen angle to the four colours.
Tints Spot Colours can be either printed solid or as a percentage tint Charts are also available that show Pantone colours in a range of percentage tints. Type can be combined with a tint of flat colour in three ways: surprinted, with the tint and in the same colour; reversed out of a tint block; or overprinted in another colour. Legibility is an issue, so check with printed samples to what will work with different percentages of tint.
A tint is a shade of colour that has been diluted, through the addition of white, in order to create a paler variation of it. Colour reproduction is usually achieved by screening the three trichromatic process colour; cyan, magenta and yellow, in increments of 10%. This produces 1,330 available tints for the designer to use, and this increases to almost 15, 000 when black is included as well.
s Tonal Image Duotone A halftone cannot always reproduce the full tonal range of a photograph. A duotone is superimposition of a contrasty black halftone over one-colour half tone, which is shot for highlights and middle tones, using the same image. the most commonly used colours are yellows, browns, and reds. The intention is to create a rich range of colours, and at the same time add coloured tint to the result. For high quality work, where cost is no constraint, a duotone may even comprise two printings in black, or in black with a shade of grey.
A tonal image is akin to a black-and-white photograph in which the white tone have been replaced by one of, or a combination of, the other CMY process colours.