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Desktop Publishing & Prepress


This page is an introduction to some of the terminology, processes and equipment used in the DTP / Prepress industries (the Graphic Arts). A kind of simple glossary - come facts sheet perhaps.

It is written primarily for people that have never touched on the tutorial subjects, or even experienced the DTP and Printing industries before. (comments and suggestions are most welcome and helpful)

screened picHalftone dots
and Dithers are both a nightmare and a blessing. We can not reproduce a controlled and repeatable infinite variation of tone or density in a truly continuous tone manner. We can reproduce or apply a solid colour (painting the bedroom walls for instance!) and we can easily reproduce no colour (too easily sometimes!).

So the only way we can virtually reproduce a strength of a given colour somewhere between nothing and 100% is use lots of tiny solids so small that they appear to the human eye as one tone. These small solid colour images can be laid out in a set pattern or randomly placed. The set pattern of predictable Halftone dots is still the easiest and cheapest method available to us today.

The WWW or Internet uses two types of bitmapped images, GIF and JPG, and both use an array of pixel information as described below, as apposed to the printers halftone dots. However, because GIF images contain a colour palette of 256 colours or less, to attain the apparent inclusion of extra colours, they can have a dithered or random pattern of existing colours applied to some areas. JPG bitmaps use an RGB palette (16.77 million colours).

It may be of some interest to note that even photographers use the principle of random solids. In the emulsions of their films and final prints are millions (billions?) of tiny particles that react to light and change or retain dyes after controlled exposure and chemical development. The particle sizes vary and larger the particle, the quicker the reaction. Where more light hits a portion of the emulsion, the more particles react, to a point where their mass appears as a tone, yet as individuals they would not be noticed.

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computerNow a bit about colour (spelt 'color' by our USA friends). 4 Colour Process or CMYK colour, RGB colour and Spot Colour .

In many complex ways objects of the universe emit light (our solar sun for instance), and creatures have developed the ability to define shape and form by detecting much of this light. We, as humans, are primarily concerned with the observance of what is called Visible light. That is, our eyes can detect wave lengths of natures Primary visible light components of Red, Green and Blue. Simply, different amounts of these components reflecting off an object give us the perception of a single colour. (Portions of the three components may be absorbed by objects where as 100% reflection of each RGB component will appear white and 100% absorption will appear black).

When mankind tries to imitate nature, we can not get it quite right. Our ink pigments, television and monitor phosphors and laser printer waxes etc. are no where near perfect. Instead of an infinite variation of colour our television and computer monitors can only reproduce around 16.77 million colours. Worse still, our best printing pigments can only reproduce a portion of the millions of colours displayed on a monitor - something you must evntually understand for desktop publishing.

The primary Secondary colours are Cyan, Magenta and Yellow. By natures rules, certain combinations of R, G and B (light) will give us C, M and Y. Conversely, certain combinations of C, M and Y (surface pigments) would give us Red, Green and Blue. The sad news is that we (humans) don't even come close. You will learn why we need Black to help make up for pigment deficiencies.

The Prepress technician or DTP'er must get colour as close to perfect as possible when preparing halftone dots etc. so that the printer also has some hope of getting reproduced images to appear correctly on the printing press.

The use of Spot Colour in our designs refers to the selection of individual inks that will be mixed and matched by the printer. If a job is designed to use two spot colours, then those colours will be individually transferred to the paper and not matched by various combinations of C, M, Y and Black. However, some expensive (to print) jobs can include a fifth or sixth colour in addition to C, M, Y and Black. Varnish and Metallic colours are often added to a design and they would be treated as spot colours, as would a corporate colour that cannot be matched with process colours and must be printed using hand mixed and matched spot colour too.

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rows of pixelsPixels and Bitmaps (pictures for computers)

In order for a computer system to create, manipulate and output an image, whether the image is scanned, photographed by a digital camera or created from a blank canvas in a Painting program like PhotoShop, the image must be digitised into tiny solid colour areas called Pixels. The pixels don't actually exist, your hard disk does not contain tiny individual pictures of each pixel, but they are represented on the computer screen for identification and manipulation, from saved colour values. The individual pixel colour values record the colour strength of each channel. ie a greyscale black to white pixel will have just one channel per pixel, and an RGB pixel will have three.

When a bitmap image is to be created, the first thing that must be established is the bitmaps Resolution. That is, how many pixels or areas of individual colours there will be per inch (ppi) horizontally (or per centimetre), and this will also be the same vertically. If an image has a resolution of 200ppi and will be 3 inches wide when output at same size, there will be 600 different colour blocks across each row. If the image is 4 inches high then there will be 800 rows, and the total number of pixels or individual colour areas in the image will be 480,000. If the resolution is high enough we will not see a pixelised affect when output, similar to but exaggerated in the sample above. The sample above also displays the separate rows of pixels that help make up the whole.

You will learn that there is a definite relationship between the required image resolution and the desired output Halftone Dot resolution. The Colour Depth of a bitmap image refers to the number of colours that an individual pixel could represent (only one of those colours per pixel) and the value is based on Bits (either 1 or 0), the lowest value used by a computer system. Eight bits equal one Byte - a binary value. Because an image is an horizontally and vertically based array of information, they are called BITmaps. TIFF, GIF, JPG, PIC, BMP and a host of other file types are all bitmaps where the name describes the patented algorithms used to save the pixel and array information on to your hard disk.

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Vectorised Graphics are created in Drawing programs like: Freehand, Illustrator, CorelDRAW etc. and even CAD programs. For this entry, I had to make the screen capture quite large to show the Nodes appearing on the simple objects when selected with the Bezier Curve editing tool. The curve manipulating handles are also visible on the wiggly bit at the left.

When a curve or outline object is created in a vectorised drawing program, a picture of the result is not saved to the hard disk. Instead just information describing the vector or position of each node on the page and simple values describing the nodes relationship with any one node either side of it. The values are used to describe a virtual line between the nodes and can be manipulated with apparently simple mathematics first described by a mathematician named Bezier. Thus we have Bezier Curves, a name you will become familiar with.

The handles shown in the sample above can be altered with the mouse allowing real time changes to the shape of the curve segments, and saved with the node information will be data describing colours for the outline weight and fill options etc. I often lecture on this subject with a simple story about building a house and printing house warming party invitations including a photograph, but I won't do it here except to say that it is interesting to know that the complete plans of a house drawn by an architect using an CAD program could possibly fit on one 1.4meg floppy disk.

Yet ONE scanned image of a normal 'happy snap' photograph of the finished house, using the industry standard CMYK bitmap resolution, might require about seven 1.4meg floppy disks. Saving information for hundreds of thousands of bitmap pixels requires substantially more computer resources than thousands of objects with many nodes saved in a drawing program.

The text object was included in the sample above because you should also understand why quality fonts used for DTP and Prepress are often referred to as Outline fonts. Although their use and manipulation is very complex, they are computer files where the objects (characters) are described as nodes and outlines not unlike our drawing objects - they are not pictures of the characters.

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four colour press single colour press
Printing presses
come in many shapes and sizes, from small Single Colour presses and Label printing machines to 2, 4, 5, 6 colour presses and multi colour Screen Printing presses, up to large Flexographic presses that print on plastic for wrapping etc. and Web presses several stories high printing newspapers etc.on continuous reel paper (webs).

A printing press requires one "Unit" for each colour that it can print. A one colour press needs to print on each sheet of paper three times to print a three colour job. A four colour press could print the same job in one pass (not needing the fourth unit).

unit of a press
Conventional Offset Lithography prints from a thin metal plate that contains an image, using one plate for each colour. These clever plates are created so that the bare metal will receive and hold water, while the image will receive ink and repel water. Therefore the damp metal background will not be covered in ink when the inking rollers apply the ink to the image areas.

The rotating plate on the Plate Cylinder passes water rollers (dampeners), then inking rollers, and finaly transfers the image onto a rubber blanket on the Blanket Cylinder. The blanket then transfers the image to paper as the paper passes between the Blanket and a Pressure cylinders. This two step arrangement gives the process the name "Offset" as in "offset between the plate and paper". Other specialist processes transfer the inked image directly to the stock (i.e. plastics). The rubber blanket (with the offset image) enables the transfer of images onto various types of even and uneven surfaces without damaging the plate.

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e-mail 1997 '98. Last Revised:  Friday, 31 October 2003 22:04