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An Aside...

Ppi (Pixels per inch): a measurement of the amount of information scanned from an image.

Dpi (Dots per inch): a measurement of the outp ut resolution of a printer, copier, imagesetter, or monitor.

Lpi (Lines per inch): A measurement of the frequency of the halftone screen used to print an image. Lpi is also referred to as "line screen".

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File Formats

Today's class is possibly the most fundamentally important of the program. The information you will gather here will serve you throughout your career as a graphic designer.

Dithering:

Mixing tiles of nearby colours or shades of grey. Dithering looks at the colours or greys in one row of tiles and colours or greys in an adjacent row and then averages the two rows to create a third row between them. The goal is to smooth the transition between contracting elements.

Re-screening:

When you scan photos from an offset printed piece, the halftone screen used in printing will cause image distortions in your scan. Avoid re-scanning re-printed material. If you must scan it, you may try placing the printed material diagonally on the bed of the scanner. There are also tools in Photoshop which can help reduce such distortions.

Scanning:

When scanning on a flatbed scanner, light is reflected from an original image to an electronic sensing area that records the value for each point on the image. The number of points is your resolution. The amount of grey or colour information sensed for each point is your bit depth.

A Good Scan:

To scan properly, you must obtain good resolution, colour balance, tonal range and sharpness. To scan for print, the resolution must be 1.5 to 2 times the line screen. Since the computer is the principle tool of the modern graphic designer, it is imperative that you have a firm grasp on the various file formats and their purposes. There are concepts which require explaining in order to fully understand the various file formats. They are: vector, raster, colour modes, bit depth, resolution, dimension.

Once you get a good handle on the basics, scanning -- with any software -- will be a breeze.

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The Bit Map and the Vector

File formats break down into three types: vector, raster and others. The three types have very different purposes and characteristics. Some clarification about terminology must be made at the outset. the term Vector will be used as synonymous with object oriented. bit map (or bit map) raster or pixel-based are also interchangeable.

The Bit map

The bit map is the most basic of all graphics. It is most akin to a wall of tiles. The tiles are lined up and in rows and columns. They are either black, white, shades of grey, or colour.

Black & white bit maps require only one bit of information to describe each pixel -- zero or one. There is not enough information in them to display greys or colours.

A greyscale bit map's tiles are described as 8 bit. This means that each tile can display 2 to the 8th power:

2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 greys

A colour bit map's tiles can be 8 bit or more. In this case, there would be 256 colours or more. A photographic quality full colour image requires 24 bits per pixel in order to display 16.7 000 000 colours. That is:

2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2
x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2
x 2 x 2 x 2 x 2 x 2 x 2 x 2
= 16 700 000 colours

This means that any tile in such a bit map image may be any one of 16 700 000! This said, it is easy to understand why such images may require such large amounts of disk space.

Bit map images have a grid-like structure which is distorted when skewed, re-sized, moved or rotated. When you move part of the mosaic, you tear the tiles. They get super-imposed on other tiles producing unsightly results.

Printing bit map images gives you only the resolution you started with. A low resolution bit map prints in low resolution, even on a high resolution device. If you print a 300 dpi image on a 1200 dpi device, you get 300 dpi.

The Vector

Object-oriented graphics -- also known as vector graphics -- are mathematical instructions compiled from menu choices and mouse actions in a draw program.

Think of vectors as a lot of electronic rubber bands describing the outline of the image. You can scale, distort, rotate and refill vector objects without any loss of quality. They can be stacked and potentially hidden behind other objects without being erased.

The benefits of vector graphics extend to printing. The draw programs communicates the characteristics of the shapes to the printer, allowing it to print the drawing at the highest resolution available. Therefore, a vector graphic printed on a 2400 dpi image setter will print at 2400 dpi. The same document, printed on a 300 dpi laser printer, will print at 300 dpi.

Vector graphics are the best choice for illustrations, line art and business graphics. The have smooth curves, smooth greyscale shading and tints.

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Other Formats

Word Perfect, Microsoft Word, AppleWorks, AutoCAD and many, many more formats are proprietary formats with particular functions. They are taylor made for their particular applications.

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File Extensions

File extensions are three letters which follow the "dot" in the file name on the computer. They were originally required by Microsoft DOS. The Macintosh has never needed them. In Microsoft Windows, the file extension determines which application will open the file. A ".doc" file will be opened by Microsoft Word. On the Macintosh, it is a good idea to use file extensions in case your file ends up on a Windows PC.

On a Macintosh, a filename may have an "extension" at the end that identifies the file's type. Extensions are usually hidden in Mac OS X. Changing the name does not change its extension.

If extensions are showing, changing the name also does not change the extension. To change the extension, select just the extension and type the new extension. This changes the file's type and it may not open.