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Updated 27 May 2002

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Tip of the month is edited by Peter Ring, PRC (Peter Ring Consultants, Denmark)
- consultants on how to write user friendly manuals

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If you have corrections, better texts or suggestions for improvements, please let me know.

Background

Some of the info given below may sound primitive to the professional reader, but it is included to make sure most technical writers understands the background for the various formats, and thereby each format's individual strong and weak points.

I sincerely hope you will find inspiration to a more efficient use of graphics, and maybe inspiration to new and better ways of teaching your customers how to use your products.

Basic image format types

• Raster images, often also called bitmaps. They are mainly used for photos, screen-grabs, and scanned images.


The image is described pixel by pixel with a certain number of data-bits per pixel. These data-bits are carrying the information about the colour of the specific pixel. A pixel is a defined point in the matrix defined by the resolution of the image, see "Tip of the month June 1998" . The image below shows the Microsoft Internet Explorer icon enlarged 8x so that you can see the individual pixels.



Some raster image formats are sometimes (e.g. TIFF), or always (e.g. GIF), compressed. As an example, a simple compression algorithm counts how many identical pixels there are just after each other and stores that information instead of storing the colour data for each pixel. Modern compression algorithms are much more sophisticated. See also "Versions" below.

The most popular raster image formats are TIF (TIFF), Windows/OS2 BMP (Windows & OS2, only), GIF, JPG/JPEG, and PICT (Mac, only). JPEG is a "lossy" format; that means that the compression also removes the least important information details from the image, so that you cannot re-establish the original. The rest are "lossless" formats. GIF is unpopular among the programmers, because you need to pay royalties when selling programs which can save in the GIF format.

A new format, PNG (pronounced "ping"), is maybe on its way to replace GIF and JPEG. It offers lossless compression, 24 bit indexed colours, and it has build-in gamma, alpha-channel transparency, and two level interlacing (like GIF version 89a, interlaced, the image starts blurred and becomes crispier while it loads from e.g. the Internet). PNG is supported by MS Internet Explorer 4.0+, Netscape Communicator 4.04+ and Adobe's latest products. For more information, see Adobe Magazine No. 2 - 1998 "Look out GIF and JPEG". GIF (and TIFF-LZW) is unpopular among the programmers because you must pay royalties if you are using the LZW-compression. For more information on the GIF licence problem see http://www.unisys.com/unisys/lzw/. The original LZW patent from 20 June 1983 runs-out on 20 June 2003.

• Vectorised images are mainly used for drawings.


They are build from closed and open curves going from point to point. A curve can have a defined colour, width, and ending type (e.g. rounded or squared). A (closed) curve can be assigned a fill colour, filling the interior of the curve. Vectorised curves has a sort of resolution, too, here defined as the number of digits describing the position of a point, or the data for a vector or line width.

Some formats has only a very limited number of widths, endings, fixed shapes, etc., other formats permits a lot of variations. That's why conversion between vectorised formats sometimes gives some less pretty results.



Most of the vectorised formats permit curved lines, and have fixed shapes (like rectangles and ellipsis), e.g. AutoCAD, Corel Draw (.CDR) and Adobe Illustrator (.AI). The shape of the line is then described by two "vectors", one in each end of the line. The angle of the vector describes the direction of the line at the ending point, and the length of the vector describes how fast changes from the initial direction are permitted.

Some primitive vectorised formats builds curved lines incl. circles and ellipsis from short pieces of straight lines. When exporting a vectorized drawing from e.g. Corel Draw to e.g. Windows metafile (.WMF) (most other programmes or output formats gives the same bad results), the export filter converts curved lines to short sections of straight lines even WMF can handle curved lines.

• Combined formats.


A combined format may contain a mixture of raster image elements, vectorised image elements, and text. Examples are Windows metafile (.WMF), Corel Draw (.CDR) and Adobe Illustrator (.AI).

When saving in a combined format, you are often asked if you want to save the texts as text or as curves. Saving as text makes it easy to edit the text, incl. the font later on. Saving as curves solves the frequent problems with including special characters and/or fonts.

PostScript (.PS and .EPS) is a special combined format, closely related to the printer files send to PostScript printers.

Note, that some programmes convert your image to an internal low printing quality format (e.g. MS Word to WMF), unless you use EPS files, preferably with a high-resolution colour thumbnail for almost correct screen display of the image.

Variations

• Version


Many graphics formats exist in a large number of versions. The versions are typically related to different compression schemes (e.g. TIFF), but not all import filters can handle all of these compression schemes. That is why e.g. MS Word or Adobe PageMaker will accept some TIF's and reject other TIF's.

• Colour system


All image formats has fixed or variable limitations to the number of colours per pixel (point), e.g. GIF which is limited to max. 256 different colours in a standard or local colour palette . Other formats offer up to 32 bits per pixel (2³² = 4,294,967,296 colours) or more.

There are four basic colour systems: RGB, CMYK, ink colours, and B/W (grey scale). All colours can be displayed or printed with different intensities. In computer formats there are typically 2⁸ = 256 levels (intensity 0..255 = 0..100%) per colour.

• RGB ( Red- Green- Blue) is the system used for displaying light-emitting colours, e.g. on a TV or PC screen.
• CMYK is the system used for displaying screened reflective colours, e.g. printed colour photos, where the colour is build from small Cyan, Magenta, Yellow, and blacK dots (colour B means Blue, therefore the K). When printed, the size of the dot relative to the number of dots per mm or inch is used to control the intensity of the dot's colour. Colour printers are almost always using the CMYK system. Photosetters can work with ink colours, too, see below
• Ink colours are printed with a printing ink having the exactly correct colour. Common systems are e.g. Pantone and Europe. They are used a lot for printed matter, most often black + a "company colour".
• B/W (Black/White). A grey scale represents various intensities of black. "Black &white" is a 2 colour (1 bit/pixel) grey scale.
When transforming colours between RGB and CMYK colours (e.g. using Paint Shop Pro), the result frequently looks somewhat disappointing, especially on the PC screen. The printing/viewing quality can often be somewhat improved by adjusting the "gamma" settings, see the manuals for your application, video monitor, scanner and/or printer.

An indexed colour palette is a system, where each colour has a number. There are three types of palettes: calculated, standard and local. When using standard or local palettes on a video monitor, test what your colours will look like on a video screen set to 256 or (worse) 16 colours - there are lots of them out there! If the colour can't be matched exactly, you can choose between "nearest colour" or "dithering ".

• In a calculated palette , the colour number is calculated by the intensities of each basic colour. Example: in a 24 bit RGB system, a yellowish colour has the intensities of [R,G,B] = [255,228,8] = [hexFFE408]. The calculated colour number is then = 255 x 256² + 228 x 256 + 8 x 1 = 16,770,056.
• In a standard palette , a number of standard colours has been selected from the total number of colours possible, and each defined colour has then been given an arbitrary number. The Windows colour palette and the Pantone system are good examples.
• A local palette is user defined for each individual image file (or animation frame). The palette colour definitions are stored together with the image. Using local palettes are e.g. often the way to fairly nice and not too large (animated) GIF files.

• Layers, selection, and other information


Some formats permit saving of layers and/or selections. When an image is build in layers, you can work on one layer without disturbing the other layers. It is very common in vectorised and combined formats, and it is becoming more and more common with raster type formats. Selection is the part of an image currently selected. Desktop icon files (e.g. Windows .ICO) are small raster files containing information about the computer application it is intended to start.

• Transparent colour


Some formats like GIF offers you the possibility to select a "transparent colour". In a application which can handle transparent colours, this specific colour will then be replaced by the background colour. It is e.g. used a lot for Internet applications and DTP work.

• Thumbnails


A "thumbnail" is a raster copy of the image - often low resolution or grey-scale - stored at the start of an image file. Not all formats permit the inclusion of a thumbnail. Thumbnails are used for two purposes:

• Quick preview before loading the image file into an application.
• Displaying an image in an application, which can't display the file in its basic format. This is typically the case with EPS files, which only very few programmes can display directly on-screen. As an example, PageMaker displays the (internally generated!) thumbnail of an EPS image, but when printed on a PostScript printer, the full content original e.g. vectorised PostScript image is printed.

• 3-dimentional (3-D) formats


Some vectorised formats are 3-dimentional, and all true 3-D formats are vectorised. They were originally developed (and still mainly used) for computer games (see animated formats below) and CAD (Computer Aided Design) systems. A subject is build by a network of lines, which are combined to describe curved planes. In most 3-D vectorised formats, light sources may be applied for light and shadow effects.

When a subject is described in a 3-D vectorised format, it can be viewed from any angle as you please. When you are happy with the viewing angle, you can save it in a 2-dimentional format showing the subject seen from that specific viewing angle.

This is very useful when making cartoon-style manuals, e.g. for the assembly of complex products.

• Animated formats


Animated formats and systems exist for both raster and 2-D or 3-D vectorised images.

The simplest animated format is the animated GIF, frequently used on websites. It simply displays a number of single GIF images as "frames" in a sequence, where the position (e.g. a moving person relative to the first full size background frame) and duration is determined for each frame.

"Photo" type animated raster formats are mainly used for PC video purposes, e.g. .AVI and .MPEG, which both exists in a number of variations with different compression schemes and with/without sound. They are well suitable for "live" instructions. Their main disadvantage is the size of the files, now to some extend solved by the development of fast CD-ROM drives. AVI and MPEG are compressed formats relying on the fact, that most parts of a movie frame is identical to the previous frame, and that it consequently takes less data to describe the difference from the previous frame, rather than saving the complete data set for each frame. Many modern high-end screen-grabbers like SnagIt/32 can record e.g. AVI "movies" from a computer screen.

Complex 3-D simulators combine a large number of individual 3-D item files, and changes their relative size, position, and sometimes even shape with time. A good example are the popular flight simulator and adventure games. Another example is the interactive 3-D shopping malls on the Internet.

Animated graphic formats can be used for MultiMedia manuals describing e.g. complex mechanical processes, or how to use a computer programme.

Ideas for new "Tip of the month" subjects are VERY welcome, too!

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