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August 03, 2018

Digital images can usually be divided into two distinct categories. They are either bitmap files or vector graphics. If you work in prepress, you need a good understanding of the advantages and disadvantages of both types of data. These pages try to explain the differences.

  • As a general rule, digital pictures and scanned images are bitmap files. These are sometimes also called raster images.
  • Drawings made in applications like Adobe Illustrator or Corel Draw are saved as vector graphics.
Technically both data formats are completely different. The end result, however, can look virtually identical in either format. As a general rule bitmaps are typically used to depict lifelike images whereas vector graphics are more often used for abstract images such as logos. There are however numerous exceptions to this rule. It is often impossible to determine whether an image is a bitmap or a vector file just by looking at it.
  • Vexel art, for instance, are bitmap images that have been manipulated to look as if they are vector data.  The technique is used to create attention-grabbing realistic images that have an artificial and sharpened look to them.
  • Talented artists like Yukio Miyamoto can draw photorealistic images using vectors.

Photorealistic vector art from Yukio Miyamoto
You can convert a bitmap image into a vector file. A vector image can be transformed into a bitmap. There are even file formats that can combine both types of data into a single file.

Bitmap images

Bitmap images are exactly what their name says they are: a collection of bits that form an image. The image consists of a matrix of individual dots (or pixels) that all have their own color (described using bits, the smallest possible units of information for a computer).
Let’s take a look at a typical bitmap image to demonstrate the principle:
Example of a bitmap image
Example of a bitmap image
To the left you see an image and to the right a 250 percent enlargement of the top of one of the mountains. As you can see the image consists of hundreds of rows and columns of small elements that all have their own color. One such element is called a pixel – short for picture element. The human eye is not capable of seeing each individual pixel so we perceive a picture with smooth gradations.
The number of pixels you need to get a realistic looking image depends on the way the image will be used. One of the next pages goes into more detail on this.

Types of bitmap images

Bitmap images can contain any number of colors but there are four main categories:
  1. Line-art. These are images that only contain two colors, usually black and white. Sometimes these images are referred to as bitmaps because a computer has to use only 1 bit (on=black, off=white) to define each pixel.
    Example of a lineart image
    Example of a lineart image
  2. Grayscale images, which contain various shades of gray as well as pure black and white.Typically 256 shades of gray (8-bit) are used even though the human visual system needs only 100 tints to perceive an image as life-like.
    Example of a greyscale image
    Example of a grayscale image
  3. Multitones: such images contain shades of two or more colors. The most popular multitone images are duotones, which usually consist of black and a second spot color (often a Pantone color). The example below contains black and Pantone Warm Red.
    Example of a duotone image
    Example of a duotone image
  4. Full-color images. The color information can be described using a number of color spaces: RGB, CMYK or Lab for instance.
Example of a color image
Example of a color image

Characteristics of bitmap data

Bitmap data can take up a lot of room. A CMYK A4-size picture that is optimized for medium quality printing (150 lpi) takes up 40 MB. Compression can reduce the size of the file.
The image with the enlargement showed one of the main disadvantages of bitmap images: once they are enlarged too much, they look unnatural and blocky. Reducing their sizes also has an impact on image quality as images lose a bit of sharpness.
Bitmaps are fairly simple to output, as long as your RIP or printer has sufficient memory.

Applications that can handle bitmap data

There are hundreds of applications on the market that can be used to create or modify bitmap data. In prepress, one application – Adobe Photoshop – completely dominates the market. This doesn’t mean that cheaper alternatives like Corel Photo-Paint, should be disregarded.

File formats that are used for bitmap data

Bitmap data can be saved in a wide variety of file formats. Among these are:
  • BMP: an outdated and limited file format that is not suitable for use in prepress.
  • EPS: a flexible file format that can contain both bitmap and vector data. It is gradually being replaced by PDF.
  • GIF: mainly used for internet graphics
  • JPEG: or rather the JFIF file format, which is mainly used for internet graphics
  • PDF: versatile file format that can contain just about any type of data including complete pages, it is not yet widely used to exchange just images
  • PICT: a file format that can contain both bitmap and vector data but that is mainly used on Macintosh computers and is not very suitable for prepress.
  • PSD: the native file format of Adobe Photoshop (which can also contain vector data such as clipping paths)
  • TIFF: a popular and versatile bitmap file format

Vector graphics

Vector graphics are images that are completely described using mathematical definitions. The image below shows the principle. To the left, you see the image itself and to the right, you see the actual lines that make up the drawing.
Example of a vector based image, drawn using bézier curves
Example of a vector image
Each individual line is made up of either a vast collection of points with lines interconnecting all of them or just a few control points that are connected using so-called Bézier curves. It is this latter method that generates the best results and that is used by most drawing programs.
Example of a bezier curve
To the right is an example of using Bézier curves
This drawing demonstrates the two principles. To the left, a circle is formed by connecting a number of points using straight lines. To the right, you see the same circle that is now drawn using 4 points (nodes) only.

Characteristics of vector drawings

Vector drawings are usually pretty small files because they only contain data about the Bézier curves that form the drawing. The EPS file format that is often used to store vector drawings includes a bitmap preview image along the Bézier data. The file size of this preview image is usually larger than the actual Bézier data themselves.
Vector drawings can usually be scaled without any loss in quality. This makes them ideal for company logos, maps or other objects that have to be resized frequently. Please note that not all vector drawings can be scaled as much as you like:
  • Drawings containing trapping information can only be scaled up to 20 percent larger or smaller.
  • Thin lines may disappear if a vector drawing is reduced too much.
  • Small errors in a drawing may become visible as soon as it is enlarged too much.
It is fairly easy to create a vector based drawing that is very difficult to output. Especially the use of tiles (small objects that are repeated dozens or hundreds of times) and Corel Draw lens effects can lead to very complex files.

Applications that can handle vector data

There are hundreds of applications on the market that can be used to create or modify vector data. In prepress, Adobe Illustrator and Corel Draw are the most popular programs.

File formats that are used for vector data

Bitmap data can be saved in a wide variety of file formats. Oddly enough the most relevant formats for the printing industry are also capable of storing bitmap information:
  • EPS: the most popular file format to exchange vector drawings even though PDF is quickly gaining ground.
  • PDF: a versatile file format that can contain just about any type of data including complete pages.
  • PSD: the native file format of Adobe Photoshop.
  • AI: the native file format of Adobe Illustrator.

How to convert bitmap data to vector data and back

It is sometimes necessary to transform images from bitmap data to vector data or back. Some possible uses include:
  • If you scan or photograph a logo, it is a bitmap image. If it is going to be used often in a layout it is more practical to have that logo as a vector drawing. That reduces its file size and you can change the image size without worrying about any loss in quality.
  • Vector drawings often have to be converted to bitmaps if they will be used on a web page.
  • Vector drawings are sometimes too complicated for a RIP to be output on film or plate. Converting them to a bitmap simplifies the file.
Luckily it is fairly easy to convert images from one mode to the other:
  • From bitmap data to vector graphics: the process to convert a bitmap image to vector data is called outlining or vectorizing. Some drawing applications such as Adobe Illustrator and Corel Draw have this option built in. There are also separate programs available to vectorize bitmap images. For simple jobs the easiest solution is to put the bitmap image on the background of the canvas of a drawing application and manually draw over it.
  • From vector graphics to bitmap data:
    • Many drawing applications can store vector data as bitmap files as well (usually this option is hidden in the Export menu option).
    • You can always view a vector file on screen, then take a screen capture and save this screen capture as a bitmap image.
    • Photoshop can open some vector file formats and rasterize the file so that it becomes a bitmap file. A pop-up menu allows you to define the resolution and color mode of the bitmap data.
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August 03, 2018
TIFF or the Tagged Image File Format is a file format that is strictly used for bitmap data. TIFF files don’t contain text or vector data, even though the file format theoretically would permit additional tags to handle such data. Despite being one of the earliest file formats for images, it is still very popular today. It is a highly flexible and platform-independent format which is supported by numerous image processing applications and virtually all prepress software on the market.
The file extension for TIFF files is .tif even though .tiff is also used occasionally.

File format specifications

As the name implies, TIFF images make use of tags, keywords defining the characteristics of the image that is included in the file. For example, a picture that contains 320 by 240 pixels would include a ‘width’ tag followed by the number ‘320’ and a ‘depth’ tag followed by the number ‘240’.
The flexibility of TIFF makes it very easy to write a TIFF-writer, but very difficult to create a fully TIFF compliant reader. The need for well-defined rules has caused a few TIFF-substandards to appear. For prepress, TIFF/ITTIFF/EP, a version of TIFF optimized for digital photography.
 is a prime example but that file format is no longer actively used. Another substandard is

Color spaces

TIFF images can contain more or less anything:
  • Line-art (pure black-and-white)
  • Grayscale
  • Pseudocolor, from 1-bit to 8-bit (also called palette color or indexed color in Photoshop)
  • RGB
  • YCbCr
  • CMYK
  • CIELab
For grayscale, RGB and CMYK images, 8 bits (256 levels) are used per channel but this is not a limitation of the TIFF file format. The file specifications also allow 16-bit channels. Although this feature is also supported by recent versions of Photoshop, many layout applications and drivers cannot yet support these data types.

Compression

TIFF supports a large number of compression algorithms. The lossless algorithms that can be used are:
  • PackBits
  • LZW (Lempel-Ziv-Welch), popular for grayscale or color images (although it is not very efficient for CMYK images)
  • CCITT Fax group 3 & 4, mainly used for line-art images (especially screened data coming from a RIP or copydot application).
Officially TIFF also supports lossy JPEG compression. Unfortunately, the specs were not worked out correctly and JPEG never gets used in TIFF files, at least not for prepress use.

File size

TIFF files cannot have more than 4 Gigabyte of raster data. However, this is 4GB of compressed data, and so if the compression ratio is high enough, theoretically a TIFF image could be much larger (in fact, 2**32-1 pixels square).

How to edit TIFF files

All professional image editing applications on the market are capable of opening TIFF files. My favorite is Adobe Photoshop.

How to convert TIFF files

There are tons of converters that can convert a TIFF file to a JPEG, PNG, EPS, PDF or other file format. Google is your friend.
  • I have in the past had good experiences with GraphicConverter, a shareware tool for Macintosh that can import about 200 file types and export 80.
  • For occasional file conversions I stick to Photoshop – it is not too difficult to write an action that does a batch conversion of a series of files.
  • To convert a series of TIFF files to a PDF using Adobe Acrobat Professional 9: select File > Combine > Merge Files into a Single PDF. The Combine Files dialog box pops up. If you want to preserve the original image resolution, be sure to select the largest page icon that appears in the most lower right-hand corner listed next to ‘File size:’.

The history of TIFF

TIFF was developed as a universal image file format by Aldus (makers of PageMaker) in 1987. The most recent specifications, TIFF 6, were released in 1992. It is pointless to study older versions of the format since everybody sticks to the TIFF 6 specs. Since then, Aldus has been bought by Adobe so Adobe now holds the copyright. They have not released any new versions of TIFF which is not necessarily a bad thing since standards that last long are well supported and understood in the market.

Other sources of information

Niles Ritter maintains an unofficial TIFF home page. It covers various aspects of the file format and also points to the rather hefty TIFF 6 specifications you can download from the Adobe web site.
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August 03, 2018

PNG or Portable Network Graphics is a file format that was designed to replace GIF. Not only is GIF a technically limited file format but LZW, the compression algorithm it uses, is owned by Unisys who are more than happy to charge for the privilege of using it. PNG is patent-free and offers enough features to also make it a valid alternative to the TIFF file format in some cases. The file format is meant to store bitmap data.
PNG was developed around 1995 by an Internet working group led by Thomas Boutell. Its popularity got a big boost when W3C, the organization that defines web standards, started promoting its use in 1996. Major graphic arts applications such as Photoshop and InDesign fully support PNG, although the file format isn’t that popular in prepress because it doesn’t support CMYK. I often use InDesign to create presentations and for this type of application, PNG can be very useful.
There is a ‘sister file format’ called MNG which is meant for video applications.

File format specs

Color spaces

PNG supports the following image types:
  • Line-art – pure black-and-white, essentially 1-bit grayscale
  • Grayscale – with up to 65536 shades of gray (16-bit) are supported although often 256 shades are used.
  • indexed color – from 1-bit to 8-bit (also called palette-based color or pseudocolor)
  • RGB – up to 48-bit although 24-bit (16 million colors) is most popular.

Compression

PNG’s compression is fully lossless. No image information is lost by compressing the image.

Alpha channels

Alpha channels are comparable to Photoshop masks. It is a way of making sure that part of the image is transparent so that a colored background underneath a PNG image with alpha channel remains visible.

Gamma correction

Images created on Macs have a tendency to look too dark on a PC screen. The reverse is also true: PC images look too light on a Mac. This is due to a difference in gamma (image brightness) between both systems. A PNG image can contain the gamma value used by the authoring system so that applications can compensate for this if needed. A full blown color management system is superior to a simple algorithm such as gamma curves. PNG can support this through extensions, but its use is not widespread (yet).

Interlacing

Interlacing is a web-specific feature. It is a mechanism that makes images appear faster on-screen by first displaying a low-res version of the image and gradually showing the full version. This feature cannot be used by prepress software.

Limitations

PNG files cannot contain ICC profiles (a mechanism which describes what the color space or gamut of the image is). Metadata (who made this images, what is it about, who holds the copyright,…) also aren’t supported. Even though the image resolution of a PNG file can be stored within the pHYs chunk (in pixels per meter), some design application (like Adobe InDesign CS3) don’t support this properly and seem to assume PNG images use 72 dpi. This makes PNG unsuitable for use in print publishing.

Specsheet

Name: PNG
Developer: independent workgroup
Release date: 1995 for the original file format, 2003/2004 for the second version
Type of data: bitmap only
Number of colors: 2 to 65536 per channel
Color spaces: line-art, grayscale, indexed color, RGB
Compression algorithms: lossless (5 types of filter supported)
Ideal use: web publishing
Extension on PC-platform: .PNG
Macintosh file type: PNGf
WWW mime type: image/png
Special features: support for alpha channels, gamma correction, and interlacing
Remarks: A good reference site about PNG can be found here. PNG should be pronounced ‘ping’, not ‘pee and gee’.
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August 03, 2018

JPEG stands for Joint Photographic Experts Group, which is a standardization committee. It also stands for the compression algorithm that was invented by this committee.
The JPEG algorithm has been conceived to reduce the file size of natural, photographic-like true-color images as much as possible without affecting the quality of the image as experienced by the human sensory engine.
  • JPEG compression is lossy: when you decompress a JPEG compressed image and compare the result to the original image that was compressed, there will be differences and a loss of detail and sharpness in the decompressed image. Other compression algorithms like LZW are lossless: the decompressed image is identical to the original image.
  • We perceive small changes in brightness more readily than we do small changes in color. The human visual system is also not that good at dealing with high frequency data – if image details change very quickly in part of an image, our brain cannot really process all those changes.  These two aspects of our perception are used in JPEG compression to reduce the file size of an image.
  • JPEG compression does not use a single fixed way of compressing data. The algorithm can compress image data heavily with a significant loss of detail, or it can limit the loss of detail by lowering the compression ratio.  In applications like Adobe Photoshop, a slider allows you to select the compression ratio.
  • There are two JPEG compression algorithms: the oldest one is simply referred to as ‘JPEG’ within this page. A separate page discusses the newer JPEG 2000 algorithm.
  • You have to make a distinction between the JPEG compression algorithm and the corresponding JFIF file format, which many people refer to as JPEG files and which is discussed in the file format section.

How JPEG works

The JPEG algorithms perform compression in phases:
  • Our visual system is less sensitive to changes in color (chrominance) than changes in brightness (luminance). The JPEG algorithm is based on this difference in perception. It does not analyze RGB or CMYK color values but instead uses a luminance/chrominance based color space called YCbCr. In this color space, the Y channel carries luminance data while the Cb and Cr channels contain color information. This allows for separate compression of these two factors. Since luminance is more important than chrominance for our visual system, the algorithm retains more of the luminance in the compressed file and downsamples the color information, typically by a factor of 2 in both the horizontal and vertical direction.
Please note that the above step is not mandatory in the JPEG specs: the full specification also allows the use of the RGB color model but in practice most applications do perform that conversion to the YCbCr color space.
The YCbCr color space is only used internally. You do not notice its use as a user. After decompressing a file that was JPEG compressed, you once again have an image that has the same color space as the source image.
  • The JPEG algorithms then cut up the image in separate blocks of 8×8 pixels. The compression algorithm is calculated for each separate block, which explains why these blocks or groups of blocks become visible when too much compression is applied. Compression is also applied separately to the Y, Cb and Cr channels.
  • For each 8×8 block, a Discrete Cosine Transform (DCT) transformation is applied. In this complex process, the intensity levels of all 64 pixels are replaced by values that describe how those values could be calculated using a mix of mathematical functions. This operation does not compress the file, it simply replaces 8×8 pixel values by an 8×8 matrix of DCT coefficients.
  • Once this is done, the actual compression can start. First the compression software looks at the JPEG image quality the user requested (e.g. Photoshop settings like ‘low quality’ or ‘medium quality’) and selects an appropriate quantization table. This is a matrix of 8×8 numbers. Then the software divides each of the 8×8 DCT coefficients by its corresponding constant in the 8×8 quantization table and rounds off to the nearest integer. The quantization table favors small or gradual changes over high frequency changes. As was stated early in this topic, our visual system cannot easily cope with rapid repetivite changes in image data, so the algorithm favors gradual tonal changes. The result of this quantizing step is that you go from a matrix that contains 64 numbers that are potentially all different to a new table that contains much lower numbers, which are more likely to either have the same value or, for high frequency patterns, be zeros. The new 8*8 matrix still roughly describes how to reproduce the original 8*8 tonal values, but the rounding-off has caused a loss in image quality.
  • A table that contains many numbers that are either identical or zero is easy to compress. The last step in the process is to compress these coefficients using either a Huffman or arithmetic encoding scheme. Usually Huffman encoding is used. This lossless compression leads to yet another reduction in file size.

Advantages

By combining several compression algorithms, JPEG achieves remarkable compression ratios. Even for prepress use, you can easily compress a file to one-fifth of its original size. For web publishing or e-mail exchange, even better ratios up to 20-to-1 can be achieved.
JPEG decompression is supported in PostScript level 2 and 3 RIPs. This means that smaller files can be sent across the network to the RIP which frees the sending station faster, minimizes overhead on the print server and speeds up the RIP.

Disadvantages

The downside of JPEG compression is that the algorithm is only designed for continuous tone images (remember that the P in JPEG stands for Photographic). JPEG does not lend itself for images with sharp changes in tone. There are some typical types of images where JPEG should be avoided:
  • images that have had a mask and shadow effect added to them in applications like Photoshop.
  • screen dumps or diagrams.
  • blends created in Photoshop.
  • images containing 256 (or less) colors.
  • images generated by CAD-CAM software or 3D applications like Maya or Bryce.
  • images that lack one or more of the process colors. Sometimes images are created that use for instance only the magenta and black plate. If such an image is compressed using JPEG compression, you may see artifacts show up on the cyan and yellow plate.
Because of its lossy nature, JPEG should only be used during the production stage of prepress (making PostScript or PDF, imposing, proofing, outputting). During the create process when images are still edited, cropped and colour corrected, each new SAVE-command leads to extra loss of image quality when JPEG is used.

Where is JPEG compression used

JPEG compression can be used in a variety of file formats that are commonly used in graphic arts:
  • EPS files
  • EPS DCS files
  • JFIF files
  • PDF files
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August 03, 2018
GIF is a file format that is meant for use on the internet. It should not really be used for prepress. Unfortunately, GIF images keep popping up in pages made by amateurs so it is worthwhile to know a bit about the format. You can also use this description to explain to people why GIF is not suited for prepress use.

General information

GIF is the abbreviation of Graphics Interchange Format. It was originally developed by CompuServe (an on-line service that was pretty successful in the early nineties). The format includes some key features which make it a unique and valuable format for the internet. These features include file compression, transparency, interlacing and storage of multiple images within a single file which allows for a primitive form of animation.
There are two versions of the GIF format; versions 87a and 89a. These versions were released in 1987 and 1989 respectively.
  • GIF 87a: the initial version of the GIF file format supported LZW file compression, interlacing, 256-color palettes and multiple image storage.
  • Version 89a added background transparency and a few other additions such as delay times and image replacement parameters to make the multiple image storage feature more useful for animation.
Because the LZW compression algorithm that is used in GIF is copyright protected, a new standard has been developed, based on a free compression algorithm. This successor, called PNG, has basically replaced GIF except when GIFs animation features are useful.

Features of the GIF format

This is an overview of the different features of the GIF file format, from the perspective of a prepress operator.

Limited color palette

A GIF image can contain 2, 4, 8, 16, 32, 64, 128 or 256 colors which are stored in a color palette or color lookup table within the image file. Each color in the GIF color table is described in RGB values, with each value having a range of 0 to 255. CMYK colors are not possible in GIF. Although the GIF format has access to over 16.8 million colors, only a maximum of 256 can be referenced within a single GIF image. While this limited palette keeps down file size and is perfectly acceptable for on-screen viewing, it leads to posterized images when they are printed. Most preflight tools like PitStop can generate a warning when they encounter images with a fixed color palette.

Dithering

The limited number of colors in GIF is used to limit the file size of images. While a small image using 256 colors may take up 9.5 K, the same image using 32 colors takes up only 4.4 K and going down to 16 colors get it down to 1.9 K. Another trick that is used to limit the file size is dithering. This technique is used to create the illusion of greater color depth by blending a smaller number of colored ‘dots’ together. When fewer colors are able to be displayed than are present in the original image, then patterns of adjacent pixels are used to simulate the appearance of the underrepresented colors. Dithering is not really a feature of GIF, it is simply a technique that is often used within GIF images. Dithering adds noise to the image and it reduces sharpness.

LZW compression

GIF supports LZW compression, which is a lossless compression algorithm that is also used frequently in prepress. TIFF images, for example, are also often LZW compressed.

Transparency

Transparency is the feature of the GIF89a format which allows for the specification of one of the colors in the palette to be ignored while processing the image for your display device. While this feature works great on the internet, it is not supported by layout applications, which rely on PSD-files or EPS-images with an included mask to achieve the same functionality (but with a much smoother edge around images).

Interlacing

Interlacing is another web-specific feature of GIF. It is a mechanism that makes images appear faster on-screen by first displaying a low-res version of the image and gradually showing the full version. Physically, an interlaced GIF just has the scanlines stored in an unusual order:
  • The first pass has pixel rows 1, 9, 17, etc (every eighth row)
  • The second pass has rows 5, 13, 21, etc. (every remaining fourth row)
  • The third pass has rows 3, 7, 11, 15, etc. (every remaining odd row)
  • The last pass has rows 2, 4, 6, etc. (all the even-numbered rows).
How the web browser chooses to display this is up to the browser. This feature cannot be used by prepress software.

Animation

The GIF89a specifications add a few enhancements to the file header which allows browsers such as Netscape to display multiple GIF images in a timed and/or looped sequence. This mechanism allows for small, rather crude animations and it is a very popular feature that used often used in banners. This feature is of no use for prepress software.

Resolution

Although GIF does not require a specific resolution, most GIF images have a resolution between 72 and 90 dpi, ideal for on-screen viewing but insufficient for prepress use.

Specsheet

Name: GIF
Developer: CompuServe
Release date: 1987
Type of data: bitmap
Number of colors: 2, 4, 8, 16, 32, 64, 128 or 256
Color spaces: RGB
Compression algorithms: LZW
Ideal use: internet publishing
Extension on PC-platform: .gif
Macintosh file type: ?
Special features: support for transparency, interlacing, and animation
Remarks: –
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August 03, 2018
DCS stands for Desktop Color Separation. It is a file format that is based on the EPS file format. In fact, you could regard DCS files as a collection of EPS files.
DCS files are mainly used to exchange bitmap images between prepress applications. Occasionally DCS files are also used for vector data or text. The main advantage of DCS over its parent EPS file format is that it adds a kind of OPI-functionality to the file format. Since DCS files contain separate EPS-files for each plate, an application can generate and print color separations faster when the DCS format is used. This was a valid approach when Macs, PCs and software were not as powerful as they are today and when all output was done from the layout application. In today’s world with the increased popularity of in-rip separations as well as improved support of EPS-files within applications like QuarkXPress, DCS can be a very inefficient file format.
As already noted, DCS-files are actually EPS-files that have to follow the Adobe specifications (Appendices G and H of the PostScript Language Reference Manual, 2nd ed.). The only differences are some changes in the header comment area as well as additional comment lines in the main section of the file that describe the separation data. DCS-files contain a preview image, just like EPS-files.
There are two different versions of the DCS file format: version 1 and version 2.0.

DCS 1.0

DCS 1 was developed by Quark to add a file format that could easily and efficiently be separated to their main application, QuarkXPress. This file format is usually referred to as DCS.
A DCS 1 file is composed of 5 separate files. Below you see such a file: the main file has the extension .eps while the 4 other files have an extension that marks the color data they contain. The file size shows that the main file does not contain any actual image data but only a preview image and pointers to the other 4 corresponding highres files.

Because the main file has lines in it that refer to the other files, you cannot simply rename DCS-files in the Macintosh Finder or Windows Explorer. If you want to change the name of a DCS-file, it is best to open it in Photoshop and use a SAVE AS to save the file using a different name.
The image data in the four CMYK-files can be compressed using JPEG compression. This has often posed problems with OPI-systems and older RIPs sometimes also choke on the decompression.

DCS 2.0

Development on the DCS-2 format started in 1993 and it became available in 1995 or so. These are 2 new features in DCS 2.0:
  • The option to choose a multiple- or single-file version. DCS originally required that the separation files be separate. With DCS 2.0, these files may now be combined into one. Please note that this does not turn DCS-2 files into true composite files, just look at single file DCS-2 files as a collection of separate files that are glued together to form one big file.
  • The ability to specify additional plate colors. DCS 2.0 can point to spot color plates in addition to the standard cyan, magenta, yellow, and black. This ability makes DCS-2 the ideal file format for hexachrome images. These contain 6 colors: cyan, magenta, yellow and black as well as orange and green.
Early design applications did not support DCS-2 files. This includes versions of QuarkXPress prior to 3.32 and PageMaker 6.5. Because DCS-files are a kind of EPS-files, they can be imported in these applications but the output on film or plate is incorrect.

DCS must die

DCS made a lot of sense 10 years ago but in today’s world it has become a real nuisance. The main problem is that popular applications like QuarkXPress and InDesign (before the CS release if I am correct) do not properly support DCS-files when they print composite PostScript files (files that are not yet separated, with the separation option switched ‘off’ in the PRINT-menu). Instead of reading the high-res data in the DCS-file, they only include the lowres screen preview in their composite print files. If you fail to notice this, the output will contain ugly 72-dpi images. Some more recent layout applications can merge DCS-data when generating their output file.
Although there are extensions on the market to solve this problem (SmartXT for XPress) and most OPI-servers can merge the highres data when OPI is used, DCS is simply a nuisance and deserves to disappear, at least for pure CMYK images.
If you need more than 4 colors in an image (e.g. hexachrome) or you wish to process copydot files, then DCS is still a valid file format. Using native Photoshop files is a better alternative if you use Adobe InDesign for page layout. For simple CMYK images, DCS should never be used anymore.

Specsheet

Name: EPS DCS
Developer: Quark
Release date: ?
Type of data: bitmap
Number of colors: ?
Color spaces in practice: DCS-1: CMYK or 1-bit copydot, DCS-2: CMYK, hexachrome or copydot
Compression algorithms: JPEG
Ideal use: exchange of images between prepress applications
Extension on PC-platform: .DCS, .C, .M, .Y and .K for the separate colour files.
Macintosh file type: ?
Special features: can be troublesome in composite workflows
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August 03, 2018
EPS or Encapsulated PostScript is a standard graphics file format for exchanging images, drawings (such as a logo or map) or even layouts of complete pages. An EPS file internally contains a  description of such an object or layout using the PostScriptpage description language. It can include both bitmap and vector data.  The purpose of an EPS file is to be included in other pages. Sometimes EPS files are called EPSF files. EPSF simply stands for Encapsulated PostScript Format. EPS files have the extension .eps or .epsf.
This page discusses:
  • the basics of the EPS file format
  • how to create EPS files
  • how to view EPS thumbnails
  • how to reduce the file size of EPS files
  • More technical details about the file format
EPS is still in widespread use, but it is essentially an outdated file format that no longer evolves.
  • For exchanging logos or drawings, it has been replaced by the native file formats of Adobe applications.Adobe has made it easy to drop a native Illustrator or Photoshop file in an InDesign document. Given that most people work with the Adobe Creative Cloud or Creative Suite software, EPS no longer makes sense as an intermediate file format.
  • For exchanging complete pages or advertisements, it has been replaced by PDF (just like PostScript itself is also being phased out and replaced by PDF).
Even though PDF and native file formats are the way to go, your existing library of EPS files will still remain usable for a long time. Here is what Dov Isaacs from Adobe said in a discussion on a PrintPlanet forum: “ …Adobe will continue to support EPS as a legacy graphics format for import of non-color managed, opaque graphical data into Adobe applications (such as InDesign and Illustrator). Although we certain do not recommend that new graphical content be stored in EPS format (except to satisfy the need to import data into page layout programs that aren’t quite PDF-centric — no need to mention names here!), our user base should feel comfortable that there is no need to worry about a need to convert their very sizable libraries of EPS-based graphic assets.”

The basics of EPS files

An EPS file can contain any combination of text, graphics, and images. Since it is actually a PostScript file, it is one of the most versatile file formats that are available. EPS-files usually contain a small preview image that is used to visualize the content of the file. This is done so that applications don’t need a PostScript interpreter to display the content of the EPS file. Even office applications such as Microsoft Word can display the preview image. If an EPS file is sent to a printer that doesn’t support PostScript, it is once again this preview image that is printed. The quality will not equal that of the read EPS artwork but at least there is an image on the print-out. There are millions of people working with *.eps files without realizing how complex the artwork they are using really is.

How to create EPS files

EPS files can be generated by all professional drawing applications as well as most layout applications.
  • The most widely used application to create EPS files is Adobe Illustrator. Illustrator’s native file format is called AI. An AI file is smaller than the corresponding EPS file and it retains all of the editing capabilities of Illustrator. The advantage of saving as an EPS is that it is easier to use the file with other (non-Adobe) applications. If you need to send artwork to another company and you do not know what software they will use to process your creation, use EPS or PDF.
  • Image manipulation programs like Adobe Photoshop can also save bitmap images as EPS-files.
  • Some printer drivers are capable of generating EPS-files as well as PostScript files.

How to view EPS files

Viewing placed EPS files

When you place an EPS file on a page in a layout or word processing application, that application needs to visualize the content of the EPS. It can do this in two different ways:
  • It can display the preview image that is embedded in the EPS file.
  • It can attempt to render the content of the EPS file and generate a preview image that is optimized for the current screen size and magnification. Only a few applications, such as Adobe InDesign, are capable of doing this. Since this operation can be fairly processor intensive, InDesign will only do this if the display quality selected by the user is set to High.

Viewing EPS thumbnails in operating systems

Seeing the content of an EPS can be a real hassle, both on PCs and on Macintosh.
  • EPS thumbnails in WindowsWhen an EPS-file is viewed in the thumbnail view of Windows Explorer, a generic icon is used. Below are for example 2 EPS-files viewed in Windows XP.

EPS-files in Windows XP
For other file formats such as JPG or PNG, Explorer shows a thumbnail of the actual image content. This can be very practical when dealing with large amounts of files. There is a little tool called PS+Ai Thumb which at least partly solves this problem. It works on some images but not on all types of EPS files, as shown below. I’ve only used it with Windows XP. It does not work on the 64-bit version of Windows 7.
EPS files in Explorer with installed
EPS files in Explorer with PS+Ai Thumb installed
The best solution is to use a more dedicated image browser or viewer. Below is what Adobe Bridge displays. Bridge is bundled with applications such as the Adobe Creative Suite or Photoshop.
EPS files in Adobe Bridge
EPS files in Adobe Bridge
  • EPS previews in Mac OS XIn Leopard the situation is a bit similar to that of Windows: viewing EPS thumbnails works fine in applications like Adobe Bridge but in the Finder or QuickLook you don’t see a preview. Fortunately there is an excellent plugin called EPSQLPlugIn which fixes this. It can be downloaded here.
    Apple eliminated this limitation in OS X 10.6. Below is how the above two icons look in the Finder in Snow Leopard.
Dinges
EPS files in OS X 10.6 Snow Leopard

How to convert an EPS file to another file format

An EPS file can be converted to PDF, TIFF, JPG, PNG or other graphic file formats.
  • If you are an Adobe Illustrator or Photoshop user, just open the EPS file and use SAVE AS to export to another file format. CorelDraw and other drawing applications can also be used to do the same.
  • If you do not have any of the above-mentioned applications, there are web sites that can convert an EPS to an image. Google and you shall find.
Keep in mind that converting an EPS that contains vector data to an image file format that only can contain bitmap data means you are converting to a file that is optimized to be used at one specific size. The page on bitmap versus vector graphics explains this in more detail.

How to reduce the file size of EPS files

Logos and other types of artwork are often saved as EPS files. It makes sense to try and keep the file size down. There are a number of things you can do to reduce the size of EPS data:
  • Often the preview image that is embedded in an EPS file makes up a large part of the EPS data. If you originally filled a landscape A4 or letter size canvas with a logo, the A4 or letter sized preview image can easily exceed half a megabyte. If the logo is a vector based drawing, there is no disadvantage in reducing its size and making it 10 centimeters or 4 inches wide. That may halve the file size of the EPS file.
  • An application like Adobe Illustrator saves additional information in its EPS output. When saving as an EPS file, you can select the file format. By selecting Illustrator 9 instead of Illustrator CS5, I shaved 200K off the file size of an EPS. Take into account that saving in an older file format may impact your ability to edit the EPS file afterward.
  • Get rid of unnecessary data: In its Action window, Adobe Illustrator has an option to delete unused palette items. This deletes unused color swatches, brushes, symbols, and styles. That may reduce the file size by an extra 200 to 400 K. Also make sure that there isn’t any irrelevant artwork hidden in a deactivated layer or the pasteboard area.
  • Try optimizing the design by simplifying paths or merging multiple paths into a single one.
  • If the EPS file needs to contain bitmap images, make sure to use the optimum resolution for these.
Additional information can be found on this excellent page.

More in-depth information on the EPS file format

An EPS file must conform to the Adobe Document Structuring Conventions (DSC). These are a set of rules that define how PostScript data should be organized.
At a minimum, it must include a header comment, %!PS-Adobe-3.0 EPSF-3.0, and a bounding box comment, %%BoundingBox: llx lly urx ury, that describes the bounds of the illustration. (The specification does not require the EPSF version, but many programs will reject a file that does not have it.)
The EPS program must not use operators that initialize or permanently change the state of the machine in a manner that cannot be undone by the enclosing application’s use of save and restore (e.g.. the operators starting with ‘init’ like initgraphics). As a special case, the EPS program may use the showpage operator. The importing application is responsible for disabling the normal effects of showpage. The EPS program should make no environment-sensitive decisions (the importing application may be trying to attain some special effect, and the EPS program shouldn’t screw this up), although it can use some device-dependent tricks to improve appearance such as a snap-to-pixel algorithm.
There are some operators that should not be used within an EPS file: banddevice, cleardictstack, copypage, erasepage, exitserver, framedevice, grestoreall, initclip, initgraphics, initmatrix, quit, renderbands, setglobal, setpagedevice, setshared and startjob. These also include operators from statusdict and userdict operators like legal, letter, a4, b5, etc. There are some operators that should be carefully used: nulldevice, setgstate, sethalftone, setmatrix, setscreen, settransfer, and undefinefont.
EPS files can be encoded using 7-bits (ASCII, like PostScript data usually are) as well as 8-bits (binary, which is virtually always done on Macintosh because it decreases the size of the file significantly). 8-bit EPS-files cannot be handled properly by all operating systems or applications.

The image preview

EPS files can optionally contain a bitmapped image preview so that systems that can’t render PostScript directly can at least display a crude representation of what the graphic will look like.
There are 4 preview formats:
  • PICT, mainly used in files generated on Macs. The PICT file is stored in the resource fork of the EPS file while the actual PostScript data are stored in the data fork. PICT is the default file format of QuickDraw, the graphics model that is used by MacOS 7/8/9 applications to generate the screen display.
  • TIFF: Most EPS files created by Windows applications contain a TIFF file for preview purposes.
  • Metafile: Some EPS files originating on PC contain a Windows Metafile preview. WMF or Windows Metafile is the PC equivalent of the Macintosh PICT file format.
  • EPSI which is an EPS file with a platform device independent preview. EPSI is an all ASCII (no binary data or headers) version of EPS. EPSI provides for a hexadecimal encoded preview representation of the image that will be displayed or printed. EPSI files were documented by Adobe as a means of providing a preview for EPS files which would be cross-platform. In reality though, DOS machines and Windows favor embedding TIFF or even Windows Metafiles in the PostScript. EPSI is mainly used on Unix systems.
It is also possible to have an EPS file without a preview, though. In this case, the imported file is usually displayed as a grayed out box or a box with diagonal lines running through it.
The preview image has a fixed resolution, which is usually 72 dpi. If you enlarge an EPS file in a document, the preview image is stretched and may become ‘blocky’ and lacking in detail. This does not necessarily mean that the EPS-data themselves will degrade in quality. As long as the EPS-file only contains text and vector graphics, scaling it does not affect its quality.
If you print a file containing an EPS-image on a non-PostScript printer, it is usually the preview image that gets printed. The preview image is ignored when you print to a PostScript device.

Remarks

Although an EPS file contains PostScript data, you cannot always send it straight to a printer to have it printed. Some interpreters cannot handle the preview data that may be included in the EPS file. Others don’t output the file because the ‘showpage’ operator is missing. It can also happen that the printer does process the job but outputs a blank page because the content of the EPS-file was located outside the printable area.
EPS-files can contain PostScript level 2 operators that make it impossible to output the file on an old PostScript level 1 device.

Specsheet

Name: EPS
Developer: Adobe
Release date: mid 80’s
Type of data: vector, bitmap & fonts
Number of colors: unlimited
Color spaces: ?
Compression algorithms: ?
Ideal use: information exchange between prepress applications
Extension on PC-platform: .EPS
Macintosh file type : EPSF (sometimes TEXT)
Special features: –
Remarks: manual available here

Additional sources of information

Wikipedia has an elaborate but fairly technical page on the Encapsulated PostScript file format.

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