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Image Formats Explained

File Type: Raster Image, File Extension: .bmp

Bitmap files have some confusion associated with them. Some refer to any pixel-based image as a bitmap file. However, a true bitmap image file refers to the standard Windows image format. This type of file is mostly used on DOS- and Windows-based machines.

A bitmap file is a raster- (or pixel-) based format that only supports the RGB colour space and bit depths of 1, 4, 8, or 24 bits per channel. These attributes make bitmap images unsuitable for use in a high-end print production workflow.

Even though bitmap images are in the RGB colour space, they are not supported by any Web browsers or Web coding languages. Therefore, they are not suitable for use as images in a Web application. You would be able to use such a graphic in an HTML export by having the automatic conversion to a GIF or JPEG file occur. Bitmap images are best used for their intended purpose, as a system support on a PC Windows-based computer. Do not use the placement of a bitmap image when designing for a high-end print production job.

File Type: Raster Image, File Extension: .eps

An Encapsulated PostScript file is an image file is a variation of the EPS file that is generated using a multi-part format known as a Desktop Colour Separated (DCS) file.

The DCS file consists of a master channel and a separate high-resolution channel for each colour. Due to its pre-separated nature, these multi-part DCS files only work in a pre-separated workflow. Special provisions need to be made for the DCS file in a composite workflow.

DCS files are used in pre-separated workflows, and most often off a copydot scanning device.

File Type: Raster Image, File Extension: .gif

The Graphic Interchange Format (GIF) is a standard and recognised file format used in online and Web applications. GIF files can either be in greyscale or RGB colour spaces. This works to keep the GIF files used very small and quickly accepted in an HTML Web page.

Using a limited colour gamut that supports transparency creates GIF files. Transparency is the GIF file formats strong point. Using this attribute of the file format increases its appeal to Web designers. To further the uses of the file, pseudo animation can be created by using an animation feature found in flipbook style cartoons. This flipping of images can be used to create the illusion of true animation.

GIF files can only store and display 256 colours in the RGB or indexed colour gamut. In addition, the GIF file can be interlaced, where as every other row of information can be displayed at one time. As soon as that initial row of information is received, the next rows of data are downloaded. These attributes make GIF files unsuitable for use in a print production workflow, but highly desirable in a Web design situation.

File Type: Raster Image, File Extension: .jpg

Uses of the JPEG (Joint Photographic Experts Group) file format are far and wide. Initially, the file format was intended for use in Web applications, but has found a home in the high-end print production markets, as well. The JPEG file format can be your best friend if used properly, or your worst enemy if implemented incorrectly.

A JPEG file is encoded by using an adjustable compression approach. This means that to achieve smaller file sizes, image data is actually thrown away. In small doses, the JPEG compression approach can be very effective and efficient. However, in larger amounts, the resulting file will contain noise and undesired artefacts in the image. Be very careful when preparing JPEG files for use in a print production workflow.

The JPEG format will support the RGB, CMYK, and greyscale colour spaces. The use of JPEG images is supported in HTML and Web applications. However, unlike a GIF file, all of the colour information is stored in the file. There is no support for transparency in a JPEG file.

File Type: Raster Image, File Extension: .pcx

The PCX file is one of the original, pixel image file formats. It originated on the Windows platform, and is most commonly associated with the Windows Paint program. Today the file format is still commonly used in dome fax machines and some scanning applications.

A PCX file will support the use of RGB, indexed colour, and greyscale colour spaces. It does not support the CMYK colour space. The bit depths supported are:

1 for bitmap images

8 for greyscale and indexed colour

24 for RGB images

Using a PCX file for use in documents in a print production workflow is unacceptable because of the lack of support for the CMYK colour space. Additionally, using a PCX file in a Web-based application will not happen unless the image file is converted to a GIF or JPEG image.

Possible uses of the Run Length Encoded (RLE) compressed PCX image may be in a lower-end production environment to a low resolution or non-PostScript printer. This will most likely produce acceptable image quality.

File Type: Raster & Vector Image, File Extension: .psd

Adobe PhotoShop files are generally written and read by PhotoShop, however, there are several other applications that will read this format. Most page layout applications (except Adobe InDesign), do not allow native PhotoShop images to be placed.

A PhotoShop file will retain all of the original files attributes. Saved file characteristics include the following:

o Resolution

o Colour space (CMYK, RGB, greyscale)

o Spot colour channels

o Image bit depth

In order to use PhotoShop Files, you must first convert them to a TIFF.

File Type: Raster Image, File Extension: .pct

A PICT file is the default pixel image file format used on the Macintosh. The most common use of the file is for creating file icons and screen captures. The PICT file format will support the use of Run Length Encoding (RLE) or Joint Photographic Experts Group (JPEG) when QuickTime(tm) is loaded and operational on the computer.

The PICT file format will support a bit depth of up to 32 bits per channel, and operate in several colour spaces:



o Bitmap

o Greyscale

o Indexed Colour

Using a PICT file in a job for high-end print is not recommended. A PICT file is created in a device-dependent manner and does not support colour separations. In addition, Web browsers do not make provisions for the support of PICT files.

File Type: Raster Image, File Extension: .png

A Portable Network graphic (PNG) file is a pixel based image that can be used to display images on the Web (some older browsers may not support the use of this format). The start of the PNG file was in an alternative to the proprietary CompuServe GIF file format. It will support 24-bit colour, using a lossless compression approach.

The PNG file settings that can be controlled are:

o Transparency information

o File defined background colour

o Adobe Gamma correction

Use of a PNG file in a Web project is a good fit. However, because of the lack of support for the CMYK colour space, and the fact that there can be no colour separations, the PNG file is not a good fit in a print production cycle.

File Type: Raster Image, File Extension: .sct

The proprietary Scitex Continuous Tone (CT) files are generated from a Scitex workstation or from Adobe PhotoShop. These bitmap files contain all the information necessary to render images in a high-end pre-press workflow.

The Scitex CT file contains information using either the CMYK colour mode or greyscale. These files tend to be on the large side, and are not very transportable across slower networks. Incorporating the CT file into page layouts is typically achieved by using special plug-ins or a Scitex workstation.

Usually CT files are created at the printers location and can be used in an Automatic Picture Replacement (APR) workflow. HTML and Web browsers do not support these types of images.

File Type: Raster Image, File Extension: .tif

A Tagged Image File Format (TIFF) file is the most widely used file format in desktop publishing today. It is a raster-based file that supports the following:



o Greyscale

o Lab

o Indexed colour

TIFF files can be compressed by using an LZW lossless compression approach or JPEG lossy compression. For high-end print production, it is the best practice to use either LZW compression or a very small amount of JPEG compression. The JPEG compression approach is a lossy compression that will degrade image quality when used in large amounts.

When placing a TIFF file on a page, any clipping path that has been defined can be retained and applied. However, PhotoShop’s Alpha channel information will not be translated when placing a TIFF.

wmfFile Type: Raster Image, File Extension: .wmf

The Windows Meta File (WMF) format is used specifically on the PC platform. WMF files can contain either pixel-based information or vector information in 16-bit RGB colour space.

Meta files, as they are sometimes referred to, generally can not be placed into a page layout. The most common use of WMF files will be in low-end desktop printers, but never in a commercial (high-end print) workflow.

The reasons for not using these files in a high-end workflow are numerous:

o No support for colour separations

o Limitation of 16-bit colour

o Use of only the RGB colour space.



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