Monday, November 24, 2008

Image File Formats

When you are working in a program such as PowerPoint and decide that you wish to insert an image into one of your slides, often you do not consider the file format in which the image has been saved. However, there are a number of different formats in which the files might have been saved, and this entry will provide a brief introduction to each. Information presented here is from the wonderful book “How to Wow with PowerPoint.” (1)

Windows Metafile: This is a proprietary format designed by Microsoft which can be used across all their platforms: Word, Excel, PowerPoint Access, FrontPage, etc. You will see such images saved as either .wmf files (16-bit information) or .emf files (enhanced metafiles, 32-bit information). Bit here refers to the amount of gray the image supports. In the case of 16-bit, that really means 2 raised to the 16th power; and for 32-bit, 2 raised to the 32nd power.

Windows Bitmap: This is generally used across Windows, and is compatible with most applications used by Windows-based computers. The problem with this format (which will be saved as .bmp, .dib, or .rle files) is that it is not compressed, meaning that if you insert such an image into, say, PowerPoint it will cause a delay in loading, projecting or advancing a slide. For large images, it is better to use a compressed format (such as a JPEG).

Computer Graphics Metafile: CGM files are used more in technical fields such as engineering, and in the military, but is not commonly used on the internet.

Graphics Interchange Format: Known as .gif files, this was originally designed for use by CompuServe, but remains in common use. Its files are limited to 256 colors, which makes their use for reproducing photographs problematic; you are exchanging file size for clarity. GIFs are typically used for embedded transparency or basic animations.

Joint Photographic Experts Group: These are known as JPEGs, and are used predominantly for formatting large photos for use in multimedia presentations, such as a slide show. This is best suited for images with many colors, but one should know that the manner in which it is processed allows data contained in the image to eliminated or deleted, so that as it is re-used it will begin to degrade.

Portable Network Graphics: This is a format that attempts to combine the best elements of JPEGs and GIF files. It has a 24-bit image and also will allow for transparency of images (which people often use to place a logo on a master slide).Older web browsers will not support this, but it is effective in presentation technology. Files are saved as .png.

Macintosh PICT: Before Mac had its OS-X (in all its large cat versions), this was the original graphic element, saved as .pct. It is becoming less common now, but may be seen when working with older programs that have embedded graphics.

Tagged Image File Format: This is the .tif file. This is used for high-quality printing projects, though such files, because of their high image quality, typically are very large. Fortunately, they can be compressed in order to reduce file size.

Vector Markup Language: This is a relatively new language that is being advanced by Microsoft and by the W3C, which is the group who sets standards for the Web. All .vml files are supported by the newest web browsers and are based on an XML exchange (a means to share information across many platforms).

This is just an overview of something we rarely think about, but which can affect the effectiveness of our multimedia presentations. There are means within programs such as PowerPoint to convert one file format into others, if ever problems exist in projecting a graphic. The Clip Organizer can do when necessary.

Please, everyone, have a wonderful Thanksgiving break.


1. Harrington R, Reckdall S. How to Wow with PowerPoint. Berkeley, CA; Peachpit Press, 2007

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