Wednesday, April 20, 2011

common file formats

There are hundreds of image file types, way more than I will address here. I want to focus on the common formats that are used everyday for web and print work for raster files.
Referring page: http://en.wikipedia.org/wiki/Image_file_formats

JPG, GIF and PNG are the formats most often used to display images on the web; tiff is used when saving image files or placing them into page layout programs, and pdf is often used when a document is placed on a website for viewing or downloading.

jpg
JPEG stands for Joint Photographic Experts Group, and it is a method of compression. JPEG was developed to display the least visible amount of image quality loss while maintaining the greatest visible quality of the image. It is adjustable so you can balance the amount of compression (file size) against the quality of the resulting image. Each time you save over a jpg, however, it loses a little more, so it’s best restricted to the final save.

gif
GIF, for Graphics Interchange Format, is suitable for web graphics with few colors or when transparency is needed. The color space for gifs is ‘indexed color’ and is limited to 256 colors. GIFs are widely used for animation, but isn’t good for photographs.

png
PNG, or Portable Network Graphics format, supports 16 million colors. It is a ‘lossless’ format, so the file will be larger than the same image saved as a jpg. PNGs can store an optional alpha channel (commonly used for silhouetting images or selecting part of an image.) It works well on the web.

pdf
PDF is the extension for Portable Document Format, created by Adobe Systems in 1993. It is basically a graphical representation of a document, including fonts and images. It’s a great way to share documents you have created without the user having to be able to open the original, requiring that they have the same program and the same (versions of) fonts that you used to create it. It ensures every viewer sees the same thing, but still allows selection of text and ability to add comments, which a plain image (like exporting your doc as a jpg) would not.

bmp
BMP (bitmap) is a Windows format; they are uncompressed, resulting in larger files than other formats, and therefore are not a preferred choice for the web. BMPs have wide acceptance in Windows programs.

tiff
TIFF, for Tagged Image File Format is a format rarely supported by browsers, but most often used by designers and printers to save images without any loss of information or alpha channels. (TIFFs may also be compressed if you want a smaller file.) Optical Character Recognition software commonly generate TIFF images for scanned text.

To convert a file from one format or another, you need a program such as Adobe® PhotoShop® or other image editor.

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