Digital Image Formats: An Explanation Guilford County SciVis V106.01

An Explanation of Digital Formats An explanation  Why so many file formats?  How are they used?  One reason is compression, lossy or lossless.  Another reason is the number of colors to be used.

An Explanation of Digital Formats Lossy vs. lossless compression  Lossy compression loses some part of the file.  Lossless compression does not lose any part of the compressed file. LZW is an example of a lossless compression algorithm. Lossy compression will be missing part of the file to save space.

An Explanation of Digital Formats Number of colors  Images have differing numbers of color within them.  Black and white images take up the least amount of space with 1 bit of information.  Current video cards are able to display 24 bits of color per pixel.  This makes it possible for 16 million colors to be displayed (2 24 colors).  This requires a significant amount of disk space.

An Explanation of Digital Formats The file types - TIFF  TIFF can be lossless or lossy.  The details of the image storage algorithm are included as part of the file.  TIFF is used almost exclusively as a lossless image storage format that uses no compression at all.  Most graphics programs that use TIFF do not use compression.  File sizes are quite big. (Sometimes LZW is used, but it is not universally supported.)

An Explanation of Digital Formats The file types - PNG  PNG is a lossless storage format.  It looks for patterns in the image that it can use to compress file size.  The compression is exactly reversible, so the image is recovered exactly.

An Explanation of Digital Formats The file types - GIF  GIF creates a table of up to 256 colors from a pool of 16 million.  If the image has fewer than 256 colors, GIF can render the image exactly.  GIF is "lossless" only for images with 256 colors or less. For a rich, true color image, GIF may "lose" % of the colors.

An Explanation of Digital Formats The file types - JPG  JPG is optimized for photographs and similar continuous tone images that contain many, many colors.  It can achieve astounding compression ratios even while maintaining very high image quality.  It stores information as 24 bit color.  The degree of compression of a JPG is adjustable.

An Explanation of Digital Formats The file types - BMP  BMP is an uncompressed proprietary format invented by Microsoft.  There is really no reason to ever use this format.

An Explanation of Digital Formats The file types - PSD  PSD is a proprietary format used by Photoshop.  This is the preferred working format to edit images in the software  This package uses layers to build complex images, and layer information may be lost in the nonproprietary formats such as TIFF and JPG.  However, it is best to save the product as a TIFF or JPG, so it can be viewed in the future when software changes.

An Explanation of Digital Formats The file types - Summary  TIFF files are generally large.  PNG compression can be reversed if needed.  GIF results in lose of color information.  JPG are the best in terms of compressed files.  BMP should be rarely, if ever used.  PSD is exclusive to Photoshop.

An Explanation of Digital Formats File size comparisons  TIFF (901 kb) ormats/uncompr.tif ormats/uncompr.tif  JPG (301 kb) ormats/HiQ.jpg ormats/HiQ.jpg  PNG (741 kb) ormats/png.png ormats/png.png  GIF (131 kb) ormats/optmedcut.gif ormats/optmedcut.gif

An Explanation of Digital Formats When should you choose each?  TIFF – This is usually the best quality output from a digital camera.  JPG – This is the format of choice for nearly all photographs on the web.  GIF - For images with fewer than 256 colors and large areas of uniform color like cartoons. Do NOT use GIF for photographic images  PNG - For images with large areas of exactly uniform color, but more than 256 colors Also, for display of photographs exactly without loss of quality on the web.

An Explanation of Digital Formats The end

An Explanation of Digital Formats Reference  Matthews, R. (2007). Digital image file types. Retrieved June 21, 2007 from phics/formats/formats.html.