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Graphics file formats - which format to use?
Common questions are "How do I make my graphics files smaller?, and "How do I make my graphics look great?" The following attempts to explain why the wonderfully small JPG files are NOT the best choice to be the master copy of your image and why JPG files are great for web pages and e-mailing. For line art and graphic files (as opposed to photographic images), GIF files have historically been best, both for smallest size and for best quality. Briefly, the three most common graphic file formats, the most important for general purposes today, are TIF, JPG and GIF. I also propose we consider BMP and the new PNG format.

Choosing a File Format for Web Graphics.

Currently, the two principal file formats used for Web graphics are JPEG and GIF. Determining which file format to use for your Web image depends on the characteristics of the image you are saving or converting. Following are some guidelines for choosing between JPEG and GIF when creating images for the Web.

JPEG

Generally speaking, JPEG is superior to GIF for storing full-color or grayscale images of "realistic" scenes, or images with continuous variation in color. For example, use JPEG for scanned photographs and naturalistic artwork with highlights, shaded areas, and shadows. The more complex and subtly rendered the image is, the more likely it is that the image should be converted to JPEG. The JPEG file format supports millions of colors.

Do not use JPEG for illustrations, cartoons, lettering, or any images that have very sharp edges (e.g., a row of black pixels adjacent to a row of white pixels). Sharp edges in images tend to blur in JPEG unless you use only a small amount of compression when converting your image. Such images are better saved as GIFs. Figure A shows an example of how such an image looks as a JPEG file and as a GIF file. Notice that the GIF file is clearer than the JPEG file.

GIF

Use the GIF file format for images with only a few distinct colors, such as illustrations, cartoons, and images with blocks of color, such as icons, buttons, and horizontal rules.

GIF, like JPEG, is a "lossy" file format. It reduces an image's file size by removing bits of color information during the conversion process. The GIF format supports 256 colors or less. When creating images for the Web, be aware that only 216 colors are shared between Macintosh and Windows monitors. These colors, called the "Web palette," should be used when creating GIFs for the Web because colors that are not in this palette display differently on Macintosh and Windows monitors.

Choosing a File Format for Master Copy.

  Photographic images

Graphics, Logos, Line art and ScreenCaptures

Properties of Master Copy Continuous tones, 24 bit color or 8 bit Gray, no text, few lines and edges Solid colors, up to 256 colors, with text or lines and sharp edges
Best Quality for Master Copy TIF or PNG or BMP PNG or GIF or TIF
Smallest File Size Master Copy (*) JPG, 75% to 80% Quality factor is good. Normal useful range is 60% to 90% PNG or GIF, maybe TIF LZW. Graphics/logos usually permit 2 to 16 colors for smallest file
Maximum Compatibility (PC, Mac, Unix) TIF without LZW TIF without LZW

(*) But notice that scanning at reasonable values of 100 to 150 dpi produces much smaller image files than does scanning at 300 dpi. Lowering scan resolution to reasonable values is often the best size improvement you can make.


You can convert images to jpeg, prepare photos for the web with ReaJPEG - batch photo editing software and converter.
   
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