Image Acquisition and Handling- Role of Images in Web Documents
- Images add color, interest and content to html documents. It is important, however, to consider the client when deciding upon the format for incorporating images. Remember that images may alter the way text appears on different browsers, they may increase the time a page requires to load, and they may not be useful if the client does not have a high resolution monitor capable of viewing millions of colors.
- Types of Images
Images may be in-line (as is the Aggie Horticulture logo at right) or linked. In-line images display within the document, but a link has to be activated by being clicked-upon in order for the image to load.
- Image Acquisition
- Digital images acquired from a digital still or video camera, the digitized input of a video camera, scanned slides, or scanned photos can be saved in a format that will display in a Web browser such as Netscape. Image quality varies markedly with the input source, quality of the original image, and post-digitization compression and file manipulation procedures. Outstanding quality photos or slides can be scanned into outstanding quality digital images; well composed, well lighted video still or motion images can be saved into outstanding images as well. Under- or over-exposed photos or slides, with poor color balance require the touch of a master imager to correct even slightly. Improper techniques, poorly adjusted equipment, or improper file manipulation can ruin outstanding quality images!
- Common Image File Formats
- The two most common image file formats used in html documents are the Compuserve GIF format and the JPEG format(Joint Photographic Experts Group). The GIF format is proprietary to Compuserve, and may become subject to licensing. The JPEG format is rapidly becoming the standard on the Web. Some of the earlier versions of Mosaic and Netscape required a JPEG viewer/helper application, but the later versions display JPEGs directly from the browser.
- There's Still Reason to Use GIFs
- Some nifty features still are oriented to GIFs. Included are the ability to make a background or color within the file transparent, and the ability to interlace images as they load.
- More about GIFs
- The file size savings in GIFs depends upon changing the colors in the image into defined or "indexed colors." Colors are defined by resolution depth (24 bit colors are richer than 8 bit colors). An image is acquired into a scanner or video capture program (usually) as RGB color. To save the image as a GIF, the image is converted to indexed color (defined by bit depth, or number of colors; a prescribed palette; and presence or absence of dithering). GIF images can have excellent color and clarity. The Yale AIM Gallery image of the beetle is only 81 kb as a 256 color GIF, but the same file is 457 kb as an RGB color file in PhotoShop 3.0.
- JPEG Images Work Well, Too!
- JPEG images are compressed RGB color images. The degree of compression can be specified in PhotoShop, for example, as low, medium, high, or maximum quality, and the file sizes increase accordingly. The image of the beetle from the Yale Center for Advanced Instructional Media Gallery is a 54 kb file as a JPEG, but as RGB color in Adobe PhotoShop it is a whopping 457 kb file!
WebMasters was created and is taught by Dr. Dan Lineberger, WebMaster of Aggie Horticulture, Department of Horticultural Sciences, Texas A&M University, College Station, Texas 77843.
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