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Web Graphics

Web Graphics. Image File Formats Image optimization Accessibility issues Using images and colors on the web. Images in the Web pages. Images – essential part of Web design Illustrate content Convey the message Company identity and branding Used as navigational elements

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Web Graphics

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  1. Web Graphics Image File Formats Image optimization Accessibility issues Using images and colors on the web

  2. Images in the Web pages • Images – essential part of Web design • Illustrate content • Convey the message • Company identity and branding • Used as navigational elements • Two types of graphic formats: • Vector (resolution independent) – require plug-ins • Raster (or bitmap) images resamble a series of pixels • GIF, PNG, TIFF, JPEG

  3. Web Images Formats - JPEG • Joint Photographic Experts Groups - JPG (.jpg) • Contains up to 24-bits of color information -Supports 16.7 million colors) • Typically used for photographs and complex graphics • Lossy compression method • The more an image is compressed the more its quality is reduced

  4. Lossy Compression example (.JPG) Uncompressed image (left) – 43.6KB; high compression (right) - 5.01 KB

  5. Web Image formats - GIF • Graphics Interchange Format • Supports 256 colors • Best suited by line art, custom drawings, clip art, small images, such as bullets, buttons, icons. and etc. • Supports transparency • Supports interlacing • Supports Animation • Provides lossless compression – • As the image is compressed , no information is lost

  6. Lossless Compression – (.GIF) Lossless compression reduces the file size without degrading the visual quality. The checkered background represents transparency

  7. Interlacing (GIF and PNG) • The ability for an image to render gradually as it downloads. • Interlaced images give the client something to look at rather than blank space • GIF file format supports animation

  8. Portable network graphics PNG • Developed using open standards (PNG -8). Similar to GIF • Supports 256 colors (PNG-8) • Supports transparency • Interlacing • Lossless Compression • PNG Transparency is not supported by earlier versions of browsers (IE6 and earlier)

  9. Web Images and Accessibility • Using the alt attribute with images • Alt attribute specifies alternative text to appear while the graphic is loading • Used in place of the graphic in non-graphical browsers • Displays the alternative text if the image fails to load or images are disabled in the web browser Syntax: <imgsrc=“download.gif” alt=“download .pdf form” />

  10. Placing images Aligning images relative to text using deprecated align attribute: <img src =“pic.gif” alt=“twitter” align=“left” /> <img src =“pic.gif” alt=“twitter” align=“right” />

  11. Placing images using CSS Align attribute is deprecated. • Use CSS float property instead: <imgsrc=“pic.gif” alt=“ABC corp. logo” style=“float:left”/> • CSS rule: img{ float:right; } • To center an image, enclose the image inside a <div> element which can be aligned to the center: <div style=“text-align:center"><imgsrc=“logo.gif" alt=“SCF logo" /></div>

  12. Accessibility issues with images and colors W3C Web Content Accessibility Guidelines (WCAG) – provides a checklist for the accessibility guidelines • Provide equivalent alternative to visual content (alt property) • Do not rely on color alone

  13. Accessibility issues with images and colors • Section 508 of the Rehabilitation Act (U.S. Government Act) – electronic and IT accessibility standard • Based on W3C WCAG • All electronic and IT developed, procured, maintained or used by federal agencies should be comparably accessible to users with disabilities. • Examples • All non-text elements must have a text-based equivalent • Information must be equally available in color and without color

  14. Setting colors • Provide a good contrast between text and background • For no- and low-vision users who cannot see color, cues based on color alone are meaningless. Also, many people have various levels of color blindness; what may appear as red to some, appears as gray to others. • Don’ts: “fill out the fields marked in red”

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