Web Graphics

Colours and Display Graphics and Modems 28.8kbps = 3.6 KB per second 36 KB takes 10 seconds to load. Pixels and color

Pixels and colour (continued) One bit black and white display

Pixels and colour (continued) –8-bit or 256 colour displays

Pixels and colour (continued) –24-bit “true colour” displays

Pixels and colour (continued) 256 colours - “palette” “index” or CLUT (colour look up table” True-colour or 24-bit images - each bit has 24 bits dedicated to it, usually in three layers.

Graphic file format GIF files – Graphic Interchange format. 256 colours GIF File compression Lempel Zev Welch LZW - lossless Squeezes out inefficiencies in data storage eg large areas of same colour.

Non-interlaced GIF –downloads one line of pixels at a time Interlaced –stored in a format that allows browsers to begin with low resolution

JPEG Joint Photographic Experts Group 24 bit or true colour used when colour fidelity is paramount degree of compression=degree of quality JPEG discards “unnecessary” data - lossy image technique

Delete sample document icons and replace with working document icons as follows: From Insert Menu, select Object... Click “Create from File” Locate File name in “File” box Make sure “Display as Icon” is checked Click OK Select icon From Slide Show Menu, Select “Action Settings” Click “Object Action” and select “Edit” Click OK

JIF JPG Medium quality JPG Low quality

Uses for GIF and JPEG –JPEG - complex photographic illustrations etc where compression does not degrade image quality. –Advantages of GIF All graphics Web viewers support the GIF format GIFs of diagrammatic images look better GIF supports transparency and interlacing Delete sample document icons and replace with working document icons as follows: From Insert Menu, select Object... Click “Create from File” Locate File name in “File” box Make sure “Display as Icon” is checked Click OK Select icon From Slide Show Menu, Select “Action Settings” Click “Object Action” and select “Edit” Click OK

huge compression ratios for faster downloads Gives excellent results for photographs and medical images Supports full-colour images (24-bit “true colour” images) Advantages of JPEG images

–Dithering Full colour 24 bit graphic is converted to a black and white. Dithering enables pixels of varying shades to be used to give the impression of fine detail. It can also be used in colour graphics. Dithering is very useful when more colours are needed than are available. Delete sample document icons and replace with working document icons as follows: From Insert Menu, select Object... Click “Create from File” Locate File name in “File” box Make sure “Display as Icon” is checked Click OK Select icon From Slide Show Menu, Select “Action Settings” Click “Object Action” and select “Edit” Click OK

Questions What is one major consideration when displaying graphics on a web page? How is colour typically displayed on a computer? Describe one advantage of GIF. Describe one advantage of JPEG. Why is dithering used?