Images ET Sum 02 June

Images DPI - Dots per inch (72dpi is screen dpi) Windows vs Macintosh issues 1.Images are darker on Windows (because of the gamma difference) 2.Images appear larger on the Mac 3.Different standard screen resolutions

The Problem of Representing Video Digitally Break a picture into discrete units. The discrete units are called pixels. For instance the image below is broken into 640 columns and 480 rows. That’s 640x480 = 307,200 pixels in the picture

GIF Extension.gif At most 256 colors (8 bit) LZW Compression (loss less) Good for flat color images with blocks of similar color Transparency GIF Animation

Jpeg File Extension.jpg or.jpeg True color (24-bit) JPEG Compression (lossy) Good for Photorealistic images No Animation, transparency

PNG File extension.png Multiple levels of color support –256 colors (8-bit) with a palette –65,000 level grey scale (16-bit) –True color (48-bit) PKZIP compression (loss less) Text storage (for keywords, comments, copyright, etc.) Transparency (256 levels for 8-bit images, alpha channels for true color images)

Comparison Table GIFJPGPNG No. of Colors256Unlimited AnimationYesNo Loss Less CompressionYesNoYes TransparencyYesNoYes Can Contain CommentsYesNoYes CompressionLZWJPEGPkzip with prefiltering Alpha channelsNo Yes Gamma correctionNo Yes

GIFJPGPNG Browser support All browsersAll current browsers Netscape with a plugin and direct support in IE4 and Netscape 6. Use for images with... Flat colors or animation Photorealistic images Loss-less full color, transparent material Don't use for images with... Photorealistic images Sharp contrast, lines, text Large full color photographic images (because of large size).

Gif 16 colors 15KBJpeg low resolution 11KB

Gif 128 colors 36KBJpeg High, 26KB

Gif, 2KBJpeg Medium resolution, 15KB