Comp Sci - Lecture 4.docx

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Computer Science
Computer Science 1033A/B
Vivi Tryphonopolous

""'- Lecture4 ImageOptimization • Optimization Techniques 1. FileFormats:choose .jpg or .gif 2. Compression:compress the image data 3. Colour resolution: number of colours used FileFormat • GIFProperties Supports max of 8 bit colour scheme (good for clip art,not photos) Bestfor large areasof solid,flat colour Tiny file sizesin comparison with .jpg Summary:smaller file sizesbut only 8 bit colour • JPGProperties Storesfull colour information Supports max of 24 bits per pixel (1million colours} Bestfor blends of colour,softer shadow effects,subtle changesin colour Not for well defined lines or sharp contrast between colours Ddistorts them Summary:full colour scheme but larger file size Compression • Toreduce file sizeD choosea compression technique appropriate for the web • Roleof compression is to: Reducethe redundancy of image data Dgives ability to store or transmit data in an efficient form Compressasmuch asyou canwithout compromising quality CompressionTechniques • Lossless Compressthe original bits and bytes into lessbits and bytes without losing any of the image's original information When we reopen the file ,all original info is still there No information is lost Gifs Dlosslesscompression o Doesnot get blurry or messy o Usestechnique called "L2W" compression Dreduces file sizesof images by finding repeated patterns of pixels (ex:12341234 turns into 11) o Thiscompression maintains the image quality • Lossy When compression occurs,some of the original information is lost Discardsselected portions of image that are least missedor noticed Cannot get information back once it is discarded Quality degrades JPGsDlossycompression o Not all informatio n from the original image is preserved Dnot the same asthe original;discards data o Degradesimage quality each time you save o Compression is achieved by "forgetting" certain details about the image,which the JPGwill try to fill in later when being displayed o Human eye more sensitive to brightness details than to colour details 1. The quality of a JPGfile is proportional to the file size 2. A higher image quality setting has lower compression value,:. results in lessdata being distorted Colour Resolution • Colour resolution= bit depth Dithering • Definition :juxtaposing (placing side by side) pixels of two colours to create the illusion that a third colour is present (grainy look) • Can lower download time • Most common method of reducing colour range of images down to 256 (or fewer) colours seen in 8 bit .gif images • If an image uses colour hues outside of br
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