The image compression techniques are broadly classified into two categories depending whether or not an exact replica of the original image could be reconstructed using the compressed image .
- Lossless technique
- Lossy techniqhe
- Lossless compression technique
Lossless compression is compression techniques were there is no loss of data when the content is compressed. These methods are used by the web designer when the content to be compressed is very important and cannot be lost. This is an important consideration in web development and web marketing technqiues.
- Lossy compression technique
A compression technique that does not decompress data back to 100% of the original. Lossy methods provide high degrees of compression and result in very small compressed files, but there is a certain amount of loss when they are restored.
Audio, video and some imaging applications can tolerate loss, and in many cases, it may not be noticeable to the human ear or eye. In other cases, it may be noticeable, but not that critical to the application. The more tolerance for loss, the smaller the file can be compressed, and the faster the file can be transmitted over a network. Examples of lossy file formats are MP3, AAC, MPEG and JPEG.
Lossy compression is never used for business data and text, which demand a perfect “lossless” restoration.
Progressive/interlace displays are the two main categories for screens today. Put simply, progressive is better because it will minimize picture flickering.
Progressive displays re-draw every single horizontal line each cycle. For example, a 1080p (p for progressive) full HD television at 120 hz will re-draw every single one of its 1080 lines 120 times per second. Interlaced displays re-draw every other line each cycle. So, the same TV displaying 1080i (i for interlaced) will redraw 540 lines 120 times per second. This requires half the work of the progressive display, and was the original reason interlace was invented in the first place.
Back in the days of vacuum tube televisions, electronic processing was much slower than it is today. Re-drawing every single line would result in a jerky picture. Even though re-drawing every other line would result in some picture flickering, it was preferable to jerky motion. Modern displays easily have enough processing power to display smooth progressive picture.
Today, almost all new displays can display progressive output. Progressive displays can also display interlaced, while interlaced screens cannot display progressive. Thus, interlacing is an older technology which only exists in new equipment so it an be used with older equipment.
|Get Method||Post Method|
|Parameters remain in browser history because they are part of the URL.||Parameters are not saved in browser history.|
|Can be bookmarked.||Can not be bookmarked.|
|GET requests are re-executed but may not be re-submitted to server if the HTML is stored in the browser cache.||The browser usually alerts the user that data will need to be re-submitted.|
|application/x-www-form-urlencoded.||multipart/form-data or application/x-www-form-urlencoded Use multipart encoding for binary data.|
|can send but the parameter data is limited to what we can stuff into the request line (URL). Safest to use less than 2K of parameters, some servers handle up to 64K.||Can send parameters, including uploading files, to the server.|
|Easier to hack for script kiddies.||More difficult to hack.|
|Yes, only ASCII characters allowed.||No restrictions. Binary data is also allowed.|
|GET is less secure compared to POST because data sent is part of the URL. So it’s saved in browser history and server logs in plaintext.||POST is a little safer than GET because the parameters are not stored in browser history or in web server logs.|
|Yes, since form data is in the URL and URL length is restricted. A safe URL length limit is often 2048 characters but varies by browser and web server.||No restrictions.|
|GET method should not be used when sending passwords or other sensitive information.||POST method used when sending passwords or other sensitive information.|
|GET method is visible to everyone (it will be displayed in the browser’s address bar) and has limits on the amount of information to send.||POST method variables are not displayed in the URL.|
|Can be cached.||Not cached.|
|7607 character maximum size.||8 Mb max size for the POST method.|
The acronym XML stands for Extensible Markup Language which is a set of rules. XHTML stands for Extensible Hypertext Markup Language and is included in the XML markup language. HTML 4 was a new variation of HTML that developed into the first XHTML. This led to HTML 4 being written in the XML format for web pages.
XHTML contains two parts which are HTML 4 and XML. The HTML 4 piece is used to display text or documents that are used on different machines and platforms. The use of this language has grown to the display of multimedia, hypertext and document style on a web page. The XML piece was developed for the purpose of retaining the use of HTML by reducing the need for complex elements for web pages.
The use of XHTML is often found in markup tags that are included in an XHTML module that can then be used as any other type of HTML tag on a web page. This allows HTML to be displayed in a browser compatible with XHTML.