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Understanding image types

From: Dreamweaver CS5 Essential Training

Video: Understanding image types

When creating images for the Web, it is important to understand which types of images you can use online and when it's appropriate to use one over the other. Determining which file type to use is usually a decision made after considering file size and functionality. For online graphics, your goal is always going to be obtaining the best image quality in the smallest file size possible. Understanding how file types are compressed and best practices for creating them will help you keep your image quality high or keeping file size down.

Understanding image types

When creating images for the Web, it is important to understand which types of images you can use online and when it's appropriate to use one over the other. Determining which file type to use is usually a decision made after considering file size and functionality. For online graphics, your goal is always going to be obtaining the best image quality in the smallest file size possible. Understanding how file types are compressed and best practices for creating them will help you keep your image quality high or keeping file size down.

Before we discuss how Dreamweaver manages images, let's first examine the three file types you'll be using when creating your Web graphics: JPGs, GIFs and PNGs. JPG is short for Joint Photographic Experts Group and is a standard that has been around in one form or another since about 1992. The JPG file format allows you to create images with millions of colors, which makes it an ideal choice for photographs or images with subtle color differences. The downside to using JPGs is that, when saved, a lossy compression is used to save file size, resulting in image data being thrown away.

Although, you can choose the amount of compression when saving a JPG, data is discarded each and every time it is saved. For that reason, it's a good idea to save your original image in a file format other than JPG and simply overwrite the original JPG when a change is made to the source graphic. Also, JPGs do not support transparency or animation, which may make them unsuitable for certain uses. The GIF, short for Graphics Interchange Format, file format was first introduced by CompuServe in 1987 and quickly became a staple for online graphics.

Unlike the 24-bit JPG format, GIFs are 8- bits, which limits the color palette of GIF files to a maximum of 256 colors. Now GIFs do have the advantage of using a lossless compression format, which reduces file size without damaging image quality. GIFs also support animation and transparency, making them the favorite file format of icons and dancing babies worldwide. Due to their limited palette, GIFs are not well suited for images that have subtle tonal shifts or a wide range of color.

They are however, ideal for smaller thumbnails or images that have large areas of solid colors. PNG stands for Portable Network Graphics and owes its existence to the very curious case of Unisys, holding the patent for the compression algorithm used in GIFs in 1995. PNGs were designed to replace GIFs and share many of the GIF file format strings, but they have added functionality. PNGs, like GIFs, support transparency and use lossless compression to achieve smaller file sizes.

Unlike GIFs however, PNGs files are 24- bit graphics, which allow them to display millions of colors and subtle tonal shifts without using dithering or banding. So which file format should you use? Well, honestly it depends on each individual image and how that image is going to be used within your files. As a general rule, JPGs are best for large images and photographs, specifically images that have lots of color and detail. High compression settings can cause serious image degradation, so it's best to compare image quality and compression for each image before deciding on an export setting for your JPG.

As a best practice, remember to keep a source image for your JPG so that you're not editing and overwriting the JPG itself. JPG image quality degrades each time it saved, and this could result in an unusable image over time if saved over and over itself. GIFs are great for thumbnails, small icons or images that contain large areas of solid colors. They're also a standard image type for any image needing transparency or limited animation. Now keep in mind that GIFs do have a limited palette, so complex images with lots of color or tonal shifts like gradients, can suffer when saved as GIFs.

PNGs also have all the advantages of GIFs, without the liabilities. PNGs feature full 24-bit support, so millions of colors can be saved along with transparency. The downside to using PNGs is they tend, on average, to be a little bit bigger than GIFs and JPGs, so heavy use of them throughout your site tends to add to the site's overall weight. Older browsers support was spotty at best. So for Legacy sites or sites that target older browsers, PNGs files are not a good choice. Also, Internet Explorer 6 did not support transparency for PNGs, resulting in having to write some pretty fancy CSS to force PNG transparency support.

As a whole, PNG usage is rapidly increasing on the Web, but if used, the file format is usually used to supplement graphics on sites or to do things that JPGs and GIFs just can't. In the end, the file format you use for Web graphics is totally up to you. Knowing more about the file types and their capabilities will help you make an informed choice.

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This video is part of

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Dreamweaver CS5 Essential Training

135 video lessons · 89129 viewers

James Williamson
Author

 
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  1. 2m 57s
    1. Welcome
      1m 8s
    2. Using the exercise files
      1m 49s
  2. 7m 50s
    1. What is Dreamweaver?
      3m 16s
    2. Learning web design
      2m 22s
    3. Current web standards
      2m 12s
  3. 43m 9s
    1. The Welcome screen
      4m 5s
    2. Windows and Mac interface differences
      2m 23s
    3. The Application toolbar
      4m 7s
    4. The Document toolbar
      4m 40s
    5. Arranging panels
      8m 19s
    6. Managing workspaces
      7m 32s
    7. The Properties Inspector
      5m 54s
    8. The Insert panel
      6m 9s
  4. 25m 45s
    1. Basic site structure
      3m 11s
    2. File naming conventions
      1m 49s
    3. Defining a new site
      4m 35s
    4. Managing sites
      4m 51s
    5. Managing files and folders
      6m 36s
    6. Working with browsers
      4m 43s
  5. 27m 21s
    1. Creating new documents
      5m 16s
    2. New document preferences
      3m 6s
    3. Setting accessibility preferences
      4m 56s
    4. Working with starter pages
      3m 46s
    5. Managing starter pages
      10m 17s
  6. 30m 2s
    1. Basic tag structure
      2m 15s
    2. Adding structure to text
      8m 20s
    3. Creating lists
      9m 59s
    4. Getting text into Dreamweaver
      5m 59s
    5. Importing Word documents
      3m 29s
  7. 1h 17m
    1. Understanding style sheets
      2m 16s
    2. The anatomy of a CSS rule
      1m 48s
    3. Setting CSS preferences
      6m 36s
    4. The CSS Styles panel
      10m 2s
    5. Controlling CSS through the Properties Inspector
      5m 14s
    6. Using the Code Navigator
      7m 21s
    7. Using CSS Enable
      6m 45s
    8. Understanding element selectors
      8m 11s
    9. Understanding class selectors
      8m 49s
    10. Understanding ID selectors
      5m 59s
    11. Understanding descendant selectors
      6m 51s
    12. Attaching external style sheets
      7m 44s
  8. 1h 47m
    1. Working with units of measurement
      7m 11s
    2. Declaring font families
      9m 39s
    3. Controlling font sizing
      9m 9s
    4. Controlling weight and style
      8m 0s
    5. Controlling line height
      8m 29s
    6. Controlling vertical spacing with margins
      12m 3s
    7. Controlling spacing with padding
      5m 39s
    8. Aligning text
      8m 26s
    9. Transforming text
      5m 36s
    10. Writing global styles
      15m 42s
    11. Writing targeted styles
      17m 37s
  9. 1h 32m
    1. Understanding image types
      5m 3s
    2. Managing assets in Dreamweaver
      12m 51s
    3. Setting image accessibility preferences
      4m 20s
    4. Setting external image editing preferences
      3m 52s
    5. Placing images on the page
      7m 37s
    6. Photoshop integration
      5m 54s
    7. Modifying Smart Objects
      5m 51s
    8. Alternate Photoshop workflows
      8m 8s
    9. Modifying image properties
      11m 14s
    10. Styling images with CSS
      7m 11s
    11. Using background graphics
      9m 3s
    12. Positioning background graphics
      11m 6s
  10. 55m 16s
    1. Link basics
      3m 37s
    2. Setting site linking preferences
      2m 14s
    3. Creating links in Dreamweaver
      11m 1s
    4. Absolute links
      5m 8s
    5. Using named anchors
      11m 19s
    6. Linking to named anchors in external files
      2m 44s
    7. Creating an email link
      5m 24s
    8. Creating CSS-based rollovers
      13m 49s
  11. 1h 34m
    1. CSS structuring basics
      2m 56s
    2. The Box Model
      13m 21s
    3. Understanding floats
      6m 53s
    4. Clearing and containing floats
      8m 56s
    5. Using relative positioning
      4m 8s
    6. Using absolute positioning
      7m 18s
    7. Creating structure with div tags
      12m 7s
    8. Styling basic structure
      10m 34s
    9. Creating a two-column layout
      10m 37s
    10. Using Live View and CSS Inspect
      7m 51s
    11. Using Browser Lab
      9m 39s
  12. 56m 22s
    1. Reviewing table structure
      7m 41s
    2. Importing tabular data
      5m 13s
    3. Creating accessible tables
      9m 56s
    4. Using thead and tbody tags
      4m 0s
    5. Basic table styling
      8m 45s
    6. Styling table headers
      7m 52s
    7. Styling column groups
      4m 22s
    8. Creating custom table borders
      5m 1s
    9. Styling table captions
      3m 32s
  13. 1h 43m
    1. How forms work
      3m 0s
    2. Reviewing form design
      3m 2s
    3. Creating accessible forms
      7m 33s
    4. Setting form properties
      4m 6s
    5. The fieldset and legend tags
      4m 32s
    6. Inserting text fields
      5m 58s
    7. Inserting list menu items
      5m 26s
    8. Inserting checkboxes
      7m 50s
    9. Inserting radio button groups
      6m 22s
    10. Inserting text areas
      4m 12s
    11. Inserting submit buttons
      3m 37s
    12. Basic form styling
      12m 0s
    13. Form element styling
      8m 52s
    14. Styling form layout
      11m 49s
    15. Adding form interactivity
      2m 47s
    16. Using Spry validation widgets
      12m 49s
  14. 1h 23m
    1. Planning for templates
      10m 51s
    2. Creating a new template
      10m 37s
    3. Using editable attributes
      13m 43s
    4. Creating optional regions
      6m 23s
    5. Creating new pages from a template
      9m 17s
    6. Applying templates to existing pages
      6m 9s
    7. Working with nested templates
      7m 56s
    8. Working with repeating regions
      12m 58s
    9. Modifying templates
      5m 41s
  15. 40m 14s
    1. Behaviors overview
      3m 47s
    2. Hiding and showing elements
      9m 18s
    3. Spry overview
      4m 4s
    4. Using Spry widgets
      11m 36s
    5. Adding Spry effects
      3m 6s
    6. Using the Widget Browser
      8m 23s
  16. 28m 18s
    1. Inserting Flash files
      5m 4s
    2. Setting properties for Flash
      6m 27s
    3. Dreamweaver and Flash integration
      6m 6s
    4. Encoding Flash video
      6m 10s
    5. Adding Flash video
      4m 31s
  17. 45m 28s
    1. Running site-wide reports
      6m 33s
    2. Checking for broken links
      5m 41s
    3. Checking for browser compatibility
      8m 3s
    4. Adding remote servers
      8m 0s
    5. Uploading files
      7m 20s
    6. Managing remote sites
      9m 51s
  18. 34s
    1. Goodbye
      34s

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