Working with Images
Quick Reference Guide

 

Why do we need to work with images?

How is working with images different from working with text?

Two Basic Image Types:
1) Vector Images
(Draw-type images, or Object-oriented images)
Pictures that are made of mathematical descriptions of objects.
Advantages:
- small file size
- very scalable (you can enlarge it without loss in quality)
Disadvantages:
- Difficult to create something that looks real
- Cannot apply special effects
2) Raster Images (Paint images, or bitmapped images)
Pictures that are made of little colored dots (pixels)
Advantages:
- Easy to create realistic images (photos and illustrations)
- Special effects can be applied by moving individual pixels around (this is done using an image-editing program)
Disadvantages:
- Large file size
- If you try to enlarge it, the quality decreases quickly

What is Bit Depth?
With Raster images, each pixel contains specific information on it's color. Each pixel can be only one color, but many pixels can be blended together to simulate lots of colors.

 Bit Depth  Terms  Colors in Image
 1-bit  Line Art  2 (black and white)
 8-bit  Grayscale or Indexed Color  256 (grays or indexed color)
 16-bit  Thousands of Colors  32,000 colors
 24-bit  RGB or Full Color  16.7 million colors

Bit Depth Affects the Image File Size:
The amount of information that each image pixel can contain (the number of possible colors) of course affects the image file size. The more information (more possible colors), the larger the file size. This is why you should make your images as few colors as possible. Color images can usually be set at 8-bit (256 colors), or even lower, without a noticeable decrease in quality.

Resolution Affects the Image File Size:
Resolution is the number of dot (pixels) per inch in your image. The most common waste of disk space is images that have higher resolution than they need. Computer monitors generally display images at only 72 dpi. Therefore, if your image is designed to be viewed on the computer (such as web graphics, presentations, email attachments, etc.), there is no point in making them more than 72 dpi. Also, most printers do not print at resolutions that justify images that are more than about 100 dpi.

But my printer can print at up to 1440 dpi shouldn't my image be at a higher resolution??
To make a complex answer brief No. In general, to get the best possible printouts, your images should be at the following resolutions:

 Printer Resolution  Optimal Photo Resolution
 300 dpi  100 dpi
 600 dpi  150 dpi
 1200 dpi  170 dpi

But remember, if the image is going to be displayed on a monitor, there is no need to go beyond 72 dpi!

Image Dimensions Affect the Image File Size:
The third factor that affects file size is the physical dimensions of the image. As a habit, you should always attempt to reduce the size of your images as much as possible. There is rarely a good reason to have an image on a web page that is larger than, say, 3 or 4 inches.

What are the Most Common Image File Formats?
TIFF
­ The most widely used format for raster images (images made of pixels)
PICT ­ A very common format used on Macintosh computers. PICT files can contain either bitmapped (raster) or object-oriented (vector) graphics.
EPS (Encapsulated PostScript) ­ The most widely used file format for vector images (images made of objects or shapes). Specifically to store graphics that will be printed with a PostScript printer.

The two following formats should be used if you will be using the image on the World Wide Web. These formats compress the images so they have smaller file size (faster download time):
GIF ­ Best for illustrations and other images that have areas of flat, uniform color. When you save an image as a GIF, you should save the image at the lowest bit-depth (lowest number of colors) that you can without losing too much quality. This greatly reduces file size. Gif images can be made to be transparent, meaning that you can see through the image background to whatever is behind the image (you can select the portion of the GIF image that is transparent).
JPEG ­ Best for photographs and other images that have subtle color changes.