Bitmap image formats

1
Bitmap images
Photo editors
Resizing bitmaps
JPG file formats
Vector images
GIF file formats
Why are there so many image file formats?
Animated and transparent .gifs
Created by Sheila MacLeod Potter, working for the
AITT, Acadia University
What are the different image file formats?
Bits and .bmp files
Getting images from the web
Resolution and Scanning
2
Bitmap images

• Bitmap refers to digital images which are
  composed of dots or pixels
• In a bitmap image, the image file has to define
  the exact colour of every pixel in the image

This bitmap image has been stretched so that the
pixels which compose it are clearly visible.
3
Resizing bitmap images

• The more pixels which compose an image, the
  larger its file size.
• The number of pixels in an image can be increased
  in two ways
• by making the image larger in a photo editor.
  Tell me more about photo editors!
• by increasing the image resolution
• One confusing thing about the term bitmap is that
  there is a windows image format (.bmp) which is
  also referred to as a bitmap. To avoid this
  confusion, some people use the term raster when
  referring to images composed of pixels.
• Bitmap file formats include .jpg, .gif, .bmp,
  .tif, .wpg, .psd, etc.
• Why are there so many image formats?
• Why does the computer ask me about number of bits
  when I go to save my image as a bmp?

4
Vector images

• Vector images are not made up of dots. Instead,
  they are composed of shapes and lines.
• Vector shapes are defined by the direction of the
  line which runs around their perimeter.
• Vectors have several useful qualities
• they are infinitely scaleable
• they usually have smaller file sizes than bitmaps
• they are crisp and clear when printed
• Note that vectors on screen appear pixilated
  because the computer screen is composed of
  pixels!

5
Why are there so many image file formats?

• There are almost as many image file formats as
  there are programs that work with images.
• There is a great deal of competition in the
  graphics world among companies trying to create
  the best artists tool.
• Companies like Adobe and Corel, for example, each
  have software packages for working with images,
  which do many of the same things, yet they each
  (Adobe Photoshop and Corel PhotoPaint) have their
  own image file format (.psd and .cpt).
• At one time, different image file formats could
  only be opened by the software in which each was
  created, however, today, most common packages
  will let you open other companys formats.
• One practical reason for different formats is
  that each saves a slightly different set of
  information with the file. For example, .jpg is a
  compressed format (small files), in which the
  minimum information is saved. By comparison, .tif
  is uncompressed and can save more file
  information.

6
Image File Formats

• .jpg and .gif are the best formats to use for
  presentations and for the web because they are
  both compressed in file size. Uncompressed
  formats such as .tif are best for print, however,
  there are many other formats which you may come
  across
• Other Bitmap formats
• .tif uncompressed image format which is good
  for use in printed projects like pamphlets and
  posters.
• .wpg format used by Corel Presentations
• .bmp windows preferred image format (your
  wallpaper is a .bmp)
• .psd layered file format used in Adobe Photoshop
• .cpt layered file format used in Corel
  Photopaint
• Vector formats
• .cdr CorelDraw format. MS PowerPoint often has
  difficulty reading this format.
• .wmf windows metafile format. this format is
  widely accepted by programs which use vectors,
  including MS PowerPoint, and Word.
• .fla animated vector file created in Macromedia
  Flash

HOME
7
Photo Editors

• although, images can be stretched, or cropped
  within PowerPoint, this does not alter their file
  size.
• in order to crop an image so that parts are
  eliminated from the image file itself (and the
  file becomes smaller), a photo editor such as MS
  Paint, Corel Photopaint, or Adobe Photoshop must
  be used.
• similarly, when you stretch or shrink an image
  in PowerPoint, the file size is not effected.
• Image file size must be changed in a photo editor
  by changing the resolution, cropping, or resaving
  in a compressed format such as jpg or gif

HOME
8
JPG file format

• JPEG is a popular format for two reasons
• It keeps photographic images clear
• It lets you adjust the amount of compression
• JPG format should be used for photographs which
  contain a lot of detail and thousands of colours
• JPG compression does change the way an image looks

uncompressed (tif) file size 152Kb
slightly compressed (jpg) file size 31Kb
highly compressed (jpg) file size 10Kb
9
GIF file format

• In a .gif image, the number of colours is reduced
  to 256 and then "runs" of same-colour pixels are
  encoded in a colour number of pixels format.
  This makes a .gif file great for storing drawings
  that have lots of same-colour pixels. gif images
  are common on the web.
• Whats an animated gif or a transparent gif?

GIF format should be used for diagrams or
drawings which contain few colours If the image
contains 256 colours or less, gif compression
does not change the way it looks
10
Why does it ask me about the of bits when I
save my .bmp files?

• The more colours in an image, the more bits are
  required to represent it digitally.
• The best-looking images contain 16 million
  colours (this is known as RGB Red Green Blue
  mode)
• Imagine a bitmap in RGB that is 400 by 400
  pixels. Saving this image requires 24 bits per
  pixel for 160,000 pixels, or 480,000 bytes.
• 480,000 bytes 480 Kb which would take
  considerable time to download from the web
• For the sake of comparison, an RGB .jpg
  containing the same number of pixels (400X400)
  might be as small as 5Kb.

11
Animated Transparent gif.s

• Whats an animated gif?
• Animated gifs are also .gif files, but they are
  actually composed of multiple gif images which
  cycle to give an animated effect. Animated gifs
  files are as big as the sum of the gif images
  which compose the animation.
• Why does this gif file look differently on my
  machine than it did on the web?
• Many photo editors will allow you to assign a
  transparent colour within a gif image. Thats
  why, when you download gifs from the web,
  sometimes an ugly background colour appears when
  you open the gif in another program. The
  transparent colour only looks transparent on
  the web!

12
Copying and Downloading images from the web

• There are two ways to get an image off the web.
• copy and paste
• save it
• Copy and Paste
• copy the whole screen (PrtSc)
• OR copy the active screen (AltPrtSc)
• AND paste (CtrlV) directly into your document
• Save
• Right Click, Save As
• you cant choose a file type, but you can give
  the file a new name

13
Resolution and Scanning

• Resolution Image resolution is expressed in dots
  (or pixels) per inch (dpi).
• When scanning an image, the resolution you
  choose depends on the way that you intend to use
  the image, or how it will be displayed.
• Suggested Minimum Resolutions
• for use on the web and in presentations 72 dpi
• for print 150-300 dpi

If you are scanning an image for use on the web
or in a presentation, you may wish to blow-up
the image by scanning it at a resolution higher
than 72 dpi. Computer screens display at 72 dpi,
therefore, small images that are scanned at
higher resolution will be displayed larger on the
computer screen.
image size 1 X 1 resolution 100dpi