Categories


Raster & Vector Images: Properties

Besides the obvious difference of raster images using pixels and vector images using objects – there are other differences to be noted.

Color

Raster images – digital pictures, scanned images or those created with a graphics program – need a large number of colors to render the image. Scanned images using 24 bit (16 million colors) are almost indistinguishable from the original while images that use 8 bit (256 colors) show some deterioration to the image. To help eliminate some of the deterioration of scanned images a process called dithering is used to approximate colors that are not in the smaller color palette. However, this creates a situation where the edges (and lines) may not be clean. Using an 8 bit image also is problematic should you want to change a color. In this case you cannot isolate the range of colors needed – and attempting to change a single color will result in a less than desirable image, i.e., some loss of shape, uneven lines and edges.

Vector images – changing the color of the vector object – since it does not use pixels – is relatively easy and much like coloring with crayons. A ‘vector’ drawing program allows the user to select the object and define the color as well as the line width in some cases.

Resolution

Raster images– resolution is defined in the number of dots per inch (dpi). This is the same method to measure the resolution of a printer or scanner. If you have a 300 dpi image and increase the size, typically the edges and lines will become jagged. This is due to the fact that the ‘squares’, i.e., pixels, got bigger. However, if you decrease the size of the 300 dpi image, the ‘squares’ get smaller and maintains the integrity of the edges and lines. In one sentence – Raster images do not ‘scale’ they only enlarge.

Vector images – are defined by ‘math’, i.e., ratios – you will find they can be ‘scaled’ up or down without loosing any image quality. For example, you have an image that is 2 inches by 2 inches in size, to double it you would use a multiplier of 2. To scale this same image down by half you would use a multiplier of .5. Since vector images can be scaled without loosing any integrity to the quality – they are the most popular graphics format for clip art and for transferring to printed material.

Files

Raster images – the most commonly used file formats are:

  • BMP (Bitmap)
  • TIFF (Tag Interleave Format)
  • JPEG (joint Photographics Expert Group)
  • GIF (Graphics Interchange Format)
  • PNG (Portable Network Graphic)
  • PSD (Adobe PhotoShop)

Vector images – the most commonly used file formats are:

  • EPS (Encapsulated PostScript)
  • WMF (Windows Metafile)
  • AI (Adobe Illustrator)
  • DXF (AutoCAD)
  • SVG (Scalable Vector Graphics)

There is a significant difference in the file sizes need to store these two images formats.

Since raster images are composed of colored pixels, the graphics program must keep track of all the information relevant to the image. Not only must it keep track of all the colors it must also keep track of the specific location of each pixel.   The larger the raster image, and the more colors used – the larger the file.  Large-sized raster images can equate to the final image being over 1 megabyte in size . . . resulting in slow uploads and downloads.

To the contrary, vector images do not need to keep track of each pixel but rather only the mathematical information.  Therefore,  these files are relatively small in size by comparison. This is to say if you had your company logo as a vector image, the file size would be the same for a 3 inch by 3 inch image as for a 3 foot by 3 foot image.   

Conversion of Images

With vector images being more desirable for use in some cases, what can you do if you have a raster image you would like to use as a vector image?   Convert it.  There are some specialty programs that  allow you to convert raster images to vector images. If you would like to see the process before making a purchase, you can try converting a raster image to a vector image  for free at the website  http://vectormagic.com/home.  If you decide to give it a try – be sure your raster image is in Bitmap format before you start the process.


Comments are closed.