Understanding Vector and Bitmap Images

Bitmap or Raster Image
A Bitmap Image--shown zoomed in with the original size in the upper right corner. Notice the individual pixels that make up the image. View Full Size. S. Chastain

It's almost impossible to discuss graphics software without first establishing an understanding of the differences between the two major 2D graphics types: bitmap and vector images. 

Facts About Bitmap Images

Bitmap images (also known as raster images) are made up of pixels in a grid. Pixels are picture elements: tiny squares of individual color that make up what you see on your screen. All these tiny squares of color come together to form the images you see. Computer monitors display pixels, and the actual number depends on your monitor and screen settings. The smartphone in your pocket can display up to several times as many pixels as your computer.

For example, the icons on your desktop are typically 32 by 32 pixels, meaning there are 32 dots of color going in each direction. When combined, these tiny dots form an image.

The icon shown in the upper right corner of the image above is a typical desktop icon at screen resolution. As you enlarge the icon, you can begin to clearly see each individual square dot of color. Note that white areas of the background are still individual pixels, even though they appear to be one solid color.

Bitmap Resolution

Bitmap images are resolution dependent. Resolution refers to the number of pixels in an image and is usually stated as dpi (dots per inch) or ppi (pixels per inch). Bitmap images are displayed on your computer screen at screen resolution: approximately 100 ppi.

However, when printing bitmaps, your printer needs much more image data than a monitor. In order to render a bitmap image accurately, the typical desktop printer needs 150-300 ppi . If you've ever wondered why your 300 dpi scanned image appears so much larger on your monitor, this is why. 

Resizing Images and Resolution

Because bitmaps are resolution dependent, is impossible to increase or decrease their size without sacrificing a degree of image quality. When you reduce the size of a bitmap image through your software's resample or resize command, pixels must be discarded.

When you increase the size of a bitmap image through your software's resample or resize command, the software has to create new pixels. When creating pixels, the software must estimate the color values of the new pixels based on the surrounding pixels. This process is called interpolation.

Understanding Interpolation

If you double the resolution of an image you add pixels. Let's assume you have a red pixel and a blue pixel beside each other. If you double the resolution you will be adding two pixels between them. What color will those new pixels be? Interpolation is the decision process that determines which color those added pixels will be; the computer is adding what it thinks are the right colors.

Scaling an Image

Scaling an image does not affect the image permanently. In other words, it does not change the number of pixels in the image. What it does is make them bigger. However, if you scale a bitmap image to a larger size in your page layout software, you are going to see a definite jagged appearance. Even if you don't see it on your screen, it will be very apparent in the printed image.

Scaling a bitmap image to a smaller size doesn't have any effect; in fact, when you do this you are effectively increasing the ppi of the image so that it will print clearer. How so? It still has the same number of pixels in a smaller area.

Popular bitmap editing programs are:

  • Microsoft Paint
  • Adobe Photoshop
  • Corel Photo-Paint
  • Corel Paint Shop Pro
  • The GIMP

All scanned images are bitmaps, and all images from digital cameras are bitmaps.

Types of Bitmap Formats

Common bitmap formats include:

  • GIF
  • PNG
  • TIFF
  • PSD (Adobe Photoshop)

Converting between bitmap formats is generally as simple as opening the image to be converted and using your software's Save As command to save it in any other bitmap format supported by your software.

Bitmaps and Transparency

Bitmap images, in general, do not inherently support transparency. A couple of specific formats – namely GIF and PNG – support transparency.

In addition, most image editing programs support transparency, but only when the image is saved in the software program's native format.

A common misconception is that the transparent areas in an image will remain transparent when an image is saved to another format, or copied and pasted into another program. That just doesn't work; however, there are techniques for hiding or blocking out areas in a bitmap that you intend to use in other software.

Color Depth

Color depth refers to the number of possible colors in the image. For example, a GIF image is an 8-bit image, which means there are 256 colors that can be used.

Other colors depths are 16-bit, in which roughly 66,000 colors are available; and 24-bit, in which roughly 16 million possible colors are available. Reducing or increasing the color depth adds more or less color information to the image with a corresponding decrease or increase in file size and image quality.

Facts About Vector Images

Although not as commonly used as bitmap graphics, vector graphics have a lot of virtues. Vector images are made up of many individuals, scalable objects.

These objects are defined by mathematical equations, called Bezier Curves, rather than pixels, so they always render at the highest quality because they are device-independent. Objects may consist of lines, curves, and shapes with editable attributes such as color, fill, and outline.

Changing the attributes of a vector object does not affect the object itself. You can freely change any number of object attributes without destroying the basic object. An object can be modified not only by changing its attributes but also by shaping and transforming it using nodes and control handles. For an example of manipulating an object's nodes, see my CorelDRAW tutorial on drawing a heart.

Advantages of Vector Images

Because they're scalable, vector-based images are resolution independent. You can increase and decrease the size of vector images to any degree and your lines will remain crisp and sharp, both on screen and in print.

Fonts are a type of vector object.

Another advantage of vector images is that they're not restricted to a rectangular shape like bitmaps. Vector objects can be placed over other objects, and the object below will show through. A vector circle and bitmap circle appear to be exactly the same when seen on a white background, but when you place the bitmap circle over another color, it has a rectangular box around it from the white pixels in the image.

Disadvantages of Vector Images

Vector images have many advantages, but the primary disadvantage is that they're unsuitable for producing photo-realistic imagery. Vector images are usually made up of solid areas of color or gradients, but they cannot depict the continuous subtle tones of a photograph. That's why most of the vector images you see tend to have a cartoon-like appearance.

Even so, vector graphics are continually becoming more advanced, and we can do a lot more with vector drawings now than we could a decade ago. Today's vector tools allow you to apply bitmapped textures to objects giving them a photo-realistic appearance, and you can now create soft blends, transparency, and shading that once was difficult to achieve in vector drawing programs.

Rasterizing Vector Images

Vector images primarily originate from software. You can't scan an image and save it as a vector file without using special conversion software. On the other hand, vector images can easily be converted to bitmaps. This process is called rasterizing.

When you convert a vector image to a bitmap, you can specify the output resolution of the final bitmap for whatever size you need. It's always important to save a copy of your original vector artwork in its native format before converting it to a bitmap; once it has been converted to a bitmap, the image loses all the wonderful qualities it had in its vector state.

If you convert a vector to a bitmap 100 by 100 pixels and then decide you need the image to be larger, you'll need to go back to the original vector file and export the image again. Also, keep in mind that opening a vector image in a bitmap editing program usually destroys the vector qualities of the image and converts it to raster data.

The most common reason for wanting to convert a vector to a bitmap would be for use on the web. The most common and accepted format for vector images on the web is SVG or Scalable Vector Graphics

Due to the nature of vector images, they are best converted to GIF or PNG format for use on the web. This is slowly changing because many modern browsers are able to render SVG images.

Common vector formats include:

  • AI (Adobe Illustrator)
  • CDR (CorelDRAW)
  • CMX (Corel Exchange)
  • SVG (scalable vector graphics)
  • CGM Computer Graphics Metafile
  • DXF AutoCAD
  • WMF Windows Metafile

Popular vector drawing programs are:

  • Adobe Illustrator
  • CorelDRAW
  • Xara Xtreme
  • Serif DrawPlus
  • Inkscape

Metafiles are graphics that contain both raster and vector data. For example, a vector image that contains an object which has a bitmap pattern applied as a fill would be a metafile. The object is still a vector, but the fill attribute consists of bitmap data.

Common metafile formats include:

  • EPS (Encapsulated PostScript)
  • PDF (Portable Document Format)
  • PICT (Macintosh)