Photoshop Foundation - The Difference between Vector and Bitmap Graphics

December 2007


Welcome to the first in a new series of articles on Photoshop - the Photoshop Foundations series. The aim of this series is to give both beginners and more experienced users all the information they need to use Photoshop as efficiently as possible. Photoshop is a huge application, and there is usually more than one way to look at a given subject, or perform a certain action. This series aims to both, guide you through the more confusing aspects of Photoshop and show you the very best ways to use this application.

In this first article we are going to look at the difference between vector and bitmap graphics, which is one of the most important principles to understand when working with graphics on a computer, inside or outside of Photoshop. Although Photoshop primarily is a bitmap image editor, it is capable of handling vector graphics to a certain extent. This can be a little confusing for people new to creating graphics on a computer, but by the end of this article you should have a clear idea of the difference between these two types of graphics.

Bitmap Graphics

Bitmap graphics are made up of colored pixels. Pixels are very small rectangles (usually square, although in some video applications they are wider than they are tall) of varying colors that once put together give you an image. You can see from the example below that zooming in on a bitmap image reveals the pixels that make up the image when viewed at 100%.


Photoshop Foundation - The Difference between Vector and Bitmap Graphics

Bitmap graphics are usually (but not always) photographic in nature, capable of subtle graduated tones - often in the range of millions of colors per image. The problem with bitmap graphics is that they don't enlarge well as Photoshop needs to guess what color the extra pixels should be - this can result is loss of definition and a dramatic lowering in quality, depending on how much you enlarge the image. Common file formats for bitmap image data include GIF, JPEG and PNG for Internet usage and TIFF for print usage. As you can see from the example below, physically enlarging an image will degrade quality.

Photoshop Foundation - The Difference between Vector and Bitmap Graphics

Pixels are also used to display the image on your computer screen. Common pixel dimensions of computer displays are 1024 wide by 768 high and 1600 wide by 1200 high. The size of a bitmap graphic when viewed on your computer screen is defined by the number of pixels that make up the image - so an image that is 50 pixels wide will look very small on your screen at 100% viewing percentage, whereas an image that is 4000 pixels wide will be larger than your screen at 100% viewing percentage.

The printable dimensions of an image are defined by the DPI (dots per inch) - this information is invisibly embedded in the image file. Digital cameras often embed information such as this, that may include the conditions the image was taken in, and even the camera model used. This information is not actually visible in the image, and requires software such as Photoshop to read it.

You should not confuse the output DPI of your printer with this figure, which may range from 600-2400DPI - this refers to the density of the dots of ink laid down on the page by the printer. You don't have to prepare your images to 2400 DPI to get the best results - in fact doing so will significantly slow down printing as your file could potentially be huge! Often an image DPI in the range of 175-250 will give very good results on home printers. Images prepared for high quality commercial print are usually prepared at 300 DPI for up to A3 in size; whereas very large images (for instance on billboards) can be as low as 50 DPI, as they are not made to be viewed as closely as a magazine or small poster. There is no need to go above 300 DPI when creating images as you will yield virtually no improvement in output quality, only increasing the size of your file when saved.

It is easy to understand the relationship between pixel dimensions and DPI - put simply, the DPI is how many pixels will be printed in an inch - so you could actually think of DPI as PPI (pixels per inch). Indeed, many experts believe this to be the true definition of DPI, and that Photoshop should refer to it as such. However, the term DPI is used throughout the professional print industry, so this is why it is referred to as DPI in Photoshop, not PPI.

The easiest way to explain this is with an example - if you have a 600x600 pixel image, at 200 DPI this image will print at 3 inches by 3 inches. At 300 DPI this image will print at 2 inches by 2 inches. You can set the DPI of an image by going up to the top menu strip and selecting Image > Image Size. Remember to uncheck the Resample Image option before typing in your new DPI, otherwise your pixel dimensions will change to reflect the same output dimensions shown in the Document Size area of this dialogue box. Using this technique, you can also see what size your image will output at different DPI settings, as the output size will change as you type in different DPI (as shown below).

Photoshop Foundation - The Difference between Vector and Bitmap Graphics

Vector Graphics

Vector graphics on the other hand are not constructed with pixels at all (although they are displayed on your computer screen as pixels). They are actually made-up of mathematical expressions and instructions that produce lines, curves and filled shapes. Company logos are often vector graphics. They are usually made up of limited colors, although they may feature sophisticated gradients and shading to produce more sophisticated looking graphics.

Vector graphics are created outside of Photoshop in applications such as Adobe Illustrator or Corel Draw. They don't have a native DPI like a bitmap image and can be scaled to any size without any loss in quality. Their size is defined by the dimensions at which they were originally created. Common file formats for vector graphics are EPS and AI (the native Illustrator file format). Vector graphics are usually much smaller in file size than bitmap graphics, unless bitmap images are embedded inside the image, in this case the file size will usually be larger. If bitmap graphics are embedded in vector files (for example, an EPS), the bitmap element of the graphic will suffer in quality if resized, whereas the vector element will always output at excellent quality regardless of this. An example of a vector graphic and its enlargement is shown below.

Photoshop Foundation - The Difference between Vector and Bitmap Graphics

Vector artwork is used within Photoshop to create paths in the paths palette, manipulate type using fonts, and create and manipulate custom shapes. These elements can be resized up and down to any scale as many times as you wish without any loss in quality. You must save your file in the native PSD Photoshop file format to retain this editability - once saved as a bitmap image file such as a TIFF this editability will be lost (unless it is an embedded path).

Vector data is also used to save EPS files out of Photoshop to create 'clipping paths' that cutout bitmap images from their background. Clipping paths have the ability to provide a much higher resolution edge to an image compared to pixels when printing. As mentioned previously, vector images don't have an embedded DPI but professional output devices often output vector data in the range of 2800 DPI.

I hope that you have enjoyed reading this article, and it has cleared up some of the issues that you may have with the two main types of graphic files. Although there are a number of different file formats for images, the data within these files will be either vector or bitmap.

Photoshop is capable of saving out a huge number of different file formats and in the next article in this series we are going to take a closer look at these file formats and explain when and why you should use a specific file format for specific purposes.

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