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What is a pixel?

You've probably heard the phrase pixels per inch or have heard an image described as pixelated, and you may have even heard images referred to as pixels. To begin to understand what all this means we need to discuss two types of graphics – vector and raster. Vector graphics are images created using mathematical calculations to determine the shape of each curve and line as well as the fill. They are easily resized and don't look jagged when they're printed or enlarged. Many times clip-art is created in a vector format simply because of the resizing capability. Raster graphics are images created using pixels or little squares of color that when put together form a complete image. Digital photos are always raster images. Sometimes a raster image is enlarged by decreasing the resolution or PPI which changes the size of each pixel making it larger. If the photo is enlarged too much the individual pixels will become visible and the image begins to look jagged. This jagged look is called pixelated.

So we now know that a pixel is a single square of color in a raster image, but what does pixels per inch, PPI, mean? PPI simply refers to how many squares, or pixels, are displayed in each inch. You may wonder, "Why that is important?" Well there are a number of reasons. For a raster image to be printed and not look pixelated, the resolution, or PPI, needs to be adjusted according to the type of printing being done. Typically for an ink jet printer the PPI can be as low as 150 and the image will still print relatively well. When I take my digital photos to the local photo center or use my favorite on-line printing service I typically like to use a resolution of at least 200 PPI, but I think 300 PPI is better. For commercial printing you may even need to use a higher resolution. The best way to know for sure is to find out your vendor's recommended resolution before you get everything ready, then you won't be disappointed with the results.

One more thing to note though is that more isn't always better. Sometimes having too high a resolution can cause blurriness. Although for the most part unless you've got a really fast computer with lots of memory and a digital camera with professional strength capturing capabilities then you probably don't need to worry about too high a resolution for printing. I just find it saves me time to find out the optimal resolution before preparing my images.

That being said, if you're creating or adjusting images for use on the web then a PPI of 72 is plenty. The more pixels you use in creating the image, the larger the file size you'll end up with. Typically for saving on the web, smaller file sizes are better. In fact that is one of the causes of the problems discussed in the answer to the question following this one (How do I prevent my images from looking pixelated/jagged?). The built in optimization adjustments in Animation Shop can cause major issues if you don't know how to adjust the settings.

In addition to digital photos, scanned images and other raster art, sometimes raster images are created by coloring individual pixels, and often times that type of image is called a pixel. It is most likely called this because the artist was “pixel painting” to create the image. Technically speaking, calling an image a pixel is incorrect, because a pixel is only a single square of color in a raster image. However, by calling an image a pixel it implies that it was created by coloring the image pixel by pixel so this slang makes sense even if it isn't completely accurate. Most pixel painted images are best used for viewing on screen. For example they work great for emails, blogs and web sites. Once they are printed they typically look jagged because they are usually designed to be very small.

If you'd like to learn to make a "pixel" be sure to check out the Froggie tutorial.

To view the answer to the second question in the August 15, 2005 issue of the newsletter, click on the following link:

How do I prevent my images from looking pixelated/jagged?