Microsoft Office Tutorials and References
In Depth Information
up less space on disk, but the quality may suffer. Therefore, it’s best to use vector graphics
when you want simple lines, shapes, or cartoons and reserve raster graphics for situations
where you need photographic quality.
The following sections explain some of the technical speciﬁ cations behind raster graphics;
you’ll need this information to make the right decisions about the way you capture the
images with your scanner or digital camera and the way you use them in PowerPoint.
The term resolution has two subtly different meanings. One is the size of an image,
expressed in the number of pixels of width and height, such as 800
600. The other mean-
ing is the number of pixels per inch when the image is printed, such as 100 dots per inch
(dpi). The former meaning is used mostly when referring to images of ﬁ xed physical size,
such as the display resolution of a monitor. In this topic, the latter meaning is mostly used.
If you know the resolution of the picture (that is, the number of pixels in it) and the reso-
lution of the printer on which you will print it (for example, 300 dpi), you can ﬁ gure out
how large the picture will be in inches when you print it at its native size. Suppose you
have a picture that is 900 pixels square and you print it on a 300 dpi printer. This makes it
3” square on the printout.
Resolution on Preexisting Graphics Files
When you acquire an image ﬁ le from an outside source, such as downloading it from a web-
site or getting it from a CD of artwork, its resolution has already been determined. Whoever
created the ﬁ le originally made that decision. For example, if the image was originally
scanned on a scanner, whoever scanned it chose the scan resolution — that is, the dpi set-
ting. That determined how many individual pixels each inch of the original picture would
be divided into. At a 100 dpi scan, each inch of the picture is represented by 100 pixels
vertically and horizontally. At 300 dpi, each inch of the picture is broken down into three
times that many.
If you want to make a graphic take up less disk space, you can use an image-editing pro-
gram to change the image size, and/or you can crop off one or more sides of the image.
If you crop or decrease the size of an image in an image-editing program, save the changes under a different i le-
name. Maintain the original image in case you ever need it for some other purpose. Decreasing the image resolution
decreases its dpi setting, which decreases its quality. You might not notice any quality degradation on-screen, but
you will probably notice a difference when you are printing the image at a large size. That’s because the average
monitor displays only 96 dpi but the average printer prints at 600 dpi or higher.
PowerPoint slides do not usually need to be printed at a professional-quality resolution, so image quality on a
PowerPoint printout is not usually an issue. However, if you use the picture for something else later, such as printing
it as a full-page color image on photo paper, then a high dpi i le can make a difference.