Slide 1 In creating PowerPoint presentations you may want to use digital images that you have scanned or taken with a digital camera, received from someone else, or found on the web. By gaining an understanding of such things as resolution, bit depth, file size and color modes, you will better be able to use these images correctly and efficiently in your presentations. Even if you have someone else create or work with the digital images for you, having a basic understanding of the concepts will help you to obtain exactly what you want and need to create your presentation. In this presentation I will explain what resolution is, how to work with it, show and define bit depth, explain how to control the size of your image files, and define what color models are.

This presentation explains only resolution as it pertains to digital images for digital display -- .meaning for 35mm slides or onscreen shows. It does not cover resolution issues for printing to a laser printer, large format printer or to a printer used by professional print houses.
Slide 2 Have you ever seen a presentation where the image looked like the one above? Sort of blurry isn’t it? This image was downloaded from a web page and then stretched to fit the larger space. Want to learn how to avoid this? (At this point you are supposed to be nodding your head up and down and emphatically saying yes!)
Slide 3 Here is the true size of the image on the previous page. Frequently we will see blurry images on peoples posters as well as in presentation slides or screen shows. By understanding the concept of resolution, you should be able to avoid this problem in your presentations. Let’s continue on…
Slide 4 There are two types of images created by computers, vector images and bitmap or rasterized images. These types of images are determined by the image’s nature and how it is created by the computer.
Slide 5 Vector images record images descriptively, in terms of geometric shapes and then the shapes are converted to bitmaps for display on a monitor. Vector images are defined in a computer file by mathematical equations. They are easy to modify, can be moved, ungrouped, resized, rotated, or partially deleted. They are easily scaleable and the file sizes can be very small. They can have irregular shapes provided they are in a program that allows for it. Clipart is the most common example of vector images.

A common vector image file that works great in PowerPoint is the CGM file format. This stands for Computer Graphics Metafile. Since this is a vector file format, bringing it into PowerPoint (which is also a vector format program) allows the image to be ungrouped, recolored, or partially deleted. It can also have an irregular shaped outline and is a small file size. Since these files are not resolution dependent, you can stretch or shrink these images to whatever size you need and they will still look good when projected or printed.

Some of the university logos available to you are CGM file formats. Using these instead of tiffs or jpegs for the logos will give you a more crisp and clean logo for printing or projection. If you insert them into PowerPoint and you get an error message, you simply have to reinstall the import filters so it knows what a CGM file format is.
Slide 6 Bitmap or rasterized images are made up of a map of bits (pieces of computer information). All web images and most painting program images are bitmapped. In a bitmapped image, each square of information must be remembered by the computer. The file sizes are larger (bitmap files are about 10 times larger in file size than vector files) and the image is always rectangular in shape. To edit the image, you have to alter each pixel directly with a bitmap editor like Photoshop.

Slide 7 Computer software programs which create graphics are considered either vector or bitmap programs. PowerPoint, along with CorelDraw and Illustrator, as well as many others, are considered vector programs and remember the drawings as mathematical equations. Bitmap or raster programs are those that produce bitmapped images. Photoshop, Irfanview, paint programs, scanners, and digital cameras all create images that are bitmapped. These programs have to remember each and every bit of information in the image so the file sizes can be quite large. Vector programs usually have small file sizes until you start inserting bitmap images into the vector programs.
Slide 8 Since vector images can be resized without any problems with resolution we are going to limit our discussion to bitmap images, whose resolution is varible.

Every file on a computer is in a specific format. There are a wide variety of image formats used for graphics. The choice of what you use is based on: 1) whether you plan to edit the image in future. 2) whether you want smallest possible file size. Or 3) what image editing tools you have available.

True image files are in a format that, when saving, records the image completely and correctly, without losing any details. In this format you can re-edit them later if you need to without losing any quality in the image. True image files are large and can’t be used over the WWW. The most common file format for true image files is a TIFF file.

Compressed Image Formats are file types or formats that, when saving, compress the information and result in the smallest possible file size for going over the internet. In the process of saving the file, information is compressed or discarded to make the image as small as possible. The two most common file types as of today are: 1) GIF - Graphics Interchange Format. And 2) JPEG - Joint Photographic Experts Group.

Both of these formats compromise the image for the sake of compression so don’t save your original artwork in this format if you may want to modify it later. Also, do not resave these files over and over again as each time the file is saved, it is compressed thus information is continually discarded. Eventually the image would look terrible since so much information would have been discarded in the saving process.

 
Slide 9 TIFF files are saved in the true image format. Again, these files are not compressed when saved so all the information in the file is retained. These files are usually either 24 or 32 bit color… which we will talk about later. Scanners and digital cameras often save files as TIFFs. If they do not, you may want to if the image is something very important and you need to keep the file forever. If the images are originally a JPEG or in another format, it’s a good idea to open the file in a program like Photoshop and save them as a TIFF so you can edit it later without degrading the quality of the image. TIFF files are also pretty universal and can be opened by a variety of software.
Slide 10 GIF files are the first type of compressed image format that we will discuss. GIF files are best for images with sharp-edged areas of flat color, line drawings, simple icons and buttons, and textual images that don’t use more that the 256 colors or are larger than 8-bit images. GIF format doesn’t work well with photographs or high color images. GIF files are very compressed so the file size is very small. GIF files are mainly used on web pages.
Slide 11

The second type of compressed image format are JPEGs. JPEG files are best for images that involve smooth color gradients like photos or naturalistic artwork. It was developed specifically to compress 24-bit photographic-type images. JPEG’s don’t work well with simple illustrations or line art or any flat-color art, type, or images with sharp, well-defined edges since when compressed, the image is sort of blurred and fine lines or text may become blurry.

The compression that is applied to the file when saved can vary. The accuracy (how well details show up) of the image depends on how much compression is applied - a value you can chose in most JPEG-savvy image-editing tools. The more you compress your JPEG image, smaller the file but more image detail you will lose. This loss is permanent.

It is not a good idea to edit JPEG’s as they are already compressed and if edit and then save as another JPEG, they get compressed again and the quality degrades further. If it is an image file you will need for possible editing use later on, save it as a TIFF.
Slide 12 This slide lists some of the file formats common to the different devices.
   
 
Last updated September 2004. These courses are copyrighted by the University of Nebraska, Lincoln, Communications and Information Technology. Contact Lana Johnson ljohnson1@unl.edu