It is always a challenge to find the right visual aids to add to a multimedia presentation. Many times we want to show how an application works. Of course, one option is to run a live application in addition to your presentation. But that is not always possible. For example, let's say you want to show an Excel worksheet in the middle of a presentation. Perhaps the laptop you are using does not have Excel installed on it, or perhaps you just want to show a portion of the data and not the entire worksheet. What to do? Well, you simply take a screen shot, edit it as you like, and embed this screen shot in your presentation. Simple no?

Well, what looks so clear in Excel suddenly looks slightly blurry in the Paint application, slightly more out of focus when the screen shot is embedded in the PowerPoint presentation, and entirely unreadable when projected. HOW DOES THIS HAPPEN?

General Image Types (Vector vs. Bitmap)

Images can be stored in many formats. Two fundamental types of images are vector and bitmap. In general, vector images are stored as a sequence of drawing instructions, while bitmap images are stored as a mosaic of picture elements.

• Vector Images
  Vector images define an image as a set of instructions composed of graphic objects. Vector graphics are well suited to computer handling because they can be mathematically manipulated. This enables individual elements to be scaled and stretched. Vector graphics reproduce at the optimum resolution for an output device and maintain a consistent size when reproduced. Original vector images are created in an application, like Adobe Photoshop. Vector images can generally be translated into bitmaps. That is essentially what happened when a vector image is displayed on a conventional computer screen.



• Bitmap Images
  Bitmapped images represent an image as a grid of pixels, each with a value mapping a particular value of color and intensity. Any image can be represented in bitmap form, although this is not necessarily always efficient. Original bitmap images can be created in applications like MS Paint.

There are certain limitations to creating bitmaps:

• A bitmap image is stored as a pattern of pixels that can be difficult to modify.
• A bitmap image is tied to the resolution at which it was created.
• A bitmap image of high resolution can require a great deal of storage.

Even with these limitations, bitmaps are part of multimedia presentations because they are so easy to create.

Capturing Bitmaps for Presentations

Resolution is the key to capturing good quality bitmaps.

• Resolution (what most people don't know)
  The quality of a bitmap reproduction is directly tied to the resolution of the bitmap. The resolution of a bitmap is a product of the dimensions of the bitmap in pixels and the amount of information that is stored for each pixel.



• Spatial resolution (what most people think of as "resolution")
  The horizontal and vertical dimensions of a bitmap in pixels determine its spatial resolution. The display size of a bitmap is generally dependent upon the resolution of the input and output devices. The resolution of an input or output device is often measured in terms of the number of dots per inch (DPI).

Resizing Bitmaps

The resolution of a bitmap is effectively fixed by the resolution at which it was captured. Therefore, bitmaps are very difficult to resize. Nevertheless, every bitmap image has a natural size at which it will reproduce at optimum quality on an output device of a given resolution.

• Going Up . . .
  A bitmap may be scaled up, if the dimensions are multiplied by a whole number, for instance twice the natural size or a 200% enlargement.



• Going Down . . .
  A bitmap may be scaled down, if the dimensions are divided by a whole number, for instance half the natural size or by 50%.

Note Of course, the aspect ratio must be maintained when resizing a bitmap.

Finally, note that the physical size of a bitmap reproduction will vary according to the resolution of the output device. Therefore a given bitmap will appear physically smaller on a display of higher resolution. When creating bitmap images for screen display it is therefore important to consider the required size of the eventual image, and how this may vary on screens of different resolution.