Common image formats can be grouped into one of two categories:

• vector graphics (line drawings), and
• bitmaps (raster graphics).

The first describes an image by a collection of lines, curves (usually described by cubic polynomials), and more complex discrete mathematical objects; while the second stores an image pixel by pixel.

Vector Graphics

Simple vector images can be created using the vector graphic tools found in PowerPoint; however, these are very primitive. Tools such as CorelDRAW and Adobe Illustrator manipulate vector graphics much more effectively. All line drawings found on the ECE 250 web site were created using CorelDRAW. The vector graphics created in PowerPoint are not saved as separate files and consequently we will

Figure 1. Some of the vector graphics which can be created in PowerPoint.

Bitmap

The most common format of image is a bitmap (or, more correctly, a raster graphic) which stores a rectangular grid of pixels where each pixel is associated with a colour or shade of grey. There are two general categories of raster graphics used in presentations:

• Photographs, and
• Converted vector graphics (graphics).

The second includes line drawings, clip art, charts, graphs, schematics, flow charts, as well as other images. When a graphic is exported from Excel, CorelDRAW, Maple, or any other image generator, it usually is converted from an internal mathematical description of the object into a bitmap.

There are three common bitmap image formats:

• JPEG/JPG (Joint Photographic Experts Group)
• PNG (Portable Network Graphic, ping)
• GIF (Graphic Interchange Format)

The name of each of these should be a clear indicator as to what type of image should be stored:

• Photographs should be stored with JPEG, while
• Graphics should be stored with PNG or GIF.

The next three subsections will describe the three formats.

JPEG

The JPEG is a lossy 24-bit image format which uses a discrete cosine transform on 8×8 blocks of pixels and removes the higher frequencies. This uses two properties which are specific to photographs:

• The eye is less sensitive to higher frequencies, and
• Photographs are generally smooth (low frequency).

JPEG allows the user to specify the quality of image and the lower the quality, the more information will be lost and the less exact the stored representation of the original. The quality Q goes from a scale of 100 (insignificant differences) to 0 (high distortion). Some applications define a compression factor as 100 − Q.

The term 24 bit indicates that twenty four bits of information are used to store the colour of each pixel.

PNG

The PNG format is a lossless 24-bit image format. It will faithfully recreate the original 24-bit image when uncompressed. PNG was a the result of a significant collaborative effort in the 1990s when Unisys began enforcing its patent on GIF. PNG is also significantly superior to GIF in most respects.

GIF

The GIF format is a lossless 8-bit image format where up to 256 (= 2⁸) separate 24-bit colours may be used within the image. The lossless only refers to the compression used: by reducing the colour space to only 256 colours, this can significantly and adversely affect many images.