Screen resolution

Bitmapped images are defined spatially by how many dots (pixels) they contain horizontally or vertically and how many colours the image contains. For example, a VGA screen is 640 dots across by 480 dots in height. This is known as screen resolution. The number of colours is referred to as colour depth and is dealt with in the next subsection.

The majority of images captured will have a greater number of pixels in width and height than can be displayed on the screen. Thus they will need resizing to a resolution suitable for inclusion in the learning material. The graphic capabilities of the delivery hardware must be taken into consideration when carrying out this process (see table 2.2).

Graphics card       Pixels (screen  No. of colours      Comments  
                      resolution) 

EGA                 640 x 350           64            No longer recommended  
VGA                 320 x 200          256            Majority of computers 
VGA                 640 x 480           16                   are VGA
SVGA, Level 1       640 x 480          256   
SVGA, Level 2       800 x 600          256            Now standard recommendation. 
SVGA, Level 3      1024 x 768          256                minimum level 1 

Whether the graphics capabilities are CGA, EGA, VGA, or one of the SVGA levels, it is essential that illustrations be always of the highest possible standard. This is of particular importance in higher or adult education where students might take the material less seriously if illustrations are of poor quality. It is also important not to confuse quality of image with complexity.

Finally, there may be occasions when you wish to enlarge a particular section of an image. Enlarging or zooming in on a bit-mapped image is not successful as the individual pixels are enlarged, giving the image a blocky appearance. In these instances it is far better to zoom in on the image before it is captured. With a camera you will need the appropriate lenses. If scanning, scan at a high resolution. The required area can then be re-sized to the appropriate screen resolution.

Again, this will depend on the type of graphics card in the delivery machines. Table 2.2 shows the number of colours available, depending on the graphics card and the screen resolution used.

As we already know, bit-mapped images are represented by a number of pixels. These pixels are given a colour (or grey) value. Images may sometimes be referred to as 8 bit images, or 256 colour images. But what does this mean?

A 1 bit image, that is, an image represented by one bit per pixel, will have two values or colours per pixel; 0 or 1, off or on, black or white. A 2 bit image will have 2 values or colours per pixel; 0,1; 1,0; 0,0; 1,1. That is four combinations of the values 0 and 1 or 22 = 4 colours. A 4 bit image will thus be 2 4 = 16 colours and so on, as shown in Table 2.3.

For photorealistic images, 256 colours per pixel must be available. Furthermore, if images larger than 320 by 200 pixels are required then a minimum of SVGA, level 1, is needed, which will mean ensuring that all delivery machines have SVGA graphic capabilities. New machines are now supplied with SVGA graphic cards installed, often as part of the computer motherboard.

No. of bits per pixel        No. of colours  
     1 bit                     1^2  = 2
     2 bit                     2^2  = 4
     4 bit                     2^4 = 16
     8 bit                     2^8 = 256
    16 bit                    2^16 = 65,500
    24 bit                    2^24 = 16.7 million  

Colour Reduction and Diffusion

Palettes

Palettes only work if displaying one image at a time on different screens and even then you will get palette flash as palettes are swapped in and out of the video RAM. This is the primary limitation of working with 8 bit colour displays. If this situation is not acceptable and there is a large requirement for displaying more than one image at the same time, then 16 bit graphic cards are needed and users should be encouraged to purchase these. These are now being supplied as standard in new PCs and Macs.

Another way around the problem is to make a common palette for all your images. Some of the more sophisticated image processing software allows palette manipulation and often provide a number of palettes which you can apply. Two such programs for the PC are Windows PalEdit and BitEdit. These tools are included as part of Microsoft's Multimedia Developers Kit. PalEdit will save a palette associated with one image to a file; BitEdit can then be used to apply the palette file to a range of images. These tools can also be used to create a common palette from a number of images with differing palettes.

Here are a few guidelines that can be given when designing courseware with images for use 8 bit graphic displays:-

• Always display important images singly to make full use of the palette. In these instances, the palette can be optimised manually to gain maximum quality.
• Try to use images without areas of unwanted colour. For example, remove background information and borders - use only the object, photograph objects on a plain background, avoid mixing different types of material on the same image.
• Avoid sequences which require the changing of palettes while images are on screen. Palette flashing does not look very professional although it may be acceptable under some circumstances.
• Mask the image to black before bringing in a second image to prevent palette flash.
• Use delivery machines with 24 bit display capabilities if the above are unavoidable

Displaying Images

Photo CD

A professional version of PhotoCD, Pro PhotoCD, allows the scanning of 5 x 4" transparencies. The CD is able to hold up to 27 of these images with 6 levels of resolution, the highest being some 3000 x 4000 pixels.