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Screen resolution

What screen resolution will the computers on which I will be delivering the learning material be running at?

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
Table 2.2 showing various graphic cards and the resolution and number of colours obtainable by each

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.

Number of Colours How many colours can be displayed at any one time on the computers on which I will be delivering the learning material?

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  

Table 2.3 showing the relationship between number of bits and number of colours

Colour Reduction and Diffusion

For images captured at 24 bit (that is, containing 16.7 million colours) to be displayed properly on machines capable of only displaying 256 colours, the number of colours per image has to be reduced to 256. This process is carried out using image processing software. During this process a palette is created containing the best 256 colours from the 16.7 million available (see next section also). The resultant 256 colour images may often be referred to as indexed colour images. However, sometimes the image shows distinct borders between one colour and the next, giving a blocky appearance. By processing the image further, by applying a complex process known as diffusion (also referred to as dithering), the colour content of neighbouring pixels is taken into account when deciding what colour to assign to any given pixel in the diffused file. Although the results are good (smoother), resolution and hence detail are sacrificed.

Palettes

Palettes are something that only needs to be dealt with when using 256 (8 bit) colour displays. The palette of one image will be different to that of another and problems arise when trying to display more than one image on the same screen because only 256 distinct colours can be displayed at any one time. Thus, if one image uses palette A, to display another image with palette B, the current palette colours must change. After the change, image B will look fine, but image A will be shown with palette B and hence will appear corrupted. This is because the graphics card can only accommodate one palette at a time in the memory (video RAM) contained on the graphics card.

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

Images, especially colour reduced ones, will not always look the same when displayed on a variety of monitors. This is due to different phosphors, colour temperatures and variations in the RGB guns. Another factor affecting the display of an image is lighting. Daylight tubes should be fitted in rooms housing imaging workstations or CBL labs. Some computer software may also hog the colour palette. Monitors can be calibrated in hardware and software. Companies such as KODAK and AGFA produce calibration packages and light sensors are available to read the light output of a monitor.

Photo CD

If you require image capture for a few images (not thousands) PhotoCD may be the answer. PhotoCD provides a mechanism for transferring your normal photographic film or 35mm transparencies onto compact disc at very high resolutions. The more familiar high street film processing outlets will transfer your film for as little as 12.99 for 24 exposures or 16.99 for 36 exposures plus a single payment of 5.00 for the CD. Additional films may be added at a later date, the CD holding up to 100 images. Each image (or image pack) comprises 5 levels of resolution, the highest being 3072 by 2048 pixels. The CD also comes with the equivalent of a photographic contact sheet, showing a tiny print or "thumbnail sketch" of each of your images. PhotoCDs can be read in dedicated PhotoCD players that plug into standard television sets. They can also be read in CD-ROM drives but the drive needs to be XA compatible to read them. If you intend to add more images at a later date to create a multisession disc, the drive must also be multisession compliant. The majority of the common image processing programs are now capable of reading the PhotoCD format. Alternatively, Kodak have their own software, PhotoCD Access, which will access the images at any of the five resolutions and allow you to convert them into the common formats for inclusion in your authoring software.

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.


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