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13.4.2.4.2 Multiple Windows
The modes discussed above give the user a single magnified window (which may be the full screen). There are circumstances where a user wishes to magnify more than one area of the screen. For example, a user may wish to work in one application, whilst keeping an eye on the system’s status bar. This can be achieved in a multiple window system, in which the user can define multiple windows. Windows may be static (i.e. they magnify a fixed area of the screen, they do not track focus) or dynamic (they track focus).
A good example of where this mode could be used is when a user interacts with a spreadsheet (for example Microsoft Excel). As the user moves from cell to cell (using the cursor keys), not only is the client data updated, but also the highlighting of the row and column headers and the formula (fx) area are also updated (see Figure 13.1). If the user is using full screen magnification, it is likely that they will not be able to see the row and column headers and the formula area at the same time as viewing a cell—there is not enough room in the enlarged image. However, if the user could define three additional windows (one for each of the formula area, the column header and the row header) and these could be presented in the same enlarged image as the cell, the user can view all the relevant information on the same screen. The user will not have to leave the cell that they are working in and search for the formula area. This should greatly increase user efficiency. The formula area can be handled in a different way to the row and column headers. The interesting part of the formula area is fixed on the screen and this should be made a static area (one that does not change its view with focus changes). The row and column headers should be made active areas, which track changes in focus so that the highlighted row/column number/letter is always presented. This style of interface should greatly aid a user when they need to view information from different parts of the original image at the same time. However, it is not a trivial operation for a visually impaired user to configure such as system—determining where to place the windows and what characteristics to give them can be difficult. It is suggested that this style of interface is best used by people who use complex applications on a regular basis where configurations for particular applications can be stored and recalled.
13.4.3 Other Interface Considerations
In addition to the screen magnification modes, there are a small number of interface considerations that need to be presented; these are:
•Behaviour of the mouse pointer in the enlarged area.
•Smoothing of text.
•Colour inversion.
•Controlling the magnifier through other means than the mouse and keyboard.
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13.4.3.1 Mouse Pointer Behaviour
A screen magnifier will track the mouse pointer as it moves and will place the mouse pointer roughly in the centre of the magnified area. Suppose that full screen magnification is being used and the user moves the mouse pointer a short distance. The designer of the screen magnifier has two options. First, the enlarged image can be moved so that the pointer is always at the centre of the screen. Of course, at the edges of the original image, the pointer cannot be centralised in the enlarged image because this would mean that the enlarged image would contain an area that was not present in the original image. In this style of behaviour, every movement of the mouse (or indeed anything else that causes the focus to change—for example typing text) will cause the image to shift (see Figure 13.12).
The second approach is to move the mouse pointer in the enlarged image and not to shift the enlarged image when the mouse movement is small, i.e. it does not move close to the edge of the enlarged image. In this case, the enlarged image remains static and the mouse pointer moves (see Figure 13.13).
In our experience, users prefer the second type of behaviour—i.e. scrolling the enlarged image only when the mouse pointer moves close to the enlarged image’s edge. The reason for this is that in the first style of interaction, the enlarged image is moves whenever the mouse pointer moves. This means that the user is presented with a constantly changing image, which can prove very tiring. Allowing the enlarged image to remain constant when mouse pointer movements and other changes in focus are relatively small (i.e. well within the enlarged image) means
Figure 13.12a,b. Example of mouse movement. This shows the mouse pointer in a has been moved to the right in b. The display has scrolled to the right. The mouse pointer has centralised the image during the movement
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Figure 13.13a,b. Second example of mouse movement. The mouse pointer in a has been moved to the left. It can be seen in b that the mouse pointer has moved rather than the image (contrast with Figure 13.12)
that the user is presented with a much more stable image that is less tiring to use. Most commercial magnifiers use this style of interaction, but Microsoft Magnifier does not (see the project P.1).
13.4.3.2 Smoothing
As noted earlier, when text is enlarged, it can appear blocky; see Figure 13.2. Many users prefer to read text when it is smoothed and most screen magnifiers provide an option for smoothing. An example of unsmoothed and smoothed text is presented in Figure 13.14.
Figure 13.14a,b. Example of smoothing. a shows no smoothing (note the ‘w’ and the mouse pointer). b shows smoothing (note the slightly ‘odd’ formation to some characters). Enlargement level 6×
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You should note the reduction in aliasing and the relative smoothness of the text. You will also perhaps note that the smoothed text is not as clear as if the original text had been presented in a larger font (rather than magnified and then smoothed) and that some artefacts of the smoothing process make some aspects of the characters look a little unusual, e.g. the v and the w in Figure 13.14. The reason for this is that the algorithms that carry out smoothing operate directly on a bitmap representation of the original image—they are image processing techniques—and consequently do not have access to the information that indicates which character is being displayed and thus cannot simply replace a character with its equivalent in a larger font. If screen magnifiers used some of the techniques employed by screen readers (see later), the identity of characters could be obtained. However, it is unlikely that simply enlarging the font size of any text elements would give an entirely faithful representation of the screen and would not address the enlargement of elements of the screen that were not text so this approach is not used by screen magnifiers.
Few details of smoothing algorithms for screen magnifiers are published— the only instance known to the authors is that in (Blenkhorn et al. 2002). Most algorithms have to be able to distinguish between the foreground colour (i.e. the text colour) and the background colour and there are often techniques for user intervention to support this. Users must be able to turn smoothing on and off. Smoothing is aimed at tidying up the presentation of text and can sometimes distort images.
13.4.3.3 Colour Inversion
Most magnifiers (including Microsoft Magnifier—see the project P.1) provide the user with the option to invert colours. This is provided because some visually impaired people have conditions where white text on a black background is preferred to the more prevalent representation of black text on a white background. Colour inversion works well on a monochrome image (see Figure 13.15) where black becomes white and vice versa. However, it produces different results—which may be less useful to users—on colour images, where for example blue can become yellow or peach. Figure 13.16 shows an example of inversion on coloured images. You can explore the effect yourself using Microsoft Magnifier (see the project P.1).
Figure 13.15. Colour inversion monochrome