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3     Description of the RFV

The Restricted Focus Viewer (RFV) is a computer based tool for tracking visual attention. Its design is in part based on the human visual system, which can only focus on objects at the centre of the visual field. The region surrounding this area of sharp focus is still perceived, but the further from the centre of the visual field an object is, the more coarse is the perception of it.

The design of the RFV attempts to reflect this idea through the use of image blurring. The RFV displays a blurred stimulus image on a computer monitor, allowing the participant to see only a small region of the stimulus in focus at any time. The region in focus, called the focus window, can be moved using the computer mouse. The RFV records what the participant is focusing on at any point in time, and the data can be played back using the Replayer program.

3.1     The Focus Window

The focus window of the RFV is the region in which the stimulus is visible in full detail. In order for the focus window to look `natural', a graded blurring effect is needed, such that the transition from blurred to focus appears smooth and seamless.

A graded blurring effect is achieved by the technique illustrated in Figure 1. The outer rectangle defines the stimulus area which is fully blurred. The innermost box is the region of focus. Surrounding this focus region are three transition regions. Each transition region is slightly more blurred than the last, so that there is only a subtle difference between neighbouring regions. The overall result is the appearance of a smooth transition from the region of the image in focus, to the region which is fully blurred. Using the mouse to move the focus window therefore moves not only the focus region, but also the three transition regions.

Focus Window Boxes            
  • FR     -     Focus Region
  • T1     -     Innermost Transition Region
  • T2     -     Middle Transition Region
  • T3     -     Outermost Transition Region
  • BR     -     Blurred Region

Figure 1.   Regions of the stimulus used to achieve the graded blurring effect.

It is clear then that apart from the stimulus image in full focus, four other images are needed for the different levels of image blurring in the blurred and transition regions. At the centre of the focus window there is also a small black dot, to allow users to keep track of the focus window location when it is centred on an empty region of the image.

3.2     Stimulus Images and Blurring

The aim of the blurred region is that it should still be possible to perceive the broad structure of the original stimulus image. The individual components and finer details in the stimulus should be indiscernible, requiring the user to move the focus window over that area of the stimulus in order to determine exactly what is there. However, it should not be so blurred that the user has difficulty in navigating from one stimulus component to another. Figure 2 gives an example with an algebra expression as the stimulus, and the corresponding blurred image in which the symbols are indiscernible.

Example Visual Stimulus

Figure 2.   Example of a visual stimulus and its corresponding blurred image.

Different kinds of images have different spatial properties. Thus, different techniques are required in order to produce a blurred image that obscures the finer details, while still allowing the general form of the image to perceived. Since a human observer is needed to verify if an appropriate level of image blurring has been obtained, the RFV does not do any image blurring itself, but rather reads in image files that have already been blurred.

For each stimulus to be presented, five image files need to provided. These are the image proper (that is, the image in full focus), the image with the appropriate level of blurring for the innermost transition region, the images for middle and outermost transition regions, and finally the fully blurred image. Figure 3 gives an example of the five images used to present the stimulus shown in Figure 2, with the fully focused image at the top and fully blurred image at the bottom. Each of the three transition region images in between are slightly more blurred than the image above it. The RFV program dynamically combines these images to produce a smooth transition from the blurred region to the region in full focus (using the method described in the previous section). Figure 4 gives two examples of the focus window in different positions over the stimulus shown in Figure 2.

Five Stimulus Images

Figure 3.   An example of the five images needed to present a stimulus.

Focus Window Examples

Figure 4.   Two examples of the focus window on different regions of the stimulus.

As was mentioned before, different techniques are required to successfully blur different types of images. Consider for example, mathematical equations and circuit diagrams as stimuli. Mathematical equations usually are composed of a closely packed group of symbols. A standard blurring algorithm, which can be found on most modern computer graphics programs, is generally sufficient to successfully blur such a stimulus. (For example, on systems running Windows, the program Paint Shop Pro has a filter which allows images to be blurred. On Unix systems, the program XV has a blur algorithm.) Different levels of blurring can be achieved by varying the blur radius, or by running the blur algorithm on the image more than once.

Circuit diagrams on the other hand usually have large empty regions, and single lines representing wires that are hard to blur. One approach to effectively blur such images is to pixelize the image first (that is, decrease the image resolution without changing the image size, which again can be done by using a pixelize algorithm found on most computer graphics programs), and then run a standard blurring algorithm. By varying the size of pixelization, and the amount of blurring, and running these algorithms multiple times on a stimulus image, it is possible to obtain various levels of blurring that allow the general form of the diagram to still be perceived, but leave the individual components indiscernible.

3.3     Motion Blur

Another feature that was implemented so that the RFV would more accurately mimic the way humans perceive visual stimuli is motion blur. If the user of the RFV moves the mouse at high speed (that is, over a large distance on the screen in a small amount of time), the focus window will not achieve full focus. Once the user reduces the speed of the mouse motion back to below a certain threshold, or stops moving the mouse completely, full focus in the focus window will return. This feature helps in defining the temporal boundary between fixations and movements.

When the focus window is stationary or moving slowly, all of the regions listed in Figure 1 are present. During motion blur however, only the outermost transition region is added to the blurred stimulus. Because this region has less blurring than the rest of the image, the user is still able to track the location of the focus window on the stimulus. However, it is not possible to determine the finer details of that location without slowing or stopping the mouse. Only then will full focus be available.

3.4     Guidelines for Setting Parameters

When designing an experiment that will use the RFV, there are several important parameters that the experimenter must set. Below are some basic heuristic guidelines for setting these parameters. These are suggestions based on previous experience in using the RFV, but by no means are they intended to be strict rules for parameter settings.

Level of Blurring
The goal of blurring the stimulus is to generate an image where it is difficult to identify the finer details. The minimum level of image blurring should be sufficient that any two stimulus elements are indistinguishable, and that the connections between elements cannot be established. It should only be possible to accurately identify an element if that element is in the focus region. Of course, for different types of stimuli the size of the elements will vary, and thus different levels of blurring will be necessary. The maximum level of blurring should still allow identification of the stimulus boundaries (at least the convex hull). This is to ensure that participants can still navigate from one region of interest in the stimulus to another.

Focus Window Size
The central focus region of the focus window should allow identification of a single element of the stimulus. Thus it should not be so small that identification is difficult when the focus window is centred over an element. It also should not be so large that it allows the simultaneous identification of two or more neighbouring elements, since this will make it difficult for the experimenter to determine which element the user was focusing on. Generally, the focus region should be slightly smaller than the bounding box of a typical stimulus element. Each transition region should be only slightly larger than the region within it. The role of the transition region is to provide a smooth transition from focused to blurred, and to indicate the direction of neighbouring connected elements.

Motion Blur Speed
The motion blur feature allows for a distinction to be made between movements and fixations. The threshold speed should not be so high that users can use a `brass rubbing' strategy. That is, identifying the stimulus by rapidly moving the focus window over it. However, it should also allow for slow navigation between connected stimulus elements. The mouse speed at which motion blur onset occurs will depend largely on the type of stimulus and the nature of the task. For example, if the stimulus is a circuit diagram, than the motion blur speed should be high to allow the user to trace the path of the wires without losing focus. If instead the stimulus is mathematical equations, then motion blur onset should occur for much slower mouse speeds.

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