CHI 2005 ׀ PAPERS: Smart Interaction Techniques 1 April 2–7 ׀ Portland, Oregon, USA The Bubble Cursor: Enhancing Target Acquisition by Dynamic Resizing of the Cursor’s Activation AreaTovi Grossman, Ravin Balakrishnan Department of Computer Science University of Toronto tovi | ravin @dgp.toronto.edu www.dgp.toronto.edu ABSTRACT We present the bubble cursor – a new target acquisition technique based on area cursors. The bubble cursor improves upon area cursors by dynamically resizing its activation area depending on the proximity of surrounding targets, such that only one target is selectable at any time. We also present two controlled experiments that evaluate bubble cursor performance in 1D and 2D target acquisition tasks, in complex situations with multiple targets of varying layout densities. Results show that the bubble cursor significantly outperforms the point cursor and the object pointing technique [8], and that bubble cursor performance can be accurately modeled and predicted using Fitts’ law. ACM Classification Keywords: H.5.2 [User Interfaces]: Graphical User Interfaces, Theory and methods, Interaction styles. Keywords: Area cursor, Bubble cursor, target acquisition, Fitts’ law. INTRODUCTION Pointing to targets is a fundamental task in graphical user interfaces (GUI’s). As software gets more complex with an increasing number of selectable user interface elements being crammed into finite sized displays, improvements in pointing performance can have a significant impact on overall software usability. Recognizing this challenge, researchers have proposed several techniques [3-5, 8, 9, 14, 15, 17] that attempt to improve pointing performance by exploiting the fact that virtual pointing can surpass physical pointing by manipulating the control-display parameters. With few exceptions [8], most of these new techniques have been shown to improve pointing only in situations where targets are fairly sparsely distributed across the display space. When targets are more closely packed together, as is common in many current GUIs, the benefit of these techniques tend to degrade, and can even be detrimental, thus resulting in no advantage in the general case. In an effort to improve on these previously suggested pointing facilitation techniques we present the bubble cursor, a new technique based upon area cursors [9, 15]. While a standard point cursor has a single point of activation or hotspot, area cursors have larger hotspots defined by the boundary of the cursor (Figure 1a). Problems arise, however, when the area cursor encompasses more than one target, making it difficult to isolate the intended target (Figure 1b). The bubble cursor solves this problem of the area cursor by dynamically updating its size based on the proximity of surrounding targets, such that there is always exactly one target inside the hotspot (Figure 1c,d). In the following sections, we will review previous efforts at pointing facilitation; discuss the design and implementation of the bubble cursor; evaluate the performance of the bubble cursor in two experiments: first in a simple 1D pointing task and second in a multi-target 2D pointing task with varying target densities; show that the bubble cursor’s performance can be modeled accurately by Fitts’ law; and conclude by discussing implications for user interface design and future lines of work. Figure 1. (a) Area cursors ease selection with larger hotspots than point cursors. (b) Isolating the intended target is difficult when the area cursor encompasses multiple possible targets. (c) The bubble cursor solves the problem in (b) by changing its size dynamically such that only the target closest to the cursor centre is selected. (d) The bubble cursor morphs to encompass a target when the basic circular cursor cannot completely do so without intersecting a neighboring target. Note that the same legend is used for Figures 1 to 5. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. CHI 2005, April 2–7, 2005, Portland, Oregon, USA. Copyright 2005 ACM 1-58113-998-5/05/0004...$5.00. 281CHI 2005 ׀ PAPERS: Smart Interaction Techniques 1 April 2–7 ׀ Portland, Oregon, USA RELATED WORK The common approach for studying new selection techniques is to use Fitts’ law [6, 11], a highly successful model for predicting movement time in a pointing task. Fitts’ Law states that the time (MT) to acquire a target with width W and distance (or amplitude) A from the cursor can be predicted by the equation: ++= 1log2WAbaMT where a and b are empirically determined constants. The logarithmic term is the index of difficulty (ID) of the task. It can be seen from Fitts’ Law that if a target’s size decreases, or the distance needed to travel to acquire the given target increases, then the time taken to select it increases. Thus target selection can be facilitated by increasing the target width [5, 9, 14, 15, 17], decreasing the amplitude [3, 8], or both [4]. We now review such selection techniques. Decreasing A An interesting attempt to directly reduce A is the drag-and-pop technique developed by Baudisch et al. [3]. In this technique, the system analyzes the directional movements of the cursor and temporarily brings virtual proxies of the potential targets towards the cursor. While this technique was found to be beneficial on a large display for very large A, it can be tricky to determine when the user intends to select the remote elements versus items that are in the nearby vicinity. Falsely activated proxies may annoy or even impede the user’s selections. As a result, this technique tends to work best in an environment with a relatively sparse layout of targets. An alternative to bringing the target closer to the cursor is to jump the cursor to the target. Guiard et al. [8] designed a promising interaction technique, called object pointing, where the cursor skips across the empty space which can exist between targets, jumping from one selectable target to another. Object pointing was found to be