Target expansion techniques facilitate the pointing task by enlarging the effective sizes of targets. When the target expansion is applied to both the motor and visual spaces, the visual feedforward mechanism is key: Indeed it provides a visual aid to the user on the effective expanded targets prior to the execution or completion of the pointing task, enabling the user to take full advantage of the target expansion technique. Focusing on feedforward mechanisms, we introduce a design space that allows us to describe, classify and design target expansion techniques. To do so we first introduce and characterize the concept of atomic feedforward mechanism along three design axes. We then describe a target expansion technique as a combination of atomic feedforward mechanisms using a matrix-based notation. We provide an analytical exploration of the design space by classifying existing techniques and by designing six new techniques. We also provide a first experimental exploration of the design space in the context of distant pointing. The experimental protocol includes an innovative target layout for handling non-centroidal target expansion. The results show that feedforward dynamicity increases movement time and decreases subjective usability, while explicit expansion observability efficiently supports error prevention for distant pointing.

Addressing the challenges of distant pointing, we present the feedforward static targeting assistance technique VTE: Voronoibased Target Expansion. VTE statically displays all the activation areas by dividing the total screen space into areas such that there is only one target inside each area, also called Voronoi tessellation. The key benefit of VTE is in providing the user with an imm before the pointing task even begins: VTE then provides static targeting assistance for both phases of a pointing task, the ballistic motion and the corrective phase. With the goal of making the environment visually uncluttered, we present a first user study to explore the visual parameters of VTE that affect the performance of the technique. In a second user study focusing on static versus dynamic assistance, we compare VTE with Bubble Ray, a dynamic Voronoi-based targeting assistance technique for distant pointing. Results show that VTE significantly outperforms the dynamic assistance technique and is preferred by users both for ray-casting pointing and relative pointing with a hand-controlled cursor.

To enhance pointing tasks, target expansion techniques allocate larger activation areas to targets. We distinguish two basic elements of a target expansion technique: the expansion algorithm and the visual aid on the effective expanded targets. We present a systematic analysis of the relevance of the visual aid provided by (1) existing target expansion techniques and (2) Expansion Lens. The latter is a new continuous technique for acquiring targets. Expansion Lens namely, uses a round area centered on the cursor: the lens. The users can see in the lens the target expanded area boundaries that the lens is hovering over. Expansion Lens serves as a magic lens revealing the underlying expansion algorithm. The design rationale of Expansion Lens is based on a systematic analysis of the relevance of the visual aid according to the three goal-oriented phases of a pointing task namely the starting, transfer and validation phases. Expansion Lens optimizes (1) the transfer phase by providing a simple-shaped visual aid centered on the cursor, and (2) the validation phase regarding error rates, by displaying the target expanded area boundaries. The results of our controlled experiment comparing Expansion Lens with four existing target expansion techniques show that Expansion Lens highlights a good trade-off for performance by being the less-error prone technique and the second fastest technique. The experimental data for each phase of the pointing task also confirm our design approach based on the relevance of the visual aid according to the phase of the pointing task.