On-screen keyboard eye-typing systems are limited due to the lack of predictive text and user-centred approaches, resulting in low text entry rates and frequent recalibration. This work proposes integrating the prediction by partial matching (PPM) technique into a tree-based virtual keyboard. We developed the Flex-Tree on-screen keyboard using a two-stage tree-based character selection system with ten commands, testing it with three degree of PPM (PPM1, PPM2, PPM3). Flex-Tree provides access to 72 English characters, including upper- and lower-case letters, numbers, and special characters, and offers functionalities like the delete command for corrections. The system was evaluated with sixteen healthy volunteers using two specially designed typing tasks, including the hand-picked and random-picked sentences. The spelling task was performed using two input modalities: (i) a mouse and (ii) a portable eye-tracker. Two experiments were conducted, encompassing 24 different conditions. The typing performance of Flex-Tree was compared with that of a tree-based virtual keyboard with an alphabetic arrangement (NoPPM) and the Dasher on-screen keyboard for new users. Flex-Tree with PPM3 outperformed the other keyboards, achieving average text entry speeds of 27.7 letters/min with a mouse and 16.3 letters/min with an eye-tracker. Using the eye-tracker, the information transfer rates at the command and letter levels were 108.4 bits/min and 100.7 bits/min, respectively. Flex-Tree, across all three degree of PPM, received high ratings on the system usability scale and low-weighted ratings on the NASA Task Load Index for both input modalities, highlighting its user-centred design.

翻译：屏幕键盘视线输入系统因缺乏预测文本和以用户为中心的设计方法而受限，导致文本输入速率低下且需频繁重新校准。本研究提出将部分匹配预测（PPM）技术集成至树结构虚拟键盘中。我们开发了采用两级树结构字符选择系统（包含十个指令）的Flex-Tree屏幕键盘，并测试了三种PPM阶数（PPM1、PPM2、PPM3）。Flex-Tree可访问72个英文字符，包括大小写字母、数字及特殊符号，并提供删除指令等纠错功能。该系统由十六名健康志愿者通过两种专门设计的输入任务进行评估，包括人工选取句和随机选取句。拼写任务采用两种输入模态执行：（i）鼠标；（ii）便携式眼动仪。共开展两项实验，涵盖24种不同条件。将Flex-Tree的输入性能与字母排列的树结构虚拟键盘（NoPPM）以及面向新用户的Dasher屏幕键盘进行对比。采用PPM3的Flex-Tree表现最优，使用鼠标时平均文本输入速度达27.7字母/分钟，使用眼动仪时达16.3字母/分钟。眼动仪输入时，指令层级和字母层级的信息传输速率分别为108.4比特/分钟和100.7比特/分钟。在所有三种PPM阶数下，Flex-Tree在两种输入模态中均获得较高的系统可用性量表评分和较低的NASA任务负荷指数量表加权评分，彰显了其以用户为中心的设计理念。