Playing cards have long been a central feature in board games, despite their physical limitations, such as fragility, and repetitive content. Yet, while HCI research increasingly explores augmented board games, interactive playing cards remain underexplored. In this work, we investigate the use of e-paper displays as interactive playing cards. Based on a formative study with eight game designers, we designed and implemented InkStack---a prototype consisting of e-paper cards, a dedicated programmer, and a web application for card customisation. We then evaluated InkStack in a within-subject study with $n = 20$ participants, comparing its use across four board game mechanics against traditional paper cards and a smartphone. Results show that InkStack is preferred for more complex mechanics, whereas paper and smartphones are sufficient for simpler tasks. The findings also highlight how customisation and versatility can enhance gameplay and enable novel forms of interaction.
ACM CHI Conference on Human Factors in Computing Systems