Impact of Edge Opacity on Readability in Node-link Diagrams

Node-link diagrams are widely used to visualize network structures, but their readability often suffers from edge crossings and visual clutter. This study quantitatively evaluated how edge opacity affects performance in a pathfinding task. We independently manipulated the opacity of focus edges, which are incident to two target nodes, and interfering edges, which intersect the focus edges. A common opacity was set for the focus edge and non-interfering edge, whereas a different opacity was set for the interfering edge. A total of 32 participants performed 1,541 trials across all 100 conditions. Analysis revealed that increasing the opacity of the focus edge and non-interfering edge shortened reaction time, whereas increasing the opacity of the interfering edge lengthened reaction time. No interaction effect between the two was observed. These effects were generally linear, allowing the derivation of a simple explanatory formula that characterizes response times as a function of opacity settings. The findings demonstrate that carefully adjusting edge opacity can improve visibility and reduce visual interference in network visualizations.