Evaluating perceptual performance with pixel-based tactile images

Relief based tactile refreshable displays that are currently on the market are composed of pixel-based elements. While current devices mainly focus on making text accessible by raising pixels, some are capable of displaying tactile images as well. The dimensions of the pixels used in these devices are often based on Braille dimensions, which may not be optimal for image recognition. This study systematically investigates how pixel diameter, spacing between pixels, and whether the pixels are equidistant or not affect perceptual performance in recognizing pixelated tactile images. In Experiment 1, we varied pixel diameter (0.5 mm, 1.0 mm, 1.5 mm) and edge-to-edge spacing (0.5 mm, 1.0 mm) across equidistant grids, and found that smaller edge-to-edge spacing significantly improved recognition, with a stronger effect for larger pixels. In Experiment 2, we compared equidistant and non-equidistant (Braille-based) pixel configurations. We created a fair comparison by keeping the total number of pixels in the grid constant across conditions. Recognition rates were significantly higher for equidistant configurations. Exploratory analysis of the confusion matrices revealed that images with curved lines or oblique features were misidentified more often, especially under low-resolution and non-equidistant conditions. These findings highlight the limitations of Braille-based configurations for image rendering and offer design guidelines for future tactile displays that aim to support both text and images.