The 8th ACM/EG Expressive Symposium EXPRESSIVE 2019 C. Kaplan, A. Forbes, and S. DiVerdi (Editors) Irregular Pebble Mosaics with Sub-Pebble Detail Ali Sattari Javid, Lars Doyle, and David Mould Carleton University, Ottawa, Canada Figure 1: Our pebble mosaic algorithm renders details through variable tile sizes and textures that hint at the underlying image content. Left: abbreviated algorithm pipeline showing source, pebble layout detail, and final result; right: additional result. Abstract Pebble mosaics convey images through an irregular tiling of rounded pebbles. Past work used relatively uniform tile sizes. We show how to create detailed representations of input photographs in a pebble mosaic style; we first create pebble shapes through a variant of k-means, then compute sub-pebble detail with textured, two-tone pebbles. We use a custom distance function to ensure that pebble sizes adapt to local detail and orient to local feature directions, for an overall effect of high fidelity to the input photograph despite the constraints of the pebble style. CCS Concepts • Computing methodologies → Non-photorealistic rendering; Image processing; 1. Introduction Pebble mosaics are an artform with roots in the ancient world yet still used today. Unlike tesselated mosaics, characterized by a uni- formity and regularity of the tesserae used to construct them, peb- ble mosaics use rounded stones with potentially a high degree of variability in size and shape. This paper seeks to create a digital version of a pebble mo- saic from an input photograph. Previously existing specialized mo- saic creation systems do not yet deal with highly heterogeneous tile creation, and general stylization systems based on neural net- works struggle with creating distinct tile boundaries, often yielding blurred or malformed tiles. Two sample results, and a summary of our process, are shown in Figure 1. Our process has two main phases: pebble creation and pebble detailing. First, we coarsely approximate the image with rounded regions generated with an algorithm akin to k-means. Second, we add color and texture to each pebble to provide image detail at sub- pebble resolution. Like recent work on pebble mosaics [DACMar], this work bridges the gap between realistic and non-photorealistic graphics, computing plausible-looking pebble shapes and textures while abstracting an input photograph. This paper improves on its predecessor in two main ways. First, it achieves greater irregularity in the shape and size of pebbles, automatically adapting to local image detail. Second, it improves on the fidelity of the mosaic to the original image, incorporating sub-pebble detail by using two-tone pebbles and selecting pebble texture with reference to the image content in the pebble region. Both changes dramatically improve the appeal of the final result. c  2019 The Author(s) Eurographics Proceedings c  2019 The Eurographics Association. DOI: 10.2312/exp.20191084 https://diglib.eg.org https://www.eg.org