Abstract: Mosaic is a sketch-based system that simplifies and automates the creation of digital decorative mosaics from scratch. The creation of each tile piece of unique shape, color and orientation, in a complex mosaic is a tedious process. Our core contribution is two-fold: first, we present a new tile growing algorithm, that balances the shape and placement of tiles with need for uniform grout; second, we develop a suite of sketch-based tools on top of this algorithm to create and clone tiles and tile patterns along sketched paths, and color them efficiently. A user evaluation shows that our system makes the creation of mosaics fast and accessible to a broad audience.