Smoothing out sketches’ rough edges

MIT-developed tool improves automated image vectorization, saving digital artists time and effort.

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MIT scientists have created a tool that can help artists to create digital characters, logos, and other graphics more quickly and easily. The tool involves a new automated vectorization algorithm that traces intersections without error, greatly reducing the need for manual revision.

Controlling the tool is an altered adaptation of another numerical method in the computer-graphics network, called “frame fields,” used to manage following of ways around curves, sharp corners, and messy parts of drawings where numerous lines cross.

First author Mikhail Bessmeltsev said, “The tool could save digital artists significant time and frustration. A rough estimate is that it could save 20 to 30 minutes from automated tools, which is substantial when you think about animators who work with multiple sketches. The hope is to make automated vectorization tools more practical for artists who care about the quality of their work.”

The key advancement originated from utilizing frame fields to guide tracing. Edge fields allot two bearings to each purpose of a 2-D or 3-D shape. These bearings overlay an essential structure or topology, that can control geometric undertakings in computer illustrations.

Frame fields have been utilized, for example, to reestablish crushed verifiable archives and to change over triangle networks — systems of triangles covering a 3-D shape — into quadrangle networks — lattices of four-sided shapes. Quad meshes are generally used to make computer-generated characters in motion pictures and computer games and for a computer-aided design (CAD) for real-world design and simulation.

For the first time, scientists applied these frame fields to image vectorization. The frame fields assign two directions to every dark pixel on an image. This keeps track of the tangent directions — where a curve meets a line — of nearby drawn curves. That means, at every intersection of a drawing, the two directions of the frame field align with the directions of the intersecting curves. This drastically reduces the roughness, or noise, surrounding intersections, which usually makes them difficult to trace.

Bessmeltsev said, “At a junction, all you have to do is follow one direction of the frame field and you get a smooth curve. You do that for every junction, and all junctions will then be aligned properly.”

When given an input of a pixeled raster 2-D drawing with one color per pixel, the tool assigns each dark pixel a cross that indicates two directions. Starting at some pixel, it first chooses a direction to trace. Then, it traces the vector path along the pixels, following the directions. After tracing, the tool creates a graph capturing connections between the solid strokes in the drawn image. Using this graph, the tool matches the necessary lines and curves to those strokes and automatically vectorizes the image.

Scientists tested the tool on various sketches, such as cartoon animals, people, and plants. The tool cleanly vectorized all intersections that were traced incorrectly using traditional tools. With traditional tools, for instance, lines around facial features, such as eyes and teeth, didn’t stop where the original lines did or ran through other lines.

Nathan Carr, a principal researcher in computer graphics at Adobe Systems Inc. said, “Many artists still enjoy and prefer to work with real media (for example, pen, pencil, and paper). … The problem is that the scanning of such content into the computer often results in a severe loss of information. The work relies on a mathematical construct known as ‘frame fields,’ to clean up and disambiguate scanned sketches to gain back this loss of information. It’s a great application of using mathematics to facilitate the artistic workflow in a clean well-formed manner. In summary, this work is important, as it aids in the ability for artists to transition between the physical and digital realms.”

Scientists have published the paper in the journal ACM Transactions on Graphics.