In order to put clothes on sheep, and other characters, computer graphics artists typically simulate them by creating a thin sheet, then adding some sort of texture. But that doesn’t work for knitted items, like sweaters or mittens. To make the image realistic, a computer must model the surface right down to the intricate intertwining of yarn.
Computer scientists, in effect, have to teach computers to knit. Then graphic artists have to scrupulously model the 3-D structure of every stitch.
But the latest innovation is to create a 3-D model of a single stitch and then merge multiple copies into a mesh, like tiles in a mosaic. The computer projects the mesh onto a model of the desired shape of the garment, treating each stitch as a tiny flat polygon that stretches and bends to fit the 3-D surface.
The computer then “relaxes” the graphic image of each stitch to fit the shape of its polygon, just as real yarn would stretch and bend to fit the shape of the wearer. The result is a simulation with detail down to the yarn level.
This new method for building simulated knitted fabric out of an array of individual stitches was reported at the 39th International Conference and Exhibition of Computer Graphics and Interactive Techniques earlier this month in by Cem Yuksel of the University of Utah; Jonathan Kaldor, of Facebook; and Steve Marschner and Doug James, Cornell University professors of computer science.
“We are actually changing the shape of the yarn loops that make up the stitches,” Marschner said, “simulating how they wrap around other loops.”
The researchers tested their method with various patterns from knitting books and created images of dresses, sweaters, a shawl and a tea cozy. The simulations are very realistic, but the researchers observed that the results of knitting a particular pattern depend on the yarn and needles used, as well as the style of the individual knitter.
The technique has parameters that can be adjusted to simulate the effects of different needles or yarn, or different yarn tension used by the knitter.
The process is computationally rigorous, needing several hours to simulate a garment- cable stitching takes the longest. With today’s computing power it would not be convenient for an interactive application such as virtual reality, Marschner said, but it would be usable for movies.
Researchers used the physically based renderering software Mitsuba for rendering the results.