NPR Line Drawings Really Work!

A new study by Forrester Cole, et al. titled, "How Well Do Line Drawings Depict Shape" presented at SIGGRAPH 2009 evaluates the methods of producing line drawings developed in the past few years (which are quite comprehensive and what Forrester Cole seems to have devoted his entire graduate and PhD Student career to, so let's thank him).

In their study they had participants orient what are called gauges onto line drawings of 3D objects. Each guage is a disc with a line that represents its normal. Participants had to orient the normal of each gauge. A correct disc orientation would make it look like the disc was actually on the surface at its position. The position of each gauge was fixed and each gauge was initially superimposed over the model so as to not cue the participant. The gauges also did not interact with the models.

The study compares six different styles of rendering. Among these six styles were fully shaded images, apparent ridges, suggestive contours and an artists line drawing of the same object (the authors note that plain contours were rendered over all models other than the artist's image).

Their results are quite long and detailed and take up a majority of their paper. They show that shaded images are the best for depicting shape, however, that was an expected hypothesis and not the point of the experiment. In summary, their data shows that each line drawing method has its own strengths and weaknesses. Most were comparable and at times better than an artist's drawing. Some methods are good for some types of models while other methods are good for other types of models.

This is good news for NPR but I still want to see these methods developed into something that doesn't bog down processing and destroy interactive frame rates. Albeit, some methods achieve interactive frame rates (30-60fps, but sometimes they quote interactive frame rates as 5fps) these methods are still unsuitable for real-time environments such as games and walk-throughs that are not prerecorded.