July 4, 2015

A study of secondary feature lines in Freestyle

Filed under: Blender, Freestyle — blenderyard @ 4:29 PM

Highly detailed 3D scans of live humans and other objects have been increasingly available in the computer graphics market these days, providing an interesting venue for cost-effective generation of non-photorealistic (NPR) illustrations with a variety of artistic styles.  A follow-up of the previous case study using a 3D-scanned model, here is another Freestyle line stylization study using a full body model of a woman (RP Wom Rosy 0095) from Renderpeople.  The input mesh consists of 100K triangular faces.

rosy_sketchy_1_tn rosy_sketchy_2_tn

This time a specific focus was put on silhouettes, suggestive contours and ridges/valleys with the aim of capturing subtle surface curvatures and small bumps.  Silhouettes in Freestyle are often clean and easy to capture.  In contrast, suggestive contours and ridges/valleys tend to be jagged and noisy.  However, they are also known to be useful for highlighting minor (secondary) features of smooth surfaces.  Hence how to post-process these types of secondary feature edges is a challenge.

rosy_sketchy_3_tn rosy_sketchy_4_tn

In this study, the secondary feature edges were smoothed by a Bezier Curve geometry modifier, in combination with chain splitting conditioned by a minimum angle threshold to ease the non-linear curve fitting (prone to numerical stability that leads to unexpected results).  The smoothed secondary lines were then grouped into shorter and longer ones by a length threshold.  The shorter lines are here considered less important, so they are drawn with a light gray color.  The longer lines are instead supposed to be of more importance in delivering geometry information to the audience.  Hence they are drawn in a solid black color, with a quick color gradient toward the light gray at both ends to smoothly get connected with the shorter lines.  Silhouettes on the other hand were simply drawn in black with slight thickness variability along stroke.

rosy_sketchy_5_tn rosy_sketchy_6_tn

All the renders in this blog article are results of the same model and the documented line stylization approach, with minor case-by-case tuning of selected parameters depending on viewing geometry (i.e., camera positions).  Specifically, close-up renders often have less line density, so that the length threshold was decreased to get more secondary lines drawn in the solid black color.


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