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XRT is a raytracing based programmable rendering system for photo-realistic image synthesis built around a plug-in architecture design.


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It's a wonderful world ...

Posted: 04 Nov 2011 23:59
Tags: gallery hair

wStraight.jpg

While browsing the Internet for free models, the harvest has been very good and I wish to thank all these people who kindly made all this data available.

New eye candies

I have created new galleries for all of them:

  • The Utah 3D Animation Repository (here)
  • Deformation Transfer Data (here)
  • Hair Model Files (here)
  • McGuire GraphicsData (here)

Some of them are far from being complete (especially McGuire GraphicsData gallery). Properly setting up cameras and lights is awfully time consuming, and XRT runs out of memory when scenes contain more than 1.5 million primitives.

Hair raising (literaly)

I also spent quite some time on the Hair Models gallery. Rendering curves is not trivial but shading them so that they look like real hair is really challenging. I started with an old Renderman shader from the 1999 Siggraph course "Advanced renderman: Beyond the Companion" but it did not really cut it. A couple of Google searches later, I knew that my shader was based on a Kay-Kajiya hair model [1] which has been superseded by the Marschner hair model [2]. Not being discouraged by hairy formulas, I found a Marschner shader here itself based on an open source implementation from the Cortex project.

All pictures of the gallery are based on this shader, slightly extended: to account for hair self-shadowing, all shading calculations are modulated by ambient occlusion which I believe greatly improves realism. Nevertheless, it is still very far from what you can see in Disney's "Tangled" for instance. The state of the art seems to be Zinke hair model [3] from which Sadeghi derived the Renderman shader [4] used in that movie. I have not yet found enough courage to dig the formulas …

Bibliography
1. Kajiya J. et al., Rendering fur with three dimensional textures (1989)
2. Marschner S. et al., Light scattering from human hair fibers (2003)
3. Zinke A. et al., Dual Scattering Approximation for Fast Multiple Scattering in Hair (2008)
4. Sadeghi I. et al., Efficient Implementation of the Dual Scattering Model in RenderMan (2010)

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