An Asian Exhibition

- Apsara between two Dragons and Mount Meru -

Rendering Competition 2007

by Richard Socher and Miguel Granados

Final Image

- Apsara between two Dragons and Mount Meru -<

Features

Image Based Lighting

Example:

Median cut algorithm Resulting Image Image source

Grace Cathedral, San Francisco

Eucalyptus Grove, UC Berkeley

Code:

ImageBasedLighting.cxx

Explanation:

A fixed number of directional lights are added to the scene based on the iluminance values in a high dynamic range (HDR) image.
The algorithm implemented for computing the direction and intensity of the lights was the Median Cut Algorithm. In the input image, columns and rows correspond to the spherical coordinates phi and theta of an environmental HDR photography.

Reference:

Paul Debevec. Rendering Synthetic Objects into Real Scenes: Bridging Traditional and Image-based Graphics with Global Illumination and High Dynamic Range Photography. Computer Graphics (Proceedings of SIGGRAPH 98), 32(4):189 - 198, 1998.

Fractal Geometry

Example:

In the final image. Or in a simpler setting like:

mountEdgy (before there were smooth triangles) 131072 smooth triangles mountain with height field after tone mapping

$fractal geomtry mountains$ $fractal geomtry mountains$ $fractal geomtry mountains$

Code:

FractalGeometryObjects.hxx

Explanation:

Fractal Geometry is created by defining 4 points and a height. Then the mid points of the four points and the centroid of the four points are calculated. The latter is then offset by a random height change. In the first recursion this offset is the maximum height, which is passed as a parameter. The offset becomes smaller in each step. The centroid together with 3 mid-points forms one of four new quadrilaterals in each recursion step. Each quadrilateral is then used as a new starting point for further subdivision. This is done n times. In a final step all quadrilaterals are subdivided into 2 triangles by their four points. The first step of the algorithm is shown in the following figure:

$fractal geomtry mountains$

Depending on the height of the triangle it is assigned a different color, to have green , grey and white mountains.

Reference

Subdivision idea from Paul Bourke and the lecture. The color change is a quick modification to make it look better.

Reflection and Refraction

Example:

In the final image, on the floor, the bases for the exhibition pieces, etc.

Reflective (blue), refractive (green), and transparent (green) spheres. At an early stage, you can look behind the mountain.

Code:

Explanation:

The shader is completely dependent on the material. We tried for a while to use .mtl files directly exported from blender. However since they are not standard conform and did not use proper indexes for the principal kind of illumination, you now have to modify them a little bit to fit our ray tracer. Snell's law is used for refraction and the formula we derived as an exercise is used for reflection. The Fresnel term determines their influence.

Reference

Snell's Law and James Foley, Andries van Dam, Steven Feiner, and John Hughes. Computer Graphics - Principles and Practice, second edition in C. Addison Wesley, 1997

Thin Lense Camera

Example:

only the blue sphere is really sharp, the red one behind it is blured.

Code:

RealisticThinLenseCamera.hxx

Explanation:

We define a distance from the camera that shall be sharp. Afterwards, the origin of each ray is changed a slight bit. The new direction is then computed by substracting the new origin by the intersection point on the focal plane. This way everything on the focal plane is still sharp, but all objects in front or behind are blurred.

Reference

Andrew Glassner. Principles of Digital Image Synthesis. Morgan Kaufmann, 1995.

Tone Mapping

Example:

In the final image. We use high dynamic range illumination and textures, hence we needed to apply tone mapping in order to save a suitable image.

High luminance cut of the HDR result Low luminance cut of the HDR result Toned mapped image using Ward's operator

The rendered HDR image can be seen using the HDRView software.

Code:

ToneMappers.cxx

Explanation and Reference

We use Ward's Operator to preserv perceived contrast, as explained in G. Ward. A contrast-based scalefactor for luminance display. In Graphics Gems IV, chapter VII.2, p. 415-421. There is also a nice wrap up in this thesis. The factor that is calculated expresses the minimal discernible luminance changes.

SAH Kdtree

Code:

Explanation:

Implementation of a Kdtree structure using the Surface Area Heuristic (SAH). Both the naive and the SAH versions of the Kdtree were implemented, with the following features:

	SAH Kdtree	Naive Kdtree
Splitting location:	Minimizing traversal/intersection cost	Middle point of bounding box
Spliting plane:	Minimizing traversal/intersection cost	Round robbin
Recursion stop criteria:	Minimizing traversal/intersection cost	Maximum depth and minimum number of primitives
Implemented construction time:	O(N*log^2(N))	O(N*log(N))

Benchmark:

Benchmark perform using four different resolutions of the Stanford dragon model. Each render was perform using 9 samples per pixel and 64 light sources.

Number of triangles Render time SAH (s) Render time Naive (s) Improvement Model

11102 85.71s 146.63s 41% less time

47794 101.19s 156.39s 35% less time

202520 119.25s 171.8s 31% less time

871306 117.96s 186.7s 37% less time

Reference:

Ingo Wald and Vlastimil Havran. On building fast kd-trees for ray tracing, and on doing that in O(N*log N). In Proceedings of the 2006 IEEE Symposium on Interactive Ray Tracing, 2006. (accepted for publication, minor revision pending).

Model Sources

The woman came from http://modelsbank.3dm3.com. We changed some details.
The high dynamic range images used where taken from the Light Probe Image Gallery.
The small dragon model is taken from the The Stanford 3D Scanning Repository. The big dragon was freely available on a graphics website. The model was created in Blender by Richard Socher.

RT Fun

See some more pictures we created along the way.