Pramod Kumar Sharma
Guillaume
Poncin
The idea in this project was to render realistic pictures of a Christmas tree covered by snow at dusk. The complex lighting due to the atmosphere and to the small lamps on the tree, the modeling of pine trees and finally the modeling and rendering of snow constitute the main challenges of this project.
1. Modeling snow
2. Modeling trees
3. Lights: glow effect
4. Global illumination using photon mapping
5. Subsurface scattering in snow
6. Miscellaneous...
Show is modeled using metaballs. A metaballs is
described as an "isosurface," a surface that is defined by checking for
a constant value throughout a region in 3D space. In general metaballs
can have any kind of basic components but for modeling snow we used
only spheres and ellipsoids. Following images
demonstrate the process of metaball generations using LRT:
Sphere
Ellipsoid
Blob with 2
ellisoids
Blob
with 4 ellipsoids
And finally an interesting one:)
The basic implementation of intersections with this primitive use the
ideas developed in Persistance of Vision (POVray). We extended it by
integrating a kdtree acceleration structure in the blob class for
speeding up the intersections when there are many components. This
resulted in a speed improvement from 20 min to 3 minutes of rendering
time for a blob with 20000 components.
Snow modeling using Metaballs:
To simulate the snow on various objects in the scene we thought
of using the ray tracer itself. Snow is considered as a ray coming from
some point at infinity with direction determined by gravity and winds.
So snow fall is basically a set of parallel rays intersecting
the scene. This framework allows to simulate snow coming from any
direction and we can put the snow on any kind of object. We can easily
control the size/density of snow.
To give a more realistic impression, we are
modulating the sphere into a rotated ellipsoid to better model the
aggregation of real flakes.
Following example demonstrate the idea where snow is projected on
a sphere from above. Figure shows only snow part.
Snow on
sphere
Layered Snow Fall: To get the realistic feeling of now we did 2 layer snow projection upper layer having smaller flakes. First layer with big blob element size represent the shape of the object on which ray is projected and second layer present finer details of snow flakes. This reduces number of blob elements required to model snow on a given surface by a large amount. We did not include this in our final scene since we could not get the expected effect, unless we used too many blob components.
We really wanted to have realistic looking trees as the base for our image. We created an Lsystem parser specialized for pine trees, as described by A. Lindenmeyer. It is based on the parsing code written by N. Lambert (Stanford PhD student). We extended his code to account for gravity and to properly model the geometry of a coneshaped tree. The rules for the main tree are the following:
Axiom: \(100)O(0)D
O(t) : (t<=3) ==> ASASASASO(t+1)
S ==> L(0.9)!(0.85)+(0.5)
A ==>
E[&L(0.85)!(0.45)B]\(81)[Z&L(0.8)!(0.4)B]\(75)[&L(0.85)!(0.45)B]\(74)[Z&L(0.8)!(0.5)B]\(78)[&L(0.75)!(0.4)B]\(75)
B ==> YF[L(0.8)!(0.9)WC]L!(0.9)C
C ==> YF[+L(0.8)!(0.9)WB]L!(0.9)B
The main difficulties that we encountered in the modeling were to find this formula and to make the needles look nice. The rules we use are very simple: O accounts for the trunk, S for the transformations that occur along the trunk, A for the main branch structures, B&C, that recursively call each other for the small branches. The non standard symbols Y and W account for gravity.
The needles are modeled by elongated ellipsoids. They are pointing with a relative direction of 30 degrees with respect to the branches (omitting the gravity effect), and disposed randomly along the branches. Their size vary as a function of their position in the tree.
We roughly have 10000 objects per tree (5% branches, 95% needles).
6 stages in the Lparsing of the tree:






And finally, with needles:
After we made our first tree, it was easy to create a new model for each of the trees in the picture, by tweaking the parameters slightly.
How can we integrate so many lights in the scene without taking too much computation time ? We have approximately 150 lights in the main tree and one in the moon. Using area lights would have been suicidal, so we opted for point lights with a nice cameraglowing effect.
The glow is an approximation of both the atmospheric effect and the spread of the light on the camera film. The idea is to take the intersection of a ray from the camera with the plane of the light source and to compute the distance between this intersection and the light source. We use an exponential falloff formula on the intensity of the light.
The main issue here was to get the right parameters, so that the glow was not too weak or too strong, allowing for all the other lights in the scene. We also spent some time optimizing it since the distance computation has to be made for every point in the scene multiplied by every glowing light.
Finally, to get the soft feeling of lighting from the scattering atmosphere, we implemented photon mapping. The problem here was computation time. We put a lot of effort into optimizing this part.
First, we implemented H. Christensen's technique of irradiance precomputation. It speeds up the final gathering step by a factor of ten.
Then we limited the sampling of photons to the snow surfaces, as needles and branches would not account for much of the diffuse lighting anyways. In order to get photons from the sky, we treat it as a light source infinitely far from the scene that emits low intensity light from every position on the hemisphere. Final gathering rays that go to infinity are given the color of the sky. That is how we get the blue coloration of the whole scene.
Without global illumination (and no snow on the trees):
With global illumination:
A blob without Translucency 
With Translucency 


Snow without translucency
Snow with translucency
Snow without subsurface scattering
Snow with subsurface scattering
There are many other details in the picture that we worked on. We are just mentioning a few here...
Textures
First, we have icy water texture on the
foreground based on a very hacked version of the windy displacement
mapping in LRT. It is projected on a simple polygon, and gives the
right impression of halffrozen liquid element. We also have
cylindrical mapping of a procedurally generated texture on the trees
and a procedurally generated background for the sky (applied as a
texture).
Acceleration
We used the KdTree acceleration structure from LRT
but we had to modify it to make it work on the scene. The main issue is
that we have very large objects (quasi infinite plane, moon), and at
the same time very small details (needles). That is why we create one
KdTree per group of shapes that belong to the same object. We end up
with 20 of these KdTrees, that we put into a top level KdTree.