Bin Chen 2008
To create ‘dramatic’ lighting, I decided to put light sources in the middle of the flowers, like the magical ones in the story. For petals with diffuse material, no light can be seen through the petal, so I had to add transparency to the petal.
One method to do it is with path tracing. The difference from a diffuse surface is that when a ray is traced and hits the surface of the petal, ray can pass through the surface by a certain chance (by flipping the normal of the surface when tracing the random ray). Some ray will then be able to reach the light source in the middle of the flower:
Note this is an expensive method since there can be up to 8 layers between the camera and the light sources.
I have tried this method and got the following noisy result with a relatively long rendering time:
Then I realized this is not acceptable, and implemented photon mapping. The idea is the same, but I simply allow a certainly percentage of transmitted photon for the lotus petals, and I started to get some encouraging results:
And the most important thing is the that rendering a picture now takes a lot less time, so I can experiment with different lighting effects, which turns out to be extremely critical to produce the final image with ‘dramatic’ lighting.
During the experiment, I have tried shooting more than 10,000,000 photons, which takes some time. To make things even go faster, I implemented a feature that allows importing / exporting photon maps built to a file, which significantly reduced the waiting time when I shoot a lot of photons and need to tune the photon mapping parameters.
By tuning the two parameters (search radius and photon number), I was able to get some reasonably good looking pictures:
Now we go from the pure diffuse flowers to the transparent flowers in the final image (I put a very bright light in the far lotus to also light the leaves slightly and a dimmer light in the near lotus):
Diffuse to Transparent