Bunny Vase with Liquid
May 2, 2016 - 11:10 am by Joss Whittle C/C++ Graphics PhD
Program
The rantings of a lunatic Scientist
While working on my submission to this years SURF Research as Art Competition I realized that if I was to have any hope of rendering the final image at high resolution in a reasonable amount of time I would need more power. To do this I applied node parallelism in the form of a computer lab turned render farm.
The above image is the result of ~8 hours of rendering, at 4k resolution, over 18 machines (as described below). No colour correction or other post-processing (other than converting to jpeg for uploading) has been applied.
I try to keep the current generation of my rendering software nicely optimized but at it’s core it’s purpose is to be mathematically correct, capable of capturing a suite of internal statistics, and to be simple to extend. Speedup by cpu parallelism is only performed at the pixel (technically pixel tile) sampling level to reduce the amount of intrusion ray-packet tracing can bring to a renderer. In the future I plan to add a GPU work distributor using CUDA but for now this is quite low on my research priorities.
In order to get the speed boost I needed to render high resolution bi-directional path traced images I made use of Swansea Computer Sciences – Linux Lab, which has 30 or so i7, 8GB Ram, 256GB SSD machines running OpenSUSE. I wrote a bash script which for each ip address in a machine_file (containing all ip’s in the farm) ssh’s into each system and starts the renderer as a background process, and another to ssh into all machines and stop the current render.
The render job on each node outputs to a unique binary partials file every 10 samples per pixel (at 4k resolution) to a common network directory, overwriting the files previous values. This file contains three int’s containing width, height, and samples respectively; followed by (width * height * 3) double precision numbers representing the row-column pixel data stored in BGR format.
The data in the file represents the average luminance of each pixel in HDR. A second utility program can be run at a later time to process all compatible partials files in the same directory and turn them into a single image. Which is then properly tone mapped, gamma corrected, and saved as a .bmp file. To combine two partials, the utility program simply performs the following equation for each pixel:
P_1,2 = ((P_1 * S_1) + (P_2 * S_2)) / (S_1 + S_2)
By repeating this process one by one (each partial file can be > 500MB) all partials in a directory can be aggregated together into a single consistent and unbiased image.
After succumbing to a bit of a slump in research productivity over the last week or two it feels great to be making progress again.
Finally I have a fully functional implementation of Bidirectional path tracing with some basic multiple importance sampling for the path weights. To celebrate having this new renderer in the code base I decided to have another crack at implementing a ceramic like shader. In the past I had modeled this material in geometry by placing a diffuse textured sphere inside an ever so slightly larger glass sphere to model the glaze/polish of the material. However, this method was a clunky approximation and severely limited the complexity of the models which it could be applied to.
This time I modeled a blended BRDF between a lambertian diffuse under-layer and an anisotropic glossy over-layer to represent the painted ceramic glaze. The amount of each BRDF used for each interaction is modulated by a Fresnel term on the incident direction. This means that looking straight at the surface will show mostly the coloured under-layer, while looking at glancing angles will show mostly the glossy over-layer.
The final, most important, part of this shader however is the bump map applied to it. Originally I rendered this scene without bump mapping, and while the material seemed plausible it looked almost too perfect. To break up the edges of reflections and to allow the surface of the material to “grab” onto a bit more light the effect of the material becomes and order of magnitude more convincing.
Just a quick render after implementing an Anisotropic Metal material in my renderer. The test scene was inspired by one I saw on Kevin Beason’s worklog blog.
