C-Ray 1.1 for Parallella!

By James A Ross

Original Code

http://www.futuretech.blinkenlights.nl/c-ray.html

Prerequisites

• COPRTHR-2 SDK for host and coprocessor code
• SDL 2.0 (optional) for SDL real-time rendering view

Usage

    compile:  just type 'make'. 
...with SDL:  'USESDL=1 make'
        run:  ./c-ray-mt
    options:   -t <num>   how many threads to use (default: 1)
               -s WxH     where W is the width and H the height of the image
               -r <rays>  shoot <rays> rays per pixel (antialiasing)
               -i <file>  read from <file> instead of stdin
               -o <file>  write to <file> instead of stdout
               -h         help

Alternatively, the following syntax for scene input and image output are equivalent:

cat scene | ./c-ray-mt -t 16 > foo.ppm
./c-ray-mt -t 16 -o foo.ppm -i scene

Input Scene Format

Scene file format:
  # sphere (many)
  s  x y z  rad   r g b   shininess   reflectivity
  # light (many)
  l  x y z
  # camera (one)
  c  x y z  fov   tx ty tz

Benchmarks

Command	Execution Time (ms)
./c-ray-mt -t 16 -s 800x600 -r 1 -o foo.ppm -i scene	318
./c-ray-mt -t 1 -s 800x600 -r 1 -o foo.ppm -i scene	5069
./c-ray-mt -t 16 -s 800x600 -r 16 -o foo.ppm -i scene	4988

By comparison, the original C-Ray 1.1 code executed the first benchmark on the dual ARM Cortex-A9 cores (with four threads) in 1360 ms, 4.3x slower.

Major Modifications from Original C-Ray 1.1

As Epiphany-III is a coprocessor some modifications had to be made:

• Single Precision
• Host Code for Epiphany Offload
• Parallel Load Balancing
• Miscellaneous Supporting Code
• Image Output

Single Precision (32-bit)

The original code used 64-bit double precision instructions in the ray tracer. The Epiphany-III cores within the Parallella board do not have hardware support for double precision so it's not a completely fair comparison.

Host Code for Epiphany Offload

The host and device code use the COPRTHR-2 SDK. Unified Virtual Memory (UVA) is used to simplify sharing of data between the ARM host and Epiphany coprocessor. There are no explicit memory copies on the host with the exception of marshalling data. This is done to simplify the copy from shared DRAM to local SRAM within each Epiphany core.

Parallel Load Balancing

Parallel work is typically distributed evenly across multiple cores, but this is not ideal for applications which have data-dependent computation -- some cores would finish faster than others and go idle. One solution to this problem is to use a mutex to fetch and increment a counter located within one of the cores. The relatively minor overhead of this operation keeps all cores busy for the duration of the computation.

Miscellaneous Supporting Code

Several supporting math routines are used rather than the defaults from the compiler. The routines, _rsqrt, _sqrt, and _inv calculate the inverse square root, square root, and inverse, respectively. They presently use four newton iterations for higher precision. There are three other supporting math routines written by SUN: __powf, __copysignf, and __scalbnf that are included within the code. These routines are likely targets for optimization.

Image Output

A real-time viewer has been added with the SDL2 interface to visualizing the state of the rendering. It presently uses a 5Hz update rate and is most useful for long-running jobs. You can see the effect of the parallel load balancing. A separate thread is created for the display loop. In order to enable SDL, you must have a clean build and define the USESDL value.

make distclean
USESDL=1 make
# An example of a long-running visualization:
./c-ray-mt -t 16 -r 128 -o foo.ppm -i scene

Known Issues

Likely due to the conversion to 32-bit single precision floating point, some residual graphical errors (so called 'surface acne') may appear. One can adjust the error margin value, ERR_MARGIN, within the code, which may improve the result.