Wednesday, October 14, 2015

Real-time Raytracing part 1

In the next few posts I will talk about Real-time Raytracing. The real-time part has different interpretations. Some define it as being 'interactive', which is another vague term as any application running at 1 FPS can still be interactive. In this case we will define a "real-time constraint" as a time constraint per frame of about 16ms (which leads to 60 frames per second). This is a common time constraint used in games which have to be responsive.

Now for the raytracing part. The basic concept of raytracing is tracing the paths of light for every pixel on an image plane. This way we can simulate visual realism by by mimicking the real process. You can create pretty images without much code:

Unfortunately raytracing also has downsides. The process to trace the light paths requires global scene access, and is thus unable to work similar to a rasterizer (which draws objects one by one). This makes the whole rendering process more difficult and the standard route of rendering objects is averted. Raytracing has a very high computational cost. A basic algorithm for checking which triangle was hit already shows the problem:
for (int x = 0; x < screenwidth; x++)
 for (int y = 0; y < screenheight; y++)
 for (int t = 0; t < nTriangles; t++)
 RayTriangle(x, y, t)

The whole algorithm scales with screen size and the amount of triangles. Running this algorithm with the dragon model shown above is basically asking for your computer to explode. The model has 100.000 triangles exactly, and checking that amount of triangles for every pixel in a low resolution (1024x1024) is about 104 billion ray triangle checks. A simple line-triangle intersection already has a lot of instructions, needless to say, this algorithm isn't going to run real-time...

So, how do we make it real-time? Several decades of research show us that there are plenty of possibilities to speed up the process of raytracing. In the upcoming posts I will talk about some of these processes to speed up my path tracer. Most of the subjects will be on parallel programming on the GPU using CUDA. Even though I will try my best to keep it as readable as possible, unexplained terminology can always be found in the excellent ray tracing tutorial of scratchapixel.

If you're really interested in ray tracing, and haven't read it already: Ray tracey's blog already has a huge collection of ray tracing posts.

Part 2.

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