A Very Quick DirectX Raytracing API Primer

A raytracing API has been announced for DirectX and it seems like real time raytracing may finally be here?

MSDN: Announcing Microsoft DirectX Raytracing!
https://blogs.msdn.microsoft.com/directx/2018/03/19/announcing-microsoft-directx-raytracing/

How & where to get the new (experimental) SDK
http://forums.directxtech.com/index.php?topic=5860.0

There is some nice documentation in the SDK zip file, in the doc folder.

I’ve been lucky enough to be in a position to have played with it for a little while pre-release (about 1-2 weeks of time total) and it is pretty fun.

I’ve been playing with it from a purely triangle mesh perspective (don’t hate me! I know, I know…) and it seems like a hybrid rasterization / raytracing is the most realistic way to go there – eg, primary rays are rasterized, and maybe you do some rasterization style post processing. You actually don’t lose a whole lot going this way if you get creative. For instance, you could ray trace primary rays for non triangle based geo and take the minimum between that intersection time and the rasterized one. The only thing I feel like you lose is the ability to have the rays themselves deviate from a typical frustum setup, since you can’t really “distort rays” very easily while rasterizing.

However, I believe when looking at things from a non triangle based approach, things may be very different, especially on the performance side (better perf!). I would love to explore it myself, and know that many folks will also be exploring it. (I’m looking at you folks at the intersection of the twitter and shadertoy communities!)

Here is a very rough overview of some concepts of the Microsoft DirectX ray tracing API to help you form a mental model before trying to parse the verbose DX12 code. (It is DX12 only sadly!)

There are (useful) details missing for sure, but hopefully no misinformation. Please correct me if you see any (:

Raytracing acceleration structures:

  • Bottom Level Acceleration Structure – This is a “per object” acceleration structure. It can either be made from a triangle mesh, or you can specify that it’s a procedural shape. If it’s a procedural shape, you provide a bounding box and an intersection shader. The procedural shape will be useful for raymarching and other non triangle based ray-geometry intersection techniques.
  • Top Level Acceleration Structure – This is a “scene” acceleration structure. It contains instances of bottom level acceleration structures, each able to have their own instance data (like a transformation matrix).
  • Unfortunately the acceleration structures are made at runtime, and cannot be cached off to disk or similar. It’s a loading time cost that currently is seemingly unavoidable.

There are a few different types of shaders used for raytracing:

  • Ray Generation – This generates the primary rays. You could think of this like a compute shader that you author and run once per pixel. It’s also possible to do things like invoke it for each 2×2 pixel group in case you wanted to be able to have derivatives like when rasterizing. You call TraceRay() for each ray you want to generate, and you can use the results however you see fit. It’s typical you’d write the results to a texture or uav though. Whenever you call TraceRay(), you can provide payload data which can be read from and written to by the other shaders. This is useful for sending parameters down with the ray, or having other shaders return information like integrated fog density.
  • Any hit – Optional. As a ray traverses the acceleration structure, it will test objects in an order that is likely not front to back. You can supply an “any hit” shader that gets called during this process. You can read or write payload data in this shader, and you can also tell it to ignore a hit (useful for if you are doing alpha testing from a texture) or you can tell it to accept a hit and stop looking for other hits (useful perhaps for shadow rays). If you omit this shader, the hardware/software can make more assumptions about the ray traversal though and possibly run more quickly. So, you should only use it when necessary. You have access to barycentric information if intersecting with a triangle, and the triangle (index) itself. I’m unsure what you get in the procedural case.
  • Closest Hit – Optional. Called with the information about the closest hit. Called after all “any hit” shaders have been invoked. You have access to barycentric information if intersecting with a triangle, and the triangle (index) itself. I’m unsure what you get in the procedural case. You can call TraceRay() from this shader to spawn secondary rays.
  • Miss – Optional. Called if there are no hits for a ray. You could set some fog density to MAX_FLT, or could perhaps use this for shadow rays, assuming that there was a hit unless a miss shader was invoked.
    You can call TraceRay() from this shader to spawn secondary rays.
  • Yes, ray shaders can be recursive! a closest hit shader could spawn 3 rays which then have a closest hit which each spawn one more ray. There is a maximum stack depth, but recursive rays are totally supported.

When calling TraceRay() to shoot a ray out, you can give parameters such as…

  • Telling it to accept the first hit and end the search (useful for shadows)
  • Telling it to only test “opaque” geometry (anything that doesn’t have an any hit shaders)
  • Instance Masking – Each geometry instance can have an 8 bit mask. When you shoot a ray out, you can give an 8 bit mask that is ANDed with that instance mask, and will only consider geometry for intersection if the result is non zero. This is a bit like a stencil buffer.
  • A minimum and maximum time allowed for collision down the ray. This lets you ignore self intersection of secondary rays by setting the minimum to be greater than zero. The maximum time is useful for things like when shooting shadow rays at point light sources, to make sure you stop searching for occluding geometry at the light source.

There is the concept of a “Hit Group” which contains 0 or 1 of each shader type: intersection, any hit, closest hit.

You can specify a hit group per instance in the top level acceleration structure.

An intersection shader MUST be given for procedural geometry and MUST NOT be given for triangle based geometry.

If a shader is not specified on a hit group, it falls back to a default shader of each type that you specify. This is how you can make it so some objects have different behaviors than others for ray intersection / traversal etc.

You can also specify tables of shaders where shaders are accessible via indexing. This lets you pass numbers around to use in calculations for shader table indexing. In effect, this gives you the ability to have “function pointers” of shaders, and can even be exploited for non raytracing uses wherever having function pointers in shaders would be useful.

Lastly, this is something not super obvious when starting on raytracing, but sampling a mip mapped texture is a bit of a challenge because you no longer have automatic screen space derivatives of uv’s!

I’m sure good solutions to this will spread over time as more people dive into this raytracing API, but I personally think a good place to start is here:

Tracing Ray Differentials
http://graphics.stanford.edu/papers/trd/

That’s all for now! Anything small items think I should add, hit me up here or on twitter at https://twitter.com/Atrix256

Happy Raytracing Folks!! (:


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