Question DXR Raytracing Effect from Scratch

Hi,
I am trying to simulate a simple lidar scanner with DXR. I am running this in a custom pass volume. The problem I have is that the closestHit shader never gets triggered. I am only getting misses. I am guessing my accelerationStructure is somehow messed up. Can somebody give me a hint what I am doing wrong? I am stuck on this for 3 hours now.
To build the AS I just loop through some tagged objects and add their MeshRenderers. I then call build and pass the AS to the shader.

Custom Pass Execute Function:

Code (CSharp):

1. protected override void Execute(ScriptableRenderContext renderContext, CommandBuffer cmd, HDCamera camera, CullingResults cullingResult)
2.         {
3.             if (scanner != null)
4.             {
5.                 if (EditorApplication.isPlaying)
6.                 {          
7.                     RayTracingAccelerationStructure accelerationStructure = new RayTracingAccelerationStructure();
8.                     GameObject[] obstacles = GameObject.FindGameObjectsWithTag("ObstacleRaytraced");
9.  
10.                     foreach(GameObject obj in obstacles) {
11.                         MeshRenderer ren = obj.GetComponent<MeshRenderer>();
12.                         accelerationStructure.AddInstance(ren);
13.                     }
14.  
15.                     //accelerationStructure.Update();
16.                     accelerationStructure.Build();
17.  
18.                     //cmd.BuildRayTracingAccelerationStructure(accelerationStructure);
19.                     cmd.SetRayTracingAccelerationStructure(shader, "_RaytracingAccelerationStructure", accelerationStructure);
20.                     cmd.SetRayTracingShaderPass(shader, "LidarPass");
21.  
22.                     cmd.SetRayTracingTextureParam(shader, Shader.PropertyToID("RenderTarget"), m_renderTex);
23.                     Vector4 position = scanner.RayOrigin.position;
24.                     Vector4 dir  = scanner.RayOrigin.forward;
25.  
26.                     cmd.SetRayTracingVectorParam(shader, Shader.PropertyToID("originPos"), position);
27.                     cmd.SetRayTracingVectorParam(shader, Shader.PropertyToID("originDir"), dir);
28.  
29.                     cmd.DispatchRays(shader, "MyRaygenShader", scanner.SamplesPerScan, 1, 1);
30.  
31.                     RenderTexture.active = m_renderTex;
32.  
33.                     m_resultTex.ReadPixels(new Rect(0, 0, scanner.SamplesPerScan, 1), 0, 0);
34.                     var data = m_resultTex.GetPixels();
35.  
36.                     RenderTexture.active = null;
37.                     if (debug)
38.                     {
39.                         //Debug.Log("Size: " + m_resultTex.width);
40.                         for (int x = 0; x < m_resultTex.width; x++)
41.                         {
42.                             if (data[x] != new Color(0, 0, 0, 0))
43.                                 Debug.Log(data[x]);
44.                         }
45.                     }
46.                 }
47.             }
48.         }

Raytrace Shader:

Code (csharp):

1. RWTexture2D<float4> RenderTarget;
2. float4 originPos;
3. float4 originDir;
4.  
5. #pragma max_recursion_depth 1
6.  
7. //Includes omitted
8. //...
9.  
10. struct RayPayload { float4 color; uint2 launchIdx; float3 position;};
11.  
12. [shader("raygeneration")]
13. void MyRaygenShader(){
14.  
15.     uint2 launchIdx = DispatchRaysIndex().xy;
16.     //uint2 launchDim = DispatchRaysDimensions().xy;
17.  
18.     RayDesc ray;
19.     ray.Origin = originPos;
20.     ray.Direction = normalize(originDir);
21.     ray.TMin = 0;  
22.     ray.TMax = _RaytracingRayMaxLength;
23.  
24.     RayPayload payload;
25.     payload.color = float4(0, 0, 0, 0);
26.     payload.position = float3(0,0,0);
27.     TraceRay(_RaytracingAccelerationStructure, RAY_FLAG_NONE, 0xFF, 0, 1, 0, ray, payload);
28.     RenderTarget[launchIdx] = payload.color;
29.  
30. }
31.  
32. [shader("miss")]
33. void Miss(inout RayPayload payload : SV_RayPayload){
34.     //rayDirection = WorldRayDirection();
35.     payload.color = float4(0,0,0,0);
36. }
37.  
38.  
39. [shader("closesthit")]
40. void ClosestHit(inout RayPayload payload : SV_RayPayload, AttributeData attributeData : SV_IntersectionAttributes)
41. {
42.     payload.color= float4(1, 1, 1, 1);
43. }
44.  

Thanks a lot for your help!

 

Okay, small success: I managed to validate that my AS is correct. I can see that the Miss Shader does NOT get called when the ray intersects with geometry, but the closestHitShader does not get called either. The payload is unmodified in this case. Does the ClosestHitShader have to enabled somewhere? I can only see the flag to disable it, which I assume means its enabled by default...

 

Is there any documentation that explains how to do what OP asked about? I am looking for similar information. It seems I am even unable to define a Miss shader in my .raytrace file underneath my Ray Generation shader. An example project that demonstrates how to include your own custom raygen shader, hit shader, anyhit shader, intersection shader, and miss shader would be incredibly helpful! I am working on a project where I need to use nonlinear raytracing where the ray follows a parametric curve, so I need explicit control over the raytracing process through these different shaders. Unfortunately, I have not found documentation that has helped me do this in Unity yet!

Also, this presentation :

https://on-demand.gputechconf.com/s...started-with-directx-ray-tracing-in-unity.pdf

seems to have some conflicting info. Their closesthit shader does not use "LidarPass," so I'm confused on what info is up to date / valid and what isn't. I think this all can be cleared up with a simple demo project that implements these different shaders. I'd be happy to help put that demo project together with the help of this community.

 

@INedelcu this is exactly what I had in mind. Thank you for sharing this, it will be very helpful to me and others.



I have another question that I'd love to get your insights on:



I want to implement a non linear ray tracing shader. Basically, I want to ray trace along a parametric curve. So, I want to generate a ray with a small fixed length (TMax - TMin is small), and incrementally advance the ray along the curve until the total distance traveled by the ray reaches some threshold. At each step, the ray must check for intersections, and if there is a Miss, then the ray should calculate its new origin and direction and try again. So, kind of like ray marching along a curve.


I suppose I could implement this in an intersection shader where use the ray from the generation shader is used and updated in a loop, and checked for intersection at each iteration. However, I think this would mean I need to re-implement the optimized DX built in Triangle intersection routine because the intersection checks will now be in my own custom intersection shader which is now also responsible for curving the ray. I'd rather not do this because I want to use the optimized DX built in triangle intersection shader. Is this a callable shader? I could see this working if I am able to call the built in triangle intersection shader from my custom intersection shader at each iteration of the loop.


I could also recursively call TraceRay() in a miss shader. The ray generation shader would produce a small ray, and if the miss shader is called, it will recast the next ray with the updated direction and origin. However, it looks like there is a recursion depth limit of 31, and I do not think it will be enough, so I don't think this will work.



Maybe there's a more appropriate way of achieving this effect? Any ideas on the best way to proceed?



Thanks!

 

@INedelcu , yes this will work. Thank you for the help.

 

I have one questions?
if mtiuple AS was bind. how I can get mesh data for different AS.

 

hello， I have tried using Ray Tracing in built-in pipline, it works,but when I use TraceRay in closesthit shader, the unity crashed, I tried in many versions from 2019.4 to 2021,everytime I use the TraceRay func in closesthit shader, unity crashed, is this a bug?