Get normal with DXEngine raycast

Get normal with DXEngine raycast - Printable Version

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Get normal with DXEngine raycast - janovrom - 10-09-2020

Hi,

I'd like to ask how can I get a normal of hit point using DXEngine raycast.
I can obtain the hit result by using:

DXRayHitTestResult rayHitResult = DXScene.GetClosestHitObject(pickRay, rootNode);

It is possible to get rayHitResult.HitPosition and rayHitResult.TriangleIndex. But to what does the rayHitResult.TriangleIndex point to? MeshBase that can be accessed from rayHitResult.HitSceneNode does only have a buffer. I can cast it to SimpleMesh<PositionNormal> (or any other that I will use). Here it is possible to access IndexBufferArray.

The issue I have is that the TriangleIndex is not divisible by 3 (I expect that the IndexBufferArray is a triangle soup). So it can't be start index of the triangle. Maybe it's triangle strip?

TLDR: How can I get a normal from DXRayHitTestResult? Attached code is based on assumptions made above.

Best regards,
Janovsky Roman

Code:
MeshBase mesh = (rayHitResult.HitSceneNode as MeshObjectNode).Mesh;

int t0 = rayHitResult.TriangleIndex;

int t1 = rayHitResult.TriangleIndex + 1;

int t2 = rayHitResult.TriangleIndex + 2;

if (mesh is SimpleMesh<PositionNormal> smPN)

{

   int i0 = smPN.IndexBufferArray[t0];

   int i1 = smPN.IndexBufferArray[t1];

   int i2 = smPN.IndexBufferArray[t2];

   var p0 = smPN.VertexBufferArray[i0].Position;

   var p1 = smPN.VertexBufferArray[i1].Position;

   var p2 = smPN.VertexBufferArray[i2].Position;

   // Compute the normal (we want to stick to the plane that is the triangle)

   var e1 = p1 - p0;

   var e2 = p2 - p0;

   var n = Vector3.Cross(e1, e2);

   n.Normalize();

   forward = n;

}

Edit: If the VertexBufferArray is accessed using directly t0, t1, and t2, it produces correct results half of a time, but can be outside of array range.

RE: Get normal with DXEngine raycast - abenedik - 10-12-2020

TriangleIndex is an index of a triangle and not an index of a TriangleIndice. You can get the index in the TriangleIndices collection or in IndexBuffer by multiplying TriangleIndex by 3 - for example, triangle indices for the 3rd triangle (in this case TriangleIndex is 2) start at TriangleIndice with index 6.

You should change your calculation of t0 ... t2 to:

PHP Code:
int t0 = rayHitResult.TriangleIndex * 3;
int t1 = t0 + 1;
int t2 = t0 + 2; 