Porting from DXR/HLSL to Vulkan Ray Tracing Extension/GLSL

Question

I am working on the port of a demo from DXR to Vulkan Ray Tracing extension using GLSL. In DXR/HLSL space, there are RayTMin() and RayTCurrent() functions to retrieve the starting and the current end point of the ray (Origin + (Direction * RayTMin) and Origin + (Direction * RayTCurrent), respectively). In Vulkan RT/GLSL space, one specifies the tMin and tMax values in the traceRayEXT(...) call, but how does one retrieve the "current t" and "t min" values in an intersection shader?

Answer 1

According to this document, one should be able to extract those values through gl_RayTminEXT and gl_RayTmaxEXT GLSL built-ins for intersection shader.

Further comments are welcome.

Answer 2

As I understand GLSL_EXT_ray_tracing.txt, gl_RayTmaxEXT contains the current t in intersection shaders, due to the following pieces of information given in GLSL_EXT_ray_tracing.txt:

 In the intersection language, built-in variables are declared as follows

    ...

    // Ray parameters
    in          float  gl_RayTminEXT;
    in volatile float  gl_RayTmaxEXT;
    in          uint   gl_IncomingRayFlagsEXT;

    ...

=> due to the volatile declaration of gl_RayTmaxEXT.

Also the description of bool reportIntersectionEXT(float hitT, uint hitKind); remarks that gl_RayTmaxEXT is not constant:

If the intersection is not ignored in the any-hit shader, <hitT> is committed as the new gl_RayTmaxEXT value of the current ray, is committed as the new value for gl_HitKindEXT, and true is returned.