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?
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2 Answers
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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.
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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.