I want to simulate scene with a lot of glass and generate dataset, but I found there are many problems about UE4.24 raytracing. I understanding they are still developing and some bugs are features for better performance. In this topic, I will offer ways to solve some problems. I hope it could help somebody.
At first, I list what problems will be solved in this topic:
-
How to set IOR(Index of Refraction) of glass(transparent object);
-
Fix thick glass refraction error. (Thin glass only need to set IOR as 1.0)
-
Implement thick glass double reflection.
-
Implement multiple times reflection of glass. (It will cause serious performance problem, so be careful to use it)
-
Set IOR:
Before setting correct parameters in material, there is no refraction:
IOR_Ori.jpg
Ray tracing in UE4 actually uses “Specular” as IOR. We can use formula: Specular = sqr((IOR-1)/(IOR+1))/0.08 to convert IOR into specular. As follow, I create a material function:
IOR2Specular.jpg
“RayTracingQualitySwitchReplace” is a switch function which will choose RayTraced input as output in ray tracing mode, otherwise will choose Normal input.
Modify material as follow:
MB_Glass.jpg
Specular input doesn’t need Fresnel to adjust IOR.
Then, the refraction works and likes a solid glass ball.
IOR_New.jpg
- Thick glass refraction:
Because solid glass ball is hard to prove correctness of refraction. So I choosed thick flat glass which we can see it anywhere.
As follow image show, the refraction is error because it scale the chair so much.
FlatGlass_Ori.jpg 
Then, we need to modify the shader file in UE4 to fix the problem.
- Add defination at the top area of file “RayTracingPrimaryRays.usf”:
#define MAX_IOR_STACK 3
- In the initialization part of RAY_TRACING_ENTRY_RAYGEN function, we need to define variables:
float IorStack[MAX_IOR_STACK];
int StackIdx = 0;
- Depend on the front/back face information, save/load IOR in stack before the call of RefractRay function:
if (bIsEntering)
{
// Push Ior2 into stack
​​​​​​​ if (StackIdx < MAX_IOR_STACK)
​​​​​​​ ​​​​​​​ IorStack[StackIdx] = Ior2;
​​​​​​​ ​​​​​​​ StackIdx++;
}
else
{
​​​​​​​ // Pop stack
​​​​​​​ StackIdx--;
​​​​​​​ if (StackIdx < MAX_IOR_STACK && StackIdx >= 0)
​​​​​​​ ​​​​​​​ Ior1 = IorStack[StackIdx];
}
- In RefractRay function, when we calculate Eta variable, it should be modified as follow:
/* Origin
float Eta = Ior1 / Ior2;
if (bIsEntering)
{
Eta = 1.0f / Ior1;
}
*/
float Eta = Ior2 / Ior1;
After modification:
FlatGlass_New1.jpg 
FlatGlass_New2.jpg 
We can see the chair only offsets a little and nearly no scales, which likes the real world glass.
- Thick glass double reflection
In real world, many glass has double reflection, but in UE4 glass only has one reflection.
RealWorld_DoubleReflection.jpg
DoubleReflection_Ori.jpg
There are serval methods to solve and I saw someone suggest use two side material. However, the two side material will affect refraction, because in shader code it would make we cannot get correct front/back face information.
My method is also modification of shader.
- In “RayTracingPrimaryRays.usf” file, after done once ray tracing (in other words after the call of TraceRayAndAccumulateResults function), we add:
// Correct surface normal when hit a back surface
if (dot(-Ray.Direction, Payload.WorldNormal) < 0.0f)
{
Payload.WorldNormal = -Payload.WorldNormal;
}
After modification:
DoubleReflection_New.jpg 
The red ball and the bookcase were reflected twice in thick glass.
- Multiple times reflection of glass
Before modification:
MultiReflection_Ori1.jpg
MultiReflection_Ori2.jpg
This part need to code a lot in “RayTracingPrimaryRays.usf” file, so I only breifly explan how I implement it and attach modified “RayTracingPrimaryRays.usf”.
In the original codes of RAY_TRACING_ENTRY_RAYGEN function, it only does once reflection in every ray (the reflection does not recurse). And here only recurse the transparent face.
What I do is using stack to implement recurse of ray tracing both on reflection and refraction, because HLSL doesn’t support recursion of function.
In my attached usf file, I used macros to swith my modification:
#define FIX_IOR 1
#define FIX_DOUBLE_REFLECTION 1
#define MAX_IOR_STACK 10
#define USE_ALTERNATIVE_RENDER 1
#define MAX_ALTERNATIVE_NUM 3 //alternative between transparency object and mirror object, be careful about the 2^num rays will be detected
#define MAX_ALTERNATIVE_STACK 10
After modification:
MultiReflection_New.jpg
After all modifications, here I post comparison of the refraction of glass ball between UE4 and real world:
Attachment: RayTracingPrimaryRays.txt (If you want to run it, you should change the file extension as usf and replace the original one)
(have not tested it with decals so far, but they should work too). however, i stumbled across some problem, when i wanted to reflect the transparent objects in solid metal objects (opaque objects set to metal = 1). I made a video, there you can see it, the transparent objects render black without color, but on closeup the color comes back.