Shadow terminator artifacts on meshes with smoothed normals

[expenses]

This is highly likely to be a known issue but I thought I'd file it anyway. At the moment the assets\shaders\rt\trace_sun_shadow_mask.rgen.hlsl causes some artifacts on meshes with smoothed normals. This is most visible on the viziers scene:

Upping the bias amount seems to fix it:

diff --git a/assets/shaders/rt/trace_sun_shadow_mask.rgen.hlsl b/assets/shaders/rt/trace_sun_shadow_mask.rgen.hlsl
index fd6d98f..746be01 100644
--- a/assets/shaders/rt/trace_sun_shadow_mask.rgen.hlsl
+++ b/assets/shaders/rt/trace_sun_shadow_mask.rgen.hlsl
@@ -41,7 +41,7 @@ void main() {
     const float3 normal_ws = mul(frame_constants.view_constants.view_to_world, float4(normal_vs, 0.0)).xyz;
 
     const float3 bias_dir = normal_ws;
-    const float bias_amount = (-pt_vs.z + length(pt_ws.xyz)) * 1e-5;
+    const float bias_amount = (-pt_vs.z + length(pt_ws.xyz)) * 5e-3;
     const float3 ray_origin = pt_ws.xyz + bias_dir * bias_amount;
 
     const bool is_shadowed = rt_is_shadowed(

Not sure if that's going to break anything else or not.

[h3r2tic]

It's a pretty common thing with ray tracing, see e.g. https://wiki.povray.org/content/Knowledgebase:The_Shadow_Line_Artifact
Increasing the shadow bias is going to cause leaks, but I believe there are recent solutions to this problem -- e.g. Blender just fixed this.

[expenses]

The fix from chapter 4 of Ray Tracing Gems II is pretty good but it requires barycentric coordinates for it so I'm not sure how you'd adapt it for this.

[h3r2tic]

Aha, thanks! Should be possible to output barycentrics into the g-buffer :) Might even flip it into a v-buffer based renderer at some point anyway!