From b6d9442b456b7ce20f5a779dc966b65ac3f696fc Mon Sep 17 00:00:00 2001
From: mchughalex <25671999+mchughalex@users.noreply.github.com>
Date: Tue, 21 Jul 2026 07:57:12 +0100
Subject: [PATCH] render: add a Vulkan-only static sun receiver bias margin for
 toolchain rounding differences

---
 src/skate3_native_scene.cpp     | 10 ++++++++++
 src/skate3_native_scene_gpu.cpp | 10 +++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/skate3_native_scene.cpp b/src/skate3_native_scene.cpp
index 9494e9e..86cdece 100644
--- a/src/skate3_native_scene.cpp
+++ b/src/skate3_native_scene.cpp
@@ -753,6 +753,16 @@ REXCVAR_DEFINE_DOUBLE(skate3_native_render_scene_shadow_static_bias, 0.08,
                       "static shadows visibly detach from their casters.")
     .range(0.0, 0.5)
     .lifecycle(rex::cvar::Lifecycle::kHotReload);
+REXCVAR_DEFINE_DOUBLE(skate3_native_render_scene_shadow_static_bias_vk, 0.05,
+                      "Skate 3",
+                      "Additional static sun-shadow receiver bias in METERS "
+                      "applied on the Vulkan backend only. The caster and "
+                      "receiver depth terms compile through different "
+                      "toolchains per backend; their rounding differences "
+                      "left borderline texels flipping at map rebuilds on "
+                      "Vulkan (triangular ground flicker while moving).")
+    .range(0.0, 0.5)
+    .lifecycle(rex::cvar::Lifecycle::kHotReload);
 REXCVAR_DEFINE_BOOL(skate3_native_render_scene_char_shadow_exact, true,
                     "Skate 3",
                     "Characters sample the CSM atlas with the game's own math: "
diff --git a/src/skate3_native_scene_gpu.cpp b/src/skate3_native_scene_gpu.cpp
index d03e3bd..3d37082 100644
--- a/src/skate3_native_scene_gpu.cpp
+++ b/src/skate3_native_scene_gpu.cpp
@@ -104,6 +104,7 @@ REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_pcss_max_m);
 REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_pcss_min_texel);
 REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_pcss_sun_deg);
 REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_static_bias);
+REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_static_bias_vk);
 REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_static_radius);
 REXCVAR_DECLARE(int32_t, skate3_native_render_scene_shadow_static_size);
 REXCVAR_DECLARE(double, skate3_native_render_scene_shadow_static_strength);
@@ -7548,8 +7549,15 @@ bool RenderScene(const NativeGuestOutputRenderContext& context, void* /*user_dat
                 float(REXCVAR_GET(
                     skate3_native_render_scene_shadow_pcss_min_texel)) /
                 float(std::max(1u, g_r.static_sun_size));
-      const float bias_m = float(
+      float bias_m = float(
           REXCVAR_GET(skate3_native_render_scene_shadow_static_bias));
+      if (context.device->backend() == nrhi::Backend::kVulkan) {
+        // The two backends compile the caster/receiver depth math through
+        // different shader toolchains; their rounding differences leave
+        // borderline texels flipping at map rebuilds without this margin.
+        bias_m += float(
+            REXCVAR_GET(skate3_native_render_scene_shadow_static_bias_vk));
+      }
       cb[160] = bias_m / g_r.nsm_depth_range;  // base receiver bias
       cb[161] = bias_m / g_r.nsm_depth_range;  // slope-scaled term
       cb[162] = 0.0f;  // reserved (cascade ratios are shader constants)