Fix T51560: Black pixels on a denoising render

Once again, numerical instabilities causing the Cholesky decomposition to fail.

However, further increasing the diagonal correction just because of a few
pixels in very specific scenes and settings seems unjustified.
Therefore, this commit simply falls back to the basic NLM-filtered pixel
if the more advanced model fails.

2 files changed, 13 insertions, 0 deletions

diff --git a/intern/cycles/kernel/filter/filter_reconstruction.h b/intern/cycles/kernel/filter/filter_reconstruction.h
index 13e90f8233b..dc90f318570 100644
--- a/intern/cycles/kernel/filter/filter_reconstruction.h
+++ b/intern/cycles/kernel/filter/filter_reconstruction.h
@@ -85,9 +85,17 @@ ccl_device_inline void kernel_filter_finalize(int x, int y, int w, int h,
 	const int stride = storage_stride;
 #endif
 
+	/* The weighted average of pixel colors (essentially, the NLM-filtered image).
+	 * In case the solution of the linear model fails due to numerical issues,
+	 * fall back to this value. */
+	float3 mean_color = XtWY[0]/XtWX[0];
+
 	math_trimatrix_vec3_solve(XtWX, XtWY, (*rank)+1, stride);
 
 	float3 final_color = XtWY[0];
+	if(!isfinite3_safe(final_color)) {
+		final_color = mean_color;
+	}
 
 	ccl_global float *combined_buffer = buffer + (y*buffer_params.y + x + buffer_params.x)*buffer_params.z;
 	final_color *= sample;
diff --git a/intern/cycles/util/util_math_float3.h b/intern/cycles/util/util_math_float3.h
index a754be413fe..5327d9f7cc6 100644
--- a/intern/cycles/util/util_math_float3.h
+++ b/intern/cycles/util/util_math_float3.h
@@ -367,6 +367,11 @@ ccl_device_inline bool isequal_float3(const float3 a, const float3 b)
 #endif
 }
 
+ccl_device_inline bool isfinite3_safe(float3 v)
+{
+	return isfinite_safe(v.x) && isfinite_safe(v.y) && isfinite_safe(v.z);
+}
+
 ccl_device_inline float3 ensure_finite3(float3 v)
 {
 	if(!isfinite_safe(v.x)) v.x = 0.0;