Dynamic Paint: provide margin of error for linking neighbors across seams.

The code requires the pixel on the other side of the seam to be assigned
precisely to the expected triangle. This can cause false negatives around
vertices, where a pixel is likely to touch multiple triangles and thus
cannot be said to unambiguously belong to any one of them, so check
distance to the intended triangle and accept the result if it's close.

1 files changed, 29 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/dynamicpaint.c b/source/blender/blenkernel/intern/dynamicpaint.c
index 4f204f2892f..9e3f123d5d8 100644
--- a/source/blender/blenkernel/intern/dynamicpaint.c
+++ b/source/blender/blenkernel/intern/dynamicpaint.c
@@ -2302,6 +2302,24 @@ static void dynamic_paint_create_uv_surface_neighbor_cb(void *userdata, const in
 
 #undef JITTER_SAMPLES
 
+static float dist_squared_to_looptri_uv_edges(const MLoopTri *mlooptri, const MLoopUV *mloopuv, int tri_index, const float point[2])
+{
+	float min_distance = FLT_MAX;
+
+	for (int i = 0; i < 3; i++) {
+		const float dist_squared = dist_squared_to_line_segment_v2(
+			point,
+			mloopuv[mlooptri[tri_index].tri[(i + 0)]].uv,
+			mloopuv[mlooptri[tri_index].tri[(i + 1) % 3]].uv
+		);
+
+		if (dist_squared < min_distance)
+			min_distance = dist_squared;
+	}
+
+	return min_distance;
+}
+
 /* Tries to find the neighboring pixel in given (uv space) direction.
  * Result is used by effect system to move paint on the surface.
  *
@@ -2510,9 +2528,6 @@ static int dynamic_paint_find_neighbour_pixel(
 		/* If we ended up to our origin point ( mesh has smaller than pixel sized faces)	*/
 		if (final_index == (px + w * py))
 			return NOT_FOUND;
-		/* If found pixel still lies on wrong face ( mesh has smaller than pixel sized faces)	*/
-		if (tempPoints[final_index].tri_index != target_tri)
-			return NOT_FOUND;
 
 		/* If final point is an "edge pixel", use it's "real" neighbor instead */
 		if (tempPoints[final_index].neighbour_pixel != -1) {
@@ -2523,6 +2538,17 @@ static int dynamic_paint_find_neighbour_pixel(
 				return NOT_FOUND;
 		}
 
+		/* If found pixel still lies on wrong face ( mesh has smaller than pixel sized faces)	*/
+		if (tempPoints[final_index].tri_index != target_tri) {
+			/* Check if it's close enough to likely touch the intended triangle. Any triangle
+			 * becomes thinner than a pixel at its vertices, so robustness requires some margin. */
+			const float final_pt[2] = { ((final_index % w) + 0.5f) / w, ((final_index / w) + 0.5f) / h };
+			const float threshold = SQUARE(0.7f) / (w * h);
+
+			if (dist_squared_to_looptri_uv_edges(mlooptri, mloopuv, tempPoints[final_index].tri_index, final_pt) > threshold)
+				return NOT_FOUND;
+		}
+
 		return final_index;
 	}
 }