LineArt: Noise tolerant chaining.

Instead of splitting it at each occlusion change, it tolerates short segments of "zig-zag" occlusion incoherence and doesn't split the chain at these points, thus creating a much smoother result.

Reviewed By: Antonio Vazquez (antoniov), Aleš Jelovčan (frogstomp)

Differential Revision: https://developer.blender.org/D13851

3 files changed, 62 insertions, 0 deletions

diff --git a/source/blender/gpencil_modifiers/intern/lineart/MOD_lineart.h b/source/blender/gpencil_modifiers/intern/lineart/MOD_lineart.h
index 8c5820c9667..2691dcc8408 100644
--- a/source/blender/gpencil_modifiers/intern/lineart/MOD_lineart.h
+++ b/source/blender/gpencil_modifiers/intern/lineart/MOD_lineart.h
@@ -308,6 +308,8 @@ typedef struct LineartRenderBuffer {
   bool force_crease;
   bool sharp_as_crease;
 
+  bool chain_preserve_details;
+
   /* Keep an copy of these data so when line art is running it's self-contained. */
   bool cam_is_persp;
   float cam_obmat[4][4];
diff --git a/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c b/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
index 0deb8b1c335..78ad895c93e 100644
--- a/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
+++ b/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
@@ -571,6 +571,57 @@ static void lineart_bounding_area_link_chain(LineartRenderBuffer *rb, LineartEdg
   }
 }
 
+static bool lineart_chain_fix_ambiguous_segments(LineartEdgeChain *ec,
+                                                 LineartEdgeChainItem *last_matching_eci,
+                                                 float distance_threshold,
+                                                 bool preserve_details,
+                                                 LineartEdgeChainItem **r_next_eci)
+{
+  float dist_accum = 0;
+
+  int fixed_occ = last_matching_eci->occlusion;
+  unsigned char fixed_mask = last_matching_eci->material_mask_bits;
+
+  LineartEdgeChainItem *can_skip_to = NULL;
+  LineartEdgeChainItem *last_eci = last_matching_eci;
+  for (LineartEdgeChainItem *eci = last_matching_eci->next; eci; eci = eci->next) {
+    dist_accum += len_v2v2(last_eci->pos, eci->pos);
+    if (dist_accum > distance_threshold) {
+      break;
+    }
+    last_eci = eci;
+    /* The reason for this is because we don't want visible segments to be "skipped" into
+     * connecting with invisible segments. */
+    if (eci->occlusion < fixed_occ) {
+      break;
+    }
+    if (eci->material_mask_bits == fixed_mask && eci->occlusion == fixed_occ) {
+      can_skip_to = eci;
+    }
+  }
+  if (can_skip_to) {
+    /* Either mark all in-between segments with the same occlusion and mask or delete those
+     * different ones. */
+    LineartEdgeChainItem *next_eci;
+    for (LineartEdgeChainItem *eci = last_matching_eci->next; eci != can_skip_to; eci = next_eci) {
+      next_eci = eci->next;
+      if (eci->material_mask_bits == fixed_mask && eci->occlusion == fixed_occ) {
+        continue;
+      }
+      if (preserve_details) {
+        eci->material_mask_bits = fixed_mask;
+        eci->occlusion = fixed_occ;
+      }
+      else {
+        BLI_remlink(&ec->chain, eci);
+      }
+    }
+    *r_next_eci = can_skip_to;
+    return true;
+  }
+  return false;
+}
+
 void MOD_lineart_chain_split_for_fixed_occlusion(LineartRenderBuffer *rb)
 {
   LineartEdgeChain *ec, *new_ec;
@@ -597,6 +648,13 @@ void MOD_lineart_chain_split_for_fixed_occlusion(LineartRenderBuffer *rb)
           if (lineart_point_overlapping(next_eci, eci->pos[0], eci->pos[1], 1e-5)) {
             continue;
           }
+          if (lineart_chain_fix_ambiguous_segments(ec,
+                                                   eci->prev,
+                                                   rb->chaining_image_threshold,
+                                                   rb->chain_preserve_details,
+                                                   &next_eci)) {
+            continue;
+          }
         }
         else {
           /* Set the same occlusion level for the end vertex, so when further connection is needed
diff --git a/source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c b/source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c
index 5461b80cad1..923646a133a 100644
--- a/source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c
+++ b/source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c
@@ -3158,6 +3158,8 @@ static LineartRenderBuffer *lineart_create_render_buffer(Scene *scene,
   rb->force_crease = (lmd->calculation_flags & LRT_USE_CREASE_ON_SMOOTH_SURFACES) != 0;
   rb->sharp_as_crease = (lmd->calculation_flags & LRT_USE_CREASE_ON_SHARP_EDGES) != 0;
 
+  rb->chain_preserve_details = (lmd->calculation_flags & LRT_CHAIN_PRESERVE_DETAILS) != 0;
+
   int16_t edge_types = lmd->edge_types_override;
 
   rb->use_contour = (edge_types & LRT_EDGE_FLAG_CONTOUR) != 0;