1 files changed, 59 insertions, 19 deletions

diff --git a/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c b/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
index b666eb677eb..28fd8ff01b9 100644
--- a/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
+++ b/source/blender/gpencil_modifiers/intern/lineart/lineart_chain.c
@@ -971,27 +971,67 @@ void MOD_lineart_chain_clear_picked_flag(LineartCache *lc)
 void MOD_lineart_smooth_chains(LineartRenderBuffer *rb, float tolerance)
 {
   LISTBASE_FOREACH (LineartEdgeChain *, ec, &rb->chains) {
-    LineartEdgeChainItem *next_eci;
-    for (LineartEdgeChainItem *eci = ec->chain.first; eci; eci = next_eci) {
-      next_eci = eci->next;
-      LineartEdgeChainItem *eci2, *eci3, *eci4;
-
-      /* Not enough point to do simplify. */
-      if ((!(eci2 = eci->next)) || (!(eci3 = eci2->next))) {
-        continue;
-      }
-
-      /* No need to care for different line types/occlusion and so on, because at this stage they
-       * are all the same within a chain. */
-
-      /* If p3 is within the p1-p2 segment of a width of "tolerance". */
-      if (dist_to_line_segment_v2(eci3->pos, eci->pos, eci2->pos) < tolerance) {
-        /* And if p4 is on the extension of p1-p2 , we remove p3. */
-        if ((eci4 = eci3->next) && (dist_to_line_v2(eci4->pos, eci->pos, eci2->pos) < tolerance)) {
-          BLI_remlink(&ec->chain, eci3);
-          next_eci = eci;
+    /* Go through the chain two times, once from each direction. */
+    for (int times = 0; times < 2; times++) {
+      for (LineartEdgeChainItem *eci = ec->chain.first, *next_eci = eci->next; eci;
+           eci = next_eci) {
+        LineartEdgeChainItem *eci2, *eci3, *eci4;
+
+        if ((!(eci2 = eci->next)) || (!(eci3 = eci2->next))) {
+          /* Not enough points to simplify. */
+          next_eci = eci->next;
+          continue;
         }
+        /* No need to care for different line types/occlusion and so on, because at this stage they
+         * are all the same within a chain.
+         *
+         * We need to simplify a chain from this:
+         * 1-----------2
+         *        3-----------4
+         * to this:
+         * 1-----------2--_
+         *                 `--4 */
+
+        /* If p3 is within the p1-p2 segment of a width of "tolerance", in other words, p3 is
+         * approximately on the segment of p1-p2. */
+        if (dist_to_line_segment_v2(eci3->pos, eci->pos, eci2->pos) < tolerance) {
+          float vec2[2], vec3[2], v2n[2], ratio, len2;
+          sub_v2_v2v2(vec2, eci2->pos, eci->pos);
+          sub_v2_v2v2(vec3, eci3->pos, eci->pos);
+          normalize_v2_v2(v2n, vec2);
+          ratio = dot_v2v2(v2n, vec3);
+          len2 = len_v2(vec2);
+          /* Because this smoothing applies on geometries of different scales in the same scene,
+           * some small scale features (e.g. the "tails" on the inner ring of a torus geometry)
+           * could be completely erased if the tolerance value is set for accommodating the entire
+           * scene. Those situations typically result in (ratio << 0), looks like this:
+           *                         1---2
+           * 3-------------------------------4
+           * (this sort of long zigzag obviously are "features" that can't be erased)
+           * setting a ratio of -10 turned out to be a reasonable threshold in tests. */
+          if (ratio < len2 && ratio > -len2 * 10) {
+            /* We only remove p3 if p4 is on the extension of p1->p2. */
+            if ((eci4 = eci3->next) &&
+                (dist_to_line_v2(eci4->pos, eci->pos, eci2->pos) < tolerance)) {
+              BLI_remlink(&ec->chain, eci3);
+              next_eci = eci;
+              continue;
+            }
+            if (!eci4) {
+              /* See if the last segment's direction is reversed, if so remove that.
+               * Basically we don't need to preserve p3 if the entire chain looked like this:
+               * ...----1----3===2 */
+              if (len_v2(vec2) > len_v2(vec3)) {
+                BLI_remlink(&ec->chain, eci3);
+              }
+              next_eci = NULL;
+              continue;
+            }
+          }
+        }
+        next_eci = eci->next;
       }
+      BLI_listbase_reverse(&ec->chain);
     }
   }
 }