GPencil: Improve Interpolate AutoFlip algorithm II

Improved the calculation of the distances and changed the variable names to make it more readable.

Still, there are some corner cases that could be not handled properly, but we keep as is to test more real drawings to analyze if the small number of corner cases needs to be fixed or not.

1 files changed, 42 insertions, 15 deletions

diff --git a/source/blender/editors/gpencil/gpencil_interpolate.c b/source/blender/editors/gpencil/gpencil_interpolate.c
index 9045cc8cfda..4fdae7f9c53 100644
--- a/source/blender/editors/gpencil/gpencil_interpolate.c
+++ b/source/blender/editors/gpencil/gpencil_interpolate.c
@@ -177,44 +177,71 @@ static bool gpencil_stroke_need_flip(Depsgraph *depsgraph,
   /* calculate parent matrix */
   BKE_gpencil_layer_transform_matrix_get(depsgraph, ob, gpl, diff_mat);
   bGPDspoint *pt, pt_dummy_ps;
-  float v1a[2], v1b[2], v2a[2], v2b[2];
+  float v_from_start[2], v_to_start[2], v_from_end[2], v_to_end[2];
 
   /* Line from start of strokes. */
   pt = &gps_from->points[0];
   gpencil_point_to_parent_space(pt, diff_mat, &pt_dummy_ps);
-  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v1a[0], &v1a[1]);
+  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v_from_start[0], &v_from_start[1]);
 
   pt = &gps_to->points[0];
   gpencil_point_to_parent_space(pt, diff_mat, &pt_dummy_ps);
-  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v1b[0], &v1b[1]);
+  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v_to_start[0], &v_to_start[1]);
 
   /* Line from end of strokes. */
   pt = &gps_from->points[gps_from->totpoints - 1];
   gpencil_point_to_parent_space(pt, diff_mat, &pt_dummy_ps);
-  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v2a[0], &v2a[1]);
+  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v_from_end[0], &v_from_end[1]);
 
   pt = &gps_to->points[gps_to->totpoints - 1];
   gpencil_point_to_parent_space(pt, diff_mat, &pt_dummy_ps);
-  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v2b[0], &v2b[1]);
+  gpencil_point_to_xy_fl(gsc, gps_from, &pt_dummy_ps, &v_to_end[0], &v_to_end[1]);
 
-  if (isect_seg_seg_v2(v1a, v1b, v2a, v2b) == ISECT_LINE_LINE_CROSS) {
+  const bool isect_lines = (isect_seg_seg_v2(v_from_start, v_to_start, v_from_end, v_to_end) ==
+                            ISECT_LINE_LINE_CROSS);
+
+  /* If the vectors intersect or are almost parallel. */
+  if (isect_lines) {
+    /* For very sharp angles, check distance between extremes. */
     float v1[2], v2[2];
-    sub_v2_v2v2(v1, v1b, v1a);
-    sub_v2_v2v2(v2, v2b, v2a);
+    sub_v2_v2v2(v1, v_to_start, v_from_start);
+    sub_v2_v2v2(v2, v_to_end, v_from_end);
     float angle = angle_v2v2(v1, v2);
-    /* For very sharp angles, check distance between extremes. */
     if (angle < DEG2RADF(15.0f)) {
-      float dist_start = len_squared_v2v2(v1a, v2a);
-      float dist_end = len_squared_v2v2(v1a, v2b);
-      if (dist_end < dist_start) {
-        return true;
+      /* Check the original stroke orientation using a point of destination stroke
+       * `(S)<--??-->(E)   <--->`. */
+      float dist_start = len_squared_v2v2(v_from_start, v_to_start);
+      float dist_end = len_squared_v2v2(v_from_end, v_to_start);
+      /* Oriented with end nearer of destination stroke.
+       * `(S)--->(E) <--->` */
+      if (dist_start >= dist_end) {
+        dist_start = len_squared_v2v2(v_from_end, v_to_start);
+        dist_end = len_squared_v2v2(v_from_end, v_to_end);
+        /* `(S)--->(E) (E)<---(S)` */
+        return (dist_start >= dist_end);
+      }
+      else {
+        /* Oriented inversed with original stroke start near of destination stroke.
+         * `(E)<----(S) <--->` */
+        dist_start = len_squared_v2v2(v_from_start, v_to_start);
+        dist_end = len_squared_v2v2(v_from_start, v_to_end);
+        /* `(E)<---(S) (S)--->(E)` */
+        return (dist_start < dist_end);
       }
-      return false;
     }
 
     return true;
   }
-
+  else {
+    /* Check that both vectors have the same direction. */
+    float v1[2], v2[2];
+    sub_v2_v2v2(v1, v_from_end, v_from_start);
+    sub_v2_v2v2(v2, v_to_end, v_to_start);
+    if (((v1[0] > 0.0f && v2[0] < 0.0f) || (v1[0] < 0.0f && v2[0] > 0.0f)) &&
+        ((v1[1] > 0.0f && v2[1] < 0.0f) || (v1[1] < 0.0f && v2[1] > 0.0f))) {
+      return true;
+    }
+  }
   return false;
 }