GPencil: Curvature support for length modifier.

6 files changed, 408 insertions, 70 deletions

diff --git a/source/blender/blenkernel/BKE_gpencil_geom.h b/source/blender/blenkernel/BKE_gpencil_geom.h
index d472fd6f02b..20ee9199dba 100644
--- a/source/blender/blenkernel/BKE_gpencil_geom.h
+++ b/source/blender/blenkernel/BKE_gpencil_geom.h
@@ -114,7 +114,12 @@ void BKE_gpencil_dissolve_points(struct bGPdata *gpd,
 bool BKE_gpencil_stroke_stretch(struct bGPDstroke *gps,
                                 const float dist,
                                 const float overshoot_fac,
-                                const short mode);
+                                const short mode,
+                                const bool follow_curvature,
+                                const int extra_point_count,
+                                const float edge_point_tradeoff,
+                                const float max_angle,
+                                const bool invert_curvature);
 bool BKE_gpencil_stroke_trim_points(struct bGPDstroke *gps,
                                     const int index_from,
                                     const int index_to);
diff --git a/source/blender/blenkernel/intern/gpencil_geom.cc b/source/blender/blenkernel/intern/gpencil_geom.cc
index 8ff026231f5..2d825705ab0 100644
--- a/source/blender/blenkernel/intern/gpencil_geom.cc
+++ b/source/blender/blenkernel/intern/gpencil_geom.cc
@@ -541,64 +541,241 @@ bool BKE_gpencil_stroke_sample(bGPdata *gpd, bGPDstroke *gps, const float dist,
 }
 
 /**
+ * Give extra stroke points before and after the original tip points.
+ * \param gps: Target stroke
+ * \param count_before: how many extra points to be added before a stroke
+ * \param count_after: how many extra points to be added after a stroke
+ */
+static bool BKE_gpencil_stroke_extra_points(bGPDstroke *gps,
+                                            const int count_before,
+                                            const int count_after)
+{
+  bGPDspoint *pts = gps->points;
+
+  BLI_assert(count_before >= 0);
+  BLI_assert(count_after >= 0);
+  if (!count_before && !count_after) {
+    return false;
+  }
+
+  const int new_count = count_before + count_after + gps->totpoints;
+
+  bGPDspoint *new_pts = (bGPDspoint *)MEM_mallocN(sizeof(bGPDspoint) * new_count, __func__);
+
+  for (int i = 0; i < count_before; i++) {
+    memcpy(&new_pts[i], &pts[0], sizeof(bGPDspoint));
+  }
+  memcpy(&new_pts[count_before], pts, sizeof(bGPDspoint) * gps->totpoints);
+  for (int i = new_count - count_after; i < new_count; i++) {
+    memcpy(&new_pts[i], &pts[gps->totpoints - 1], sizeof(bGPDspoint));
+  }
+
+  if (gps->dvert) {
+    MDeformVert *new_dv = (MDeformVert *)MEM_mallocN(sizeof(MDeformVert) * new_count, __func__);
+
+    for (int i = 0; i < new_count; i++) {
+      MDeformVert *dv = &gps->dvert[CLAMPIS(i - count_before, 0, gps->totpoints - 1)];
+      int inew = i;
+      new_dv[inew].flag = dv->flag;
+      new_dv[inew].totweight = dv->totweight;
+      new_dv[inew].dw = (MDeformWeight *)MEM_mallocN(sizeof(MDeformWeight) * dv->totweight,
+                                                     __func__);
+      memcpy(new_dv[inew].dw, dv->dw, sizeof(MDeformWeight) * dv->totweight);
+    }
+    BKE_gpencil_free_stroke_weights(gps);
+    MEM_freeN(gps->dvert);
+    gps->dvert = new_dv;
+  }
+
+  MEM_freeN(gps->points);
+  gps->points = new_pts;
+  gps->totpoints = new_count;
+
+  return true;
+}
+
+/**
  * Backbone stretch similar to Freestyle.
  * \param gps: Stroke to sample.
- * \param dist: Distance of one segment.
- * \param overshoot_fac: How exact is the follow curve algorithm.
+ * \param dist: Length of the added section.
+ * \param overshoot_fac: Relative length of the curve which is used to determine the extension.
  * \param mode: Affect to Start, End or Both extremes (0->Both, 1->Start, 2->End)
+ * \param follow_curvature: True for appproximating curvature of given overshoot.
+ * \param extra_point_count: When follow_curvature is true, use this amount of extra points
  */
 bool BKE_gpencil_stroke_stretch(bGPDstroke *gps,
                                 const float dist,
                                 const float overshoot_fac,
-                                const short mode)
+                                const short mode,
+                                const bool follow_curvature,
+                                const int extra_point_count,
+                                const float edge_point_tradeoff,
+                                const float max_angle,
+                                const bool invert_curvature)
 {
 #define BOTH 0
 #define START 1
 #define END 2
 
-  bGPDspoint *pt = gps->points, *last_pt, *second_last, *next_pt;
-  int i;
-  float threshold = (overshoot_fac == 0 ? 0.001f : overshoot_fac);
+  const bool do_start = ELEM(mode, BOTH, START);
+  const bool do_end = ELEM(mode, BOTH, END);
+  float used_percent_length = overshoot_fac;
+  CLAMP(used_percent_length, 1e-4f, 1.0f);
+  if (!isfinite(used_percent_length)) {
+    /* If gps->totpoints == 2, then #overshoot_fac is allowed to be non finite. */
+    used_percent_length = 0.1f;
+  }
 
-  if (gps->totpoints < 2 || dist < FLT_EPSILON) {
+  if (gps->totpoints <= 1 || dist < FLT_EPSILON || extra_point_count <= 0) {
     return false;
   }
 
-  last_pt = &pt[gps->totpoints - 1];
-  second_last = &pt[gps->totpoints - 2];
-  next_pt = &pt[1];
-
-  if (mode == BOTH || mode == START) {
-    float len1 = 0.0f;
-    i = 1;
-    while (len1 < threshold && gps->totpoints > i) {
-      next_pt = &pt[i];
-      len1 = len_v3v3(&next_pt->x, &pt->x);
-      i++;
+  /* NOTE: When it's just a straight line, we don't need to do the curvature stuff. */
+  if (!follow_curvature || gps->totpoints <= 2) {
+    /* Not following curvature, just straight line. */
+    /* NOTE: #overshoot_point_param can not be zero. */
+    float overshoot_point_param = used_percent_length * (gps->totpoints - 1);
+    float result[3];
+
+    if (do_start) {
+      int index1 = floor(overshoot_point_param);
+      int index2 = ceil(overshoot_point_param);
+      interp_v3_v3v3(result,
+                     &gps->points[index1].x,
+                     &gps->points[index2].x,
+                     fmodf(overshoot_point_param, 1.0f));
+      sub_v3_v3(result, &gps->points[0].x);
+      if (UNLIKELY(is_zero_v3(result))) {
+        sub_v3_v3v3(result, &gps->points[1].x, &gps->points[0].x);
+      }
+      madd_v3_v3fl(&gps->points[0].x, result, -dist / len_v3(result));
+    }
+
+    if (do_end) {
+      int index1 = gps->totpoints - 1 - floor(overshoot_point_param);
+      int index2 = gps->totpoints - 1 - ceil(overshoot_point_param);
+      interp_v3_v3v3(result,
+                     &gps->points[index1].x,
+                     &gps->points[index2].x,
+                     fmodf(overshoot_point_param, 1.0f));
+      sub_v3_v3(result, &gps->points[gps->totpoints - 1].x);
+      if (UNLIKELY(is_zero_v3(result))) {
+        sub_v3_v3v3(
+            result, &gps->points[gps->totpoints - 2].x, &gps->points[gps->totpoints - 1].x);
+      }
+      madd_v3_v3fl(&gps->points[gps->totpoints - 1].x, result, -dist / len_v3(result));
     }
-    float extend1 = (len1 + dist) / len1;
-    float result1[3];
-
-    interp_v3_v3v3(result1, &next_pt->x, &pt->x, extend1);
-    copy_v3_v3(&pt->x, result1);
+    return true;
   }
 
-  if (mode == BOTH || mode == END) {
-    float len2 = 0.0f;
-    i = 2;
-    while (len2 < threshold && gps->totpoints >= i) {
-      second_last = &pt[gps->totpoints - i];
-      len2 = len_v3v3(&last_pt->x, &second_last->x);
-      i++;
+  /* Curvature calculation. */
+
+  /* First allocate the new stroke size. */
+  const int first_old_index = do_start ? extra_point_count : 0;
+  const int last_old_index = gps->totpoints - 1 + first_old_index;
+  const int orig_totpoints = gps->totpoints;
+  BKE_gpencil_stroke_extra_points(gps, first_old_index, do_end ? extra_point_count : 0);
+
+  /* The overshoot length in terms of angles with at least 1 used angle. */
+  const float overshoot_parameter = used_percent_length * (orig_totpoints - 2);
+  int overshoot_pointcount = ceil(overshoot_parameter);
+  CLAMP(overshoot_pointcount, 1, orig_totpoints - 2);
+
+  float point_edge_tradeoff = 1.0f - edge_point_tradeoff;
+
+  /* Do for both sides without code duplication. */
+  float no[3], vec1[3], vec2[3], total_angle[3];
+  for (int k = 0; k < 2; k++) {
+    if ((k == 0 && !do_start) || (k == 1 && !do_end)) {
+      continue;
     }
 
-    float extend2 = (len2 + dist) / len2;
-    float result2[3];
-    interp_v3_v3v3(result2, &second_last->x, &last_pt->x, extend2);
+    const int start_i = k == 0 ? first_old_index :
+                                 last_old_index;  // first_old_index, last_old_index
+    const int dir_i = 1 - k * 2;                  // 1, -1
 
-    copy_v3_v3(&last_pt->x, result2);
-  }
+    sub_v3_v3v3(vec1, &gps->points[start_i + dir_i].x, &gps->points[start_i].x);
+    zero_v3(total_angle);
+    float segment_length = normalize_v3(vec1);
+    float overshoot_length = 0.0f;
+
+    /* Accumulate rotation angle and length. */
+    int j = 0;
+    for (int i = start_i; j < overshoot_pointcount; i += dir_i, j++) {
+      /* Don't fully add last segment to get continuity in overshoot_fac. */
+      float fac = fmin(overshoot_parameter - j, 1.0f);
+
+      /* Read segments. */
+      copy_v3_v3(vec2, vec1);
+      sub_v3_v3v3(vec1, &gps->points[i + dir_i * 2].x, &gps->points[i + dir_i].x);
+      const float len = normalize_v3(vec1);
+      float angle = angle_normalized_v3v3(vec1, vec2) * fac;
+
+      /* Add half of both adjacent legs of the current angle. */
+      const float added_len = (segment_length + len) * 0.5f * fac;
+      overshoot_length += added_len;
+      segment_length = len;
+
+      if (angle > max_angle) {
+        continue;
+      }
+      if (angle > M_PI * 0.995f) {
+        continue;
+      }
+
+      angle *= powf(added_len, point_edge_tradeoff);
+
+      cross_v3_v3v3(no, vec1, vec2);
+      normalize_v3_length(no, angle);
+      add_v3_v3(total_angle, no);
+    }
 
+    if (UNLIKELY(overshoot_length == 0.0f)) {
+      /* Don't do a proper extension if the used points are all in the same position. */
+      continue;
+    }
+
+    sub_v3_v3v3(vec1, &gps->points[start_i].x, &gps->points[start_i + dir_i].x);
+    /* In general curvature = 1/radius. For the curvature without the
+     * weights introduced point_edge_tradeoff the calculation is
+     * curvature = delta angle/delta arclength = len_v3(total_angle) / overshoot_length */
+    float curvature = normalize_v3(total_angle) / overshoot_length;
+    /* Compensate for the weights powf(added_len, point_edge_tradeoff). */
+    curvature /= powf(overshoot_length / fminf(overshoot_parameter, (float)j),
+                      point_edge_tradeoff);
+    if (invert_curvature) {
+      curvature = -curvature;
+    }
+    const float angle_step = curvature * dist / extra_point_count;
+    float step_length = dist / extra_point_count;
+    if (fabsf(angle_step) > FLT_EPSILON) {
+      /* Make a direct step length from the assigned arc step length. */
+      step_length *= sin(angle_step * 0.5f) / (angle_step * 0.5f);
+    }
+    else {
+      zero_v3(total_angle);
+    }
+    const float prev_length = normalize_v3_length(vec1, step_length);
+
+    /* Build rotation matrix here to get best performance. */
+    float rot[3][3];
+    float q[4];
+    axis_angle_to_quat(q, total_angle, angle_step);
+    quat_to_mat3(rot, q);
+
+    /* Rotate the starting direction to account for change in edge lengths. */
+    axis_angle_to_quat(q,
+                       total_angle,
+                       fmaxf(0.0f, 1.0f - fabs(point_edge_tradeoff)) *
+                           (curvature * prev_length - angle_step) / 2.0f);
+    mul_qt_v3(q, vec1);
+
+    /* Now iteratively accumulate the segments with a rotating added direction. */
+    for (int i = start_i - dir_i, j = 0; j < extra_point_count; i -= dir_i, j++) {
+      mul_v3_m3v3(vec1, rot, vec1);
+      add_v3_v3v3(&gps->points[i].x, vec1, &gps->points[i + dir_i].x);
+    }
+  }
   return true;
 }
 
@@ -749,6 +926,7 @@ bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist, const short mo
 
   second_last = &pt[gps->totpoints - 2];
 
+  float len;
   float len1, cut_len1;
   float len2, cut_len2;
   len1 = len2 = cut_len1 = cut_len2 = 0.0f;
@@ -759,11 +937,13 @@ bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist, const short mo
     i = 0;
     index_end = gps->totpoints - 1;
     while (len1 < dist && gps->totpoints > i + 1) {
-      len1 += len_v3v3(&pt[i].x, &pt[i + 1].x);
+      len = len_v3v3(&pt[i].x, &pt[i + 1].x);
+      len1 += len;
       cut_len1 = len1 - dist;
       i++;
     }
     index_start = i - 1;
+    interp_v3_v3v3(&pt[index_start].x, &pt[index_start + 1].x, &pt[index_start].x, cut_len1 / len);
   }
 
   if (mode == END) {
@@ -771,18 +951,20 @@ bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist, const short mo
     i = 2;
     while (len2 < dist && gps->totpoints >= i) {
       second_last = &pt[gps->totpoints - i];
-      len2 += len_v3v3(&second_last[1].x, &second_last->x);
+      len = len_v3v3(&second_last[1].x, &second_last->x);
+      len2 += len;
       cut_len2 = len2 - dist;
       i++;
     }
     index_end = gps->totpoints - i + 2;
+    interp_v3_v3v3(&pt[index_end].x, &pt[index_end - 1].x, &pt[index_end].x, cut_len2 / len);
   }
 
   if (index_end <= index_start) {
     index_start = index_end = 0; /* empty stroke */
   }
 
-  if ((index_end == index_start + 1) && (cut_len1 + cut_len2 < dist)) {
+  if ((index_end == index_start + 1) && (cut_len1 + cut_len2 < 0)) {
     index_start = index_end = 0; /* no length left to cut */
   }
 
diff --git a/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c b/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
index 6aa0e6c152e..b104d4685e0 100644
--- a/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
+++ b/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
@@ -72,9 +72,14 @@ static void copyData(const GpencilModifierData *md, GpencilModifierData *target)
 }
 
 static bool gpencil_modify_stroke(bGPDstroke *gps,
-                                  float length,
+                                  const float length,
                                   const float overshoot_fac,
-                                  const short len_mode)
+                                  const short len_mode,
+                                  const bool use_curvature,
+                                  const int extra_point_count,
+                                  const float edge_point_tradeoff,
+                                  const float max_angle,
+                                  const bool invert_curvature)
 {
   bool changed = false;
   if (length == 0.0f) {
@@ -82,10 +87,18 @@ static bool gpencil_modify_stroke(bGPDstroke *gps,
   }
 
   if (length > 0.0f) {
-    BKE_gpencil_stroke_stretch(gps, length, overshoot_fac, len_mode);
+    changed = BKE_gpencil_stroke_stretch(gps,
+                                         length,
+                                         overshoot_fac,
+                                         len_mode,
+                                         use_curvature,
+                                         extra_point_count,
+                                         edge_point_tradeoff,
+                                         max_angle,
+                                         invert_curvature);
   }
   else {
-    changed |= BKE_gpencil_stroke_shrink(gps, fabs(length), len_mode);
+    changed = BKE_gpencil_stroke_shrink(gps, fabs(length), len_mode);
   }
 
   return changed;
@@ -96,12 +109,51 @@ static void applyLength(LengthGpencilModifierData *lmd, bGPdata *gpd, bGPDstroke
   bool changed = false;
   const float len = (lmd->mode == GP_LENGTH_ABSOLUTE) ? 1.0f :
                                                         BKE_gpencil_stroke_length(gps, true);
+  const int totpoints = gps->totpoints;
   if (len < FLT_EPSILON) {
     return;
   }
 
-  changed |= gpencil_modify_stroke(gps, len * lmd->start_fac, lmd->overshoot_fac, 1);
-  changed |= gpencil_modify_stroke(gps, len * lmd->end_fac, lmd->overshoot_fac, 2);
+  /* Always do the stretching first since it might depend on points which could be deleted by the
+   * shrink. */
+  float first_fac = lmd->start_fac;
+  int first_mode = 1;
+  float second_fac = lmd->end_fac;
+  int second_mode = 2;
+  if (first_fac < 0) {
+    SWAP(float, first_fac, second_fac);
+    SWAP(int, first_mode, second_mode);
+  }
+
+  const int first_extra_point_count = ceil(first_fac * lmd->point_density);
+  const int second_extra_point_count = ceil(second_fac * lmd->point_density);
+
+  changed |= gpencil_modify_stroke(gps,
+                                   len * first_fac,
+                                   lmd->overshoot_fac,
+                                   first_mode,
+                                   lmd->flag & GP_LENGTH_USE_CURVATURE,
+                                   first_extra_point_count,
+                                   lmd->edge_point_tradeoff,
+                                   lmd->max_angle,
+                                   lmd->flag & GP_LENGTH_INVERT_CURVATURE);
+  /* HACK: The second #overshoot_fac needs to be adjusted because it is not
+   * done in the same stretch call, because it can have a different length.
+   * The adjustment needs to be stable when
+   * ceil(overshoot_fac*(gps->totpoints - 2)) is used in stretch and never
+   * produce a result highter than totpoints - 2. */
+  const float second_overshoot_fac = lmd->overshoot_fac * (totpoints - 2) /
+                                     ((float)gps->totpoints - 2) *
+                                     (1.0f - 0.1f / (totpoints - 1.0f));
+  changed |= gpencil_modify_stroke(gps,
+                                   len * second_fac,
+                                   second_overshoot_fac,
+                                   second_mode,
+                                   lmd->flag & GP_LENGTH_USE_CURVATURE,
+                                   second_extra_point_count,
+                                   lmd->edge_point_tradeoff,
+                                   lmd->max_angle,
+                                   lmd->flag & GP_LENGTH_INVERT_CURVATURE);
 
   if (changed) {
     BKE_gpencil_stroke_geometry_update(gpd, gps);
@@ -117,20 +169,25 @@ static void deformStroke(GpencilModifierData *md,
 {
   bGPdata *gpd = ob->data;
   LengthGpencilModifierData *lmd = (LengthGpencilModifierData *)md;
-  if (is_stroke_affected_by_modifier(ob,
-                                     lmd->layername,
-                                     lmd->material,
-                                     lmd->pass_index,
-                                     lmd->layer_pass,
-                                     1,
-                                     gpl,
-                                     gps,
-                                     lmd->flag & GP_LENGTH_INVERT_LAYER,
-                                     lmd->flag & GP_LENGTH_INVERT_PASS,
-                                     lmd->flag & GP_LENGTH_INVERT_LAYERPASS,
-                                     lmd->flag & GP_LENGTH_INVERT_MATERIAL)) {
-    applyLength(lmd, gpd, gps);
+  if (!is_stroke_affected_by_modifier(ob,
+                                      lmd->layername,
+                                      lmd->material,
+                                      lmd->pass_index,
+                                      lmd->layer_pass,
+                                      1,
+                                      gpl,
+                                      gps,
+                                      lmd->flag & GP_LENGTH_INVERT_LAYER,
+                                      lmd->flag & GP_LENGTH_INVERT_PASS,
+                                      lmd->flag & GP_LENGTH_INVERT_LAYERPASS,
+                                      lmd->flag & GP_LENGTH_INVERT_MATERIAL)) {
+    return;
   }
+  if ((gps->flag & GP_STROKE_CYCLIC) != 0) {
+    /* Don't affect cyclic strokes as they have no start/end. */
+    return;
+  }
+  applyLength(lmd, gpd, gps);
 }
 
 static void bakeModifier(Main *UNUSED(bmain),
@@ -168,8 +225,14 @@ static void panel_draw(const bContext *UNUSED(C), Panel *panel)
 
   uiLayout *col = uiLayoutColumn(layout, true);
 
-  uiItemR(col, ptr, "start_factor", 0, IFACE_("Start"), ICON_NONE);
-  uiItemR(col, ptr, "end_factor", 0, IFACE_("End"), ICON_NONE);
+  if (RNA_enum_get(ptr, "mode") == GP_LENGTH_RELATIVE) {
+    uiItemR(col, ptr, "start_factor", 0, IFACE_("Start"), ICON_NONE);
+    uiItemR(col, ptr, "end_factor", 0, IFACE_("End"), ICON_NONE);
+  }
+  else {
+    uiItemR(col, ptr, "start_length", 0, IFACE_("Start"), ICON_NONE);
+    uiItemR(col, ptr, "end_length", 0, IFACE_("End"), ICON_NONE);
+  }
 
   uiItemR(layout, ptr, "overshoot_factor", UI_ITEM_R_SLIDER, IFACE_("Overshoot"), ICON_NONE);
 
@@ -181,11 +244,40 @@ static void mask_panel_draw(const bContext *UNUSED(C), Panel *panel)
   gpencil_modifier_masking_panel_draw(panel, true, false);
 }
 
+static void curvature_header_draw(const bContext *UNUSED(C), Panel *panel)
+{
+  uiLayout *layout = panel->layout;
+
+  PointerRNA *ptr = gpencil_modifier_panel_get_property_pointers(panel, NULL);
+
+  uiItemR(layout, ptr, "use_curvature", 0, IFACE_("Curvature"), ICON_NONE);
+}
+
+static void curvature_panel_draw(const bContext *UNUSED(C), Panel *panel)
+{
+  uiLayout *layout = panel->layout;
+
+  PointerRNA *ptr = gpencil_modifier_panel_get_property_pointers(panel, NULL);
+
+  uiLayoutSetPropSep(layout, true);
+
+  uiLayout *col = uiLayoutColumn(layout, false);
+
+  uiLayoutSetActive(col, RNA_boolean_get(ptr, "use_curvature"));
+
+  uiItemR(col, ptr, "point_density", 0, NULL, ICON_NONE);
+  uiItemR(col, ptr, "edge_point_tradeoff", 0, NULL, ICON_NONE);
+  uiItemR(col, ptr, "max_angle", 0, NULL, ICON_NONE);
+  uiItemR(col, ptr, "invert_curvature", 0, IFACE_("Invert"), ICON_NONE);
+}
+
 static void panelRegister(ARegionType *region_type)
 {
   PanelType *panel_type = gpencil_modifier_panel_register(
       region_type, eGpencilModifierType_Length, panel_draw);
   gpencil_modifier_subpanel_register(
+      region_type, "curvature", "", curvature_header_draw, curvature_panel_draw, panel_type);
+  gpencil_modifier_subpanel_register(
       region_type, "mask", "Influence", NULL, mask_panel_draw, panel_type);
 }
 
diff --git a/source/blender/makesdna/DNA_gpencil_modifier_defaults.h b/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
index 450527c7443..9d7dc50bcbf 100644
--- a/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
+++ b/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
@@ -314,9 +314,13 @@
   { \
     .start_fac = 0.1f,\
     .end_fac = 0.1f,\
-    .overshoot_fac = 0.01f,\
+    .overshoot_fac = 0.1f,\
     .pass_index = 0,\
     .material = NULL,\
+    .flag = GP_LENGTH_USE_CURVATURE,\
+    .point_density = 30.0f,\
+    .edge_point_tradeoff = 1.0f,\
+    .max_angle = DEG2RAD(170.0f),\
   }
 
 #define _DNA_DEFAULT_DashGpencilModifierData \
diff --git a/source/blender/makesdna/DNA_gpencil_modifier_types.h b/source/blender/makesdna/DNA_gpencil_modifier_types.h
index d3429329ef6..cd34717eb4b 100644
--- a/source/blender/makesdna/DNA_gpencil_modifier_types.h
+++ b/source/blender/makesdna/DNA_gpencil_modifier_types.h
@@ -493,7 +493,10 @@ typedef struct LengthGpencilModifierData {
   float overshoot_fac;
   /** Modifier mode. */
   int mode;
-  char _pad[4];
+  /* Curvature paramters. */
+  float point_density;
+  float edge_point_tradeoff;
+  float max_angle;
 } LengthGpencilModifierData;
 
 typedef enum eLengthGpencil_Flag {
@@ -501,6 +504,8 @@ typedef enum eLengthGpencil_Flag {
   GP_LENGTH_INVERT_PASS = (1 << 1),
   GP_LENGTH_INVERT_LAYERPASS = (1 << 2),
   GP_LENGTH_INVERT_MATERIAL = (1 << 3),
+  GP_LENGTH_USE_CURVATURE = (1 << 4),
+  GP_LENGTH_INVERT_CURVATURE = (1 << 5),
 } eLengthGpencil_Flag;
 
 typedef enum eLengthGpencil_Type {
diff --git a/source/blender/makesrna/intern/rna_gpencil_modifier.c b/source/blender/makesrna/intern/rna_gpencil_modifier.c
index 4fa33424994..9b8726d4491 100644
--- a/source/blender/makesrna/intern/rna_gpencil_modifier.c
+++ b/source/blender/makesrna/intern/rna_gpencil_modifier.c
@@ -74,6 +74,11 @@ const EnumPropertyItem rna_enum_object_greasepencil_modifier_type_items[] = {
      ICON_MOD_DASH,
      "Dot Dash",
      "Generate dot-dash styled strokes"},
+    {eGpencilModifierType_Length,
+     "GP_LENGTH",
+     ICON_MOD_LENGTH,
+     "Length",
+     "Extend or shrink strokes"},
     {eGpencilModifierType_Lineart,
      "GP_LINEART",
      ICON_MOD_LINEART,
@@ -120,11 +125,6 @@ const EnumPropertyItem rna_enum_object_greasepencil_modifier_type_items[] = {
      ICON_MOD_LATTICE,
      "Lattice",
      "Deform strokes using lattice"},
-    {eGpencilModifierType_Length,
-     "GP_LENGTH",
-     ICON_MOD_LENGTH,
-     "Length",
-     "Extend or shrink strokes"},
     {eGpencilModifierType_Noise, "GP_NOISE", ICON_MOD_NOISE, "Noise", "Add noise to strokes"},
     {eGpencilModifierType_Offset,
      "GP_OFFSET",
@@ -3278,13 +3278,25 @@ static void rna_def_modifier_gpencillength(BlenderRNA *brna)
 
   prop = RNA_def_property(srna, "start_factor", PROP_FLOAT, PROP_NONE);
   RNA_def_property_float_sdna(prop, NULL, "start_fac");
-  RNA_def_property_ui_range(prop, -10.0f, 10.0f, 0.1, 1);
+  RNA_def_property_ui_range(prop, -10.0f, 10.0f, 0.1, 2);
   RNA_def_property_ui_text(prop, "Start Factor", "Length difference for each segment");
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 
   prop = RNA_def_property(srna, "end_factor", PROP_FLOAT, PROP_NONE);
   RNA_def_property_float_sdna(prop, NULL, "end_fac");
-  RNA_def_property_ui_range(prop, -10.0f, 10.0f, 0.1, 1);
+  RNA_def_property_ui_range(prop, -10.0f, 10.0f, 0.1, 2);
+  RNA_def_property_ui_text(prop, "End Factor", "Length difference for each segment");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "start_length", PROP_FLOAT, PROP_DISTANCE);
+  RNA_def_property_float_sdna(prop, NULL, "start_fac");
+  RNA_def_property_ui_range(prop, -100.0f, 100.0f, 0.1f, 3);
+  RNA_def_property_ui_text(prop, "Start Factor", "Length difference for each segment");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "end_length", PROP_FLOAT, PROP_DISTANCE);
+  RNA_def_property_float_sdna(prop, NULL, "end_fac");
+  RNA_def_property_ui_range(prop, -100.0f, 100.0f, 0.1f, 3);
   RNA_def_property_ui_text(prop, "End Factor", "Length difference for each segment");
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 
@@ -3294,7 +3306,7 @@ static void rna_def_modifier_gpencillength(BlenderRNA *brna)
   RNA_def_property_ui_text(
       prop,
       "Overshoot Factor",
-      "Defines how precise must follow the stroke trajectory for the overshoot extremes");
+      "Defines what portion of the stroke is used for curvature approximation.");
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 
   prop = RNA_def_property(srna, "mode", PROP_ENUM, PROP_NONE);
@@ -3303,6 +3315,44 @@ static void rna_def_modifier_gpencillength(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Mode", "Mode to define length");
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 
+  prop = RNA_def_property(srna, "use_curvature", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_USE_CURVATURE);
+  RNA_def_property_ui_text(prop, "Use Curvature", "Follow the curvature of the stroke");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "invert_curvature", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_INVERT_CURVATURE);
+  RNA_def_property_ui_text(
+      prop, "Invert Curvature", "Invert the curvature of the stroke's extension");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "point_density", PROP_FLOAT, PROP_NONE);
+  RNA_def_property_range(prop, 0.1f, 1000.0f);
+  RNA_def_property_ui_range(prop, 0.1f, 1000.0f, 1.0f, 1);
+  RNA_def_property_ui_scale_type(prop, PROP_SCALE_CUBIC);
+  RNA_def_property_ui_text(
+      prop, "Point Density", "Multiplied by Start/End for the total added point count");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "edge_point_tradeoff", PROP_FLOAT, PROP_FACTOR);
+  RNA_def_property_range(prop, -2.0f, 3.0f);
+  RNA_def_property_ui_range(prop, 0.0f, 1.0f, 0.1f, 2);
+  RNA_def_property_ui_text(
+      prop,
+      "Smoothness",
+      "Factor to determine how much to use a theoretical smooth arc between points instead of "
+      "straight lines when determining the extrapolation shape");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "max_angle", PROP_FLOAT, PROP_ANGLE);
+  RNA_def_property_ui_text(prop,
+                           "Filter Angle",
+                           "Ignore points on the stroke that deviate from their neighbors by more "
+                           "than this angle when determining the extrapolation shape");
+  RNA_def_property_range(prop, 0.0f, DEG2RAD(180.0f));
+  RNA_def_property_ui_range(prop, 0.0f, DEG2RAD(179.5f), 10.0f, 1);
+  RNA_def_property_update(prop, NC_SCENE, "rna_GpencilModifier_update");
+
   prop = RNA_def_property(srna, "layer", PROP_STRING, PROP_NONE);
   RNA_def_property_string_sdna(prop, NULL, "layername");
   RNA_def_property_ui_text(prop, "Layer", "Layer name");