GPencil: Adding length modifier.

Reviewed By: Antonio Vazquez (antoniov)

Differential Revision: https://developer.blender.org/D8264

11 files changed, 452 insertions, 68 deletions

diff --git a/source/blender/blenkernel/BKE_gpencil_geom.h b/source/blender/blenkernel/BKE_gpencil_geom.h
index a9bd0a524c4..8fc3ce133a0 100644
--- a/source/blender/blenkernel/BKE_gpencil_geom.h
+++ b/source/blender/blenkernel/BKE_gpencil_geom.h
@@ -111,7 +111,10 @@ void BKE_gpencil_dissolve_points(struct bGPdata *gpd,
                                  struct bGPDstroke *gps,
                                  const short tag);
 
-bool BKE_gpencil_stroke_stretch(struct bGPDstroke *gps, const float dist, const float tip_length);
+bool BKE_gpencil_stroke_stretch(struct bGPDstroke *gps,
+                                const float dist,
+                                const float overshoot_fac,
+                                const short mode);
 bool BKE_gpencil_stroke_trim_points(struct bGPDstroke *gps,
                                     const int index_from,
                                     const int index_to);
@@ -135,7 +138,7 @@ bool BKE_gpencil_stroke_split(struct bGPdata *gpd,
                               struct bGPDstroke *gps,
                               const int before_index,
                               struct bGPDstroke **remaining_gps);
-bool BKE_gpencil_stroke_shrink(struct bGPDstroke *gps, const float dist);
+bool BKE_gpencil_stroke_shrink(struct bGPDstroke *gps, const float dist, const short mode);
 
 float BKE_gpencil_stroke_length(const struct bGPDstroke *gps, bool use_3d);
 float BKE_gpencil_stroke_segment_length(const struct bGPDstroke *gps,
diff --git a/source/blender/blenkernel/intern/gpencil_geom.c b/source/blender/blenkernel/intern/gpencil_geom.c
index 501ee0c2014..7f839650f33 100644
--- a/source/blender/blenkernel/intern/gpencil_geom.c
+++ b/source/blender/blenkernel/intern/gpencil_geom.c
@@ -530,14 +530,23 @@ bool BKE_gpencil_stroke_sample(bGPdata *gpd, bGPDstroke *gps, const float dist,
 
 /**
  * Backbone stretch similar to Freestyle.
- * \param gps: Stroke to sample
- * \param dist: Distance of one segment
- * \param tip_length: Ignore tip jittering, set zero to use default value.
+ * \param gps: Stroke to sample.
+ * \param dist: Distance of one segment.
+ * \param overshoot_fac: How exact is the follow curve algorithm.
+ * \param mode: Affect to Start, End or Both extremes (0->Both, 1->Start, 2->End)
  */
-bool BKE_gpencil_stroke_stretch(bGPDstroke *gps, const float dist, const float tip_length)
+bool BKE_gpencil_stroke_stretch(bGPDstroke *gps,
+                                const float dist,
+                                const float overshoot_fac,
+                                const short mode)
 {
+#define BOTH 0
+#define START 1
+#define END 2
+
   bGPDspoint *pt = gps->points, *last_pt, *second_last, *next_pt;
-  float threshold = (tip_length == 0 ? 0.001f : tip_length);
+  int i;
+  float threshold = (overshoot_fac == 0 ? 0.001f : overshoot_fac);
 
   if (gps->totpoints < 2 || dist < FLT_EPSILON) {
     return false;
@@ -547,33 +556,36 @@ bool BKE_gpencil_stroke_stretch(bGPDstroke *gps, const float dist, const float t
   second_last = &pt[gps->totpoints - 2];
   next_pt = &pt[1];
 
-  float len1 = 0.0f;
-  float len2 = 0.0f;
+  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++;
+    }
+    float extend1 = (len1 + dist) / len1;
+    float result1[3];
 
-  int i = 1;
-  while (len1 < threshold && gps->totpoints > i) {
-    next_pt = &pt[i];
-    len1 = len_v3v3(&next_pt->x, &pt->x);
-    i++;
+    interp_v3_v3v3(result1, &next_pt->x, &pt->x, extend1);
+    copy_v3_v3(&pt->x, result1);
   }
 
-  i = 2;
-  while (len2 < threshold && gps->totpoints >= i) {
-    second_last = &pt[gps->totpoints - i];
-    len2 = len_v3v3(&last_pt->x, &second_last->x);
-    i++;
-  }
-
-  float extend1 = (len1 + dist) / len1;
-  float extend2 = (len2 + dist) / len2;
-
-  float result1[3], result2[3];
+  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++;
+    }
 
-  interp_v3_v3v3(result1, &next_pt->x, &pt->x, extend1);
-  interp_v3_v3v3(result2, &second_last->x, &last_pt->x, extend2);
+    float extend2 = (len2 + dist) / len2;
+    float result2[3];
+    interp_v3_v3v3(result2, &second_last->x, &last_pt->x, extend2);
 
-  copy_v3_v3(&pt->x, result1);
-  copy_v3_v3(&last_pt->x, result2);
+    copy_v3_v3(&last_pt->x, result2);
+  }
 
   return true;
 }
@@ -702,48 +714,64 @@ bool BKE_gpencil_stroke_split(bGPdata *gpd,
  * Shrink the stroke by length.
  * \param gps: Stroke to shrink
  * \param dist: delta length
+ * \param mode: 1->Start, 2->End
  */
-bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist)
+bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist, const short mode)
 {
+#define START 1
+#define END 2
+
   bGPDspoint *pt = gps->points, *second_last;
   int i;
 
-  if (gps->totpoints < 2 || dist < FLT_EPSILON) {
+  if (gps->totpoints < 2) {
+    if (gps->totpoints == 1) {
+      second_last = &pt[1];
+      if (len_v3v3(&second_last->x, &pt->x) < dist) {
+        BKE_gpencil_stroke_trim_points(gps, 0, 0);
+        return true;
+      }
+    }
+
     return false;
   }
 
   second_last = &pt[gps->totpoints - 2];
 
-  float len1, this_len1, cut_len1;
-  float len2, this_len2, cut_len2;
-  int index_start, index_end;
-
-  len1 = len2 = this_len1 = this_len2 = cut_len1 = cut_len2 = 0.0f;
-
-  i = 1;
-  while (len1 < dist && gps->totpoints > i - 1) {
-    this_len1 = len_v3v3(&pt[i].x, &pt[i + 1].x);
-    len1 += this_len1;
-    cut_len1 = len1 - dist;
-    i++;
+  float len1, cut_len1;
+  float len2, cut_len2;
+  len1 = len2 = cut_len1 = cut_len2 = 0.0f;
+
+  int index_start = 0;
+  int index_end = 0;
+  if (mode == START) {
+    i = 0;
+    index_end = gps->totpoints - 1;
+    while (len1 < dist && gps->totpoints > i + 1) {
+      len1 += len_v3v3(&pt[i].x, &pt[i + 1].x);
+      cut_len1 = len1 - dist;
+      i++;
+    }
+    index_start = i - 1;
   }
-  index_start = i - 2;
 
-  i = 2;
-  while (len2 < dist && gps->totpoints >= i) {
-    second_last = &pt[gps->totpoints - i];
-    this_len2 = len_v3v3(&second_last[1].x, &second_last->x);
-    len2 += this_len2;
-    cut_len2 = len2 - dist;
-    i++;
+  if (mode == END) {
+    index_start = 0;
+    i = 2;
+    while (len2 < dist && gps->totpoints >= i) {
+      second_last = &pt[gps->totpoints - i];
+      len2 += len_v3v3(&second_last[1].x, &second_last->x);
+      cut_len2 = len2 - dist;
+      i++;
+    }
+    index_end = gps->totpoints - i + 2;
   }
-  index_end = gps->totpoints - i + 2;
 
-  if (len1 < dist || len2 < dist || index_end <= index_start) {
+  if (index_end <= index_start) {
     index_start = index_end = 0; /* empty stroke */
   }
 
-  if ((index_end == index_start + 1) && (cut_len1 + cut_len2 > 1.0f)) {
+  if ((index_end == index_start + 1) && (cut_len1 + cut_len2 < dist)) {
     index_start = index_end = 0; /* no length left to cut */
   }
 
@@ -753,22 +781,8 @@ bool BKE_gpencil_stroke_shrink(bGPDstroke *gps, const float dist)
     return false;
   }
 
-  pt = gps->points;
-
-  float cut1 = cut_len1 / this_len1;
-  float cut2 = cut_len2 / this_len2;
-
-  float result1[3], result2[3];
-
-  interp_v3_v3v3(result1, &pt[1].x, &pt[0].x, cut1);
-  interp_v3_v3v3(result2, &pt[gps->totpoints - 2].x, &pt[gps->totpoints - 1].x, cut2);
-
-  copy_v3_v3(&pt[0].x, result1);
-  copy_v3_v3(&pt[gps->totpoints - 1].x, result2);
-
   return true;
 }
-
 /**
  * Apply smooth position to stroke point.
  * \param gps: Stroke to smooth
diff --git a/source/blender/gpencil_modifiers/CMakeLists.txt b/source/blender/gpencil_modifiers/CMakeLists.txt
index 9690f47c862..f39306ac9d0 100644
--- a/source/blender/gpencil_modifiers/CMakeLists.txt
+++ b/source/blender/gpencil_modifiers/CMakeLists.txt
@@ -54,6 +54,7 @@ set(SRC
   intern/MOD_gpencilcolor.c
   intern/MOD_gpencilhook.c
   intern/MOD_gpencillattice.c
+  intern/MOD_gpencillength.c
   intern/MOD_gpencillineart.c
   intern/MOD_gpencilmirror.c
   intern/MOD_gpencilmultiply.c
diff --git a/source/blender/gpencil_modifiers/MOD_gpencil_modifiertypes.h b/source/blender/gpencil_modifiers/MOD_gpencil_modifiertypes.h
index e6ce7983a0f..f8a28f2e5cb 100644
--- a/source/blender/gpencil_modifiers/MOD_gpencil_modifiertypes.h
+++ b/source/blender/gpencil_modifiers/MOD_gpencil_modifiertypes.h
@@ -35,6 +35,7 @@ extern GpencilModifierTypeInfo modifierType_Gpencil_Array;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Build;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Opacity;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Lattice;
+extern GpencilModifierTypeInfo modifierType_Gpencil_Length;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Mirror;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Smooth;
 extern GpencilModifierTypeInfo modifierType_Gpencil_Hook;
diff --git a/source/blender/gpencil_modifiers/intern/MOD_gpencil_util.c b/source/blender/gpencil_modifiers/intern/MOD_gpencil_util.c
index 2dc00079f91..b28a44a0521 100644
--- a/source/blender/gpencil_modifiers/intern/MOD_gpencil_util.c
+++ b/source/blender/gpencil_modifiers/intern/MOD_gpencil_util.c
@@ -54,6 +54,7 @@ void gpencil_modifier_type_init(GpencilModifierTypeInfo *types[])
   INIT_GP_TYPE(Build);
   INIT_GP_TYPE(Opacity);
   INIT_GP_TYPE(Lattice);
+  INIT_GP_TYPE(Length);
   INIT_GP_TYPE(Mirror);
   INIT_GP_TYPE(Smooth);
   INIT_GP_TYPE(Hook);
diff --git a/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c b/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
new file mode 100644
index 00000000000..fd94ac92bc3
--- /dev/null
+++ b/source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
@@ -0,0 +1,223 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software  Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2017, Blender Foundation
+ * This is a new part of Blender
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ *
+ */
+
+/** \file
+ * \ingroup modifiers
+ */
+
+#include <stdio.h>
+
+#include "BLI_listbase.h"
+#include "BLI_math.h"
+#include "BLI_utildefines.h"
+
+#include "BLT_translation.h"
+
+#include "DNA_defaults.h"
+#include "DNA_gpencil_modifier_types.h"
+#include "DNA_gpencil_types.h"
+#include "DNA_object_types.h"
+#include "DNA_scene_types.h"
+#include "DNA_screen_types.h"
+
+#include "BKE_context.h"
+#include "BKE_gpencil_geom.h"
+#include "BKE_gpencil_modifier.h"
+#include "BKE_lib_query.h"
+#include "BKE_main.h"
+#include "BKE_modifier.h"
+#include "BKE_screen.h"
+
+#include "MEM_guardedalloc.h"
+
+#include "UI_interface.h"
+#include "UI_resources.h"
+
+#include "RNA_access.h"
+
+#include "MOD_gpencil_modifiertypes.h"
+#include "MOD_gpencil_ui_common.h"
+#include "MOD_gpencil_util.h"
+
+#include "DEG_depsgraph.h"
+
+static void initData(GpencilModifierData *md)
+{
+  LengthGpencilModifierData *gpmd = (LengthGpencilModifierData *)md;
+
+  BLI_assert(MEMCMP_STRUCT_AFTER_IS_ZERO(gpmd, modifier));
+
+  MEMCPY_STRUCT_AFTER(gpmd, DNA_struct_default_get(LengthGpencilModifierData), modifier);
+}
+
+static void copyData(const GpencilModifierData *md, GpencilModifierData *target)
+{
+  BKE_gpencil_modifier_copydata_generic(md, target);
+}
+
+static bool gpencil_modify_stroke(bGPDstroke *gps,
+                                  float length,
+                                  const float overshoot_fac,
+                                  const short len_mode)
+{
+  bool changed = false;
+  if (length == 0.0f) {
+    return changed;
+  }
+
+  if (length > 0.0f) {
+    BKE_gpencil_stroke_stretch(gps, length, overshoot_fac, len_mode);
+  }
+  else {
+    changed |= BKE_gpencil_stroke_shrink(gps, fabs(length), len_mode);
+  }
+
+  return changed;
+}
+
+static void applyLength(LengthGpencilModifierData *lmd, bGPdata *gpd, bGPDstroke *gps)
+{
+  bool changed = false;
+  const float len = (lmd->mode == GP_LENGTH_ABSOLUTE) ? 1.0f :
+                                                        BKE_gpencil_stroke_length(gps, true);
+  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);
+
+  if (changed) {
+    BKE_gpencil_stroke_geometry_update(gpd, gps);
+  }
+}
+
+static void bakeModifier(Main *UNUSED(bmain),
+                         Depsgraph *UNUSED(depsgraph),
+                         GpencilModifierData *md,
+                         Object *ob)
+{
+
+  bGPdata *gpd = ob->data;
+
+  LISTBASE_FOREACH (bGPDlayer *, gpl, &gpd->layers) {
+    LISTBASE_FOREACH (bGPDframe *, gpf, &gpl->frames) {
+      LengthGpencilModifierData *lmd = (LengthGpencilModifierData *)md;
+      LISTBASE_FOREACH (bGPDstroke *, gps, &gpf->strokes) {
+        applyLength(lmd, gpd, gps);
+      }
+    }
+  }
+}
+
+/* -------------------------------- */
+
+/* Generic "generateStrokes" callback */
+static void deformStroke(GpencilModifierData *md,
+                         Depsgraph *UNUSED(depsgraph),
+                         Object *ob,
+                         bGPDlayer *gpl,
+                         bGPDframe *UNUSED(gpf),
+                         bGPDstroke *gps)
+{
+  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);
+  }
+}
+
+static void foreachIDLink(GpencilModifierData *md, Object *ob, IDWalkFunc walk, void *userData)
+{
+  LengthGpencilModifierData *mmd = (LengthGpencilModifierData *)md;
+
+  walk(userData, ob, (ID **)&mmd->material, IDWALK_CB_USER);
+}
+
+static void 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);
+  uiItemR(layout, ptr, "mode", 0, NULL, ICON_NONE);
+
+  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);
+
+  uiItemR(layout, ptr, "overshoot_factor", UI_ITEM_R_SLIDER, IFACE_("Overshoot"), ICON_NONE);
+
+  gpencil_modifier_panel_end(layout, ptr);
+}
+
+static void mask_panel_draw(const bContext *UNUSED(C), Panel *panel)
+{
+  gpencil_modifier_masking_panel_draw(panel, true, false);
+}
+
+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, "mask", "Influence", NULL, mask_panel_draw, panel_type);
+}
+
+GpencilModifierTypeInfo modifierType_Gpencil_Length = {
+    /* name */ "Length",
+    /* structName */ "LengthGpencilModifierData",
+    /* structSize */ sizeof(LengthGpencilModifierData),
+    /* type */ eGpencilModifierTypeType_Gpencil,
+    /* flags */ eGpencilModifierTypeFlag_SupportsEditmode,
+
+    /* copyData */ copyData,
+
+    /* deformStroke */ deformStroke,
+    /* generateStrokes */ NULL,
+    /* bakeModifier */ bakeModifier,
+    /* remapTime */ NULL,
+
+    /* initData */ initData,
+    /* freeData */ NULL,
+    /* isDisabled */ NULL,
+    /* updateDepsgraph */ NULL,
+    /* dependsOnTime */ NULL,
+    /* foreachIDLink */ foreachIDLink,
+    /* foreachTexLink */ NULL,
+    /* panelRegister */ panelRegister,
+};
diff --git a/source/blender/makesdna/DNA_gpencil_modifier_defaults.h b/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
index b816a45426b..a4ab38f6022 100644
--- a/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
+++ b/source/blender/makesdna/DNA_gpencil_modifier_defaults.h
@@ -299,4 +299,14 @@
     .chaining_image_threshold = 0.001f, \
   }
 
+#define _DNA_DEFAULT_LengthGpencilModifierData \
+  { \
+    .start_fac = 0.1f,\
+    .end_fac = 0.1f,\
+    .overshoot_fac = 0.01f,\
+    .pass_index = 0,\
+    .material = NULL,\
+  }
+
+
 /* clang-format off */
diff --git a/source/blender/makesdna/DNA_gpencil_modifier_types.h b/source/blender/makesdna/DNA_gpencil_modifier_types.h
index c03b358dc8a..410212ce100 100644
--- a/source/blender/makesdna/DNA_gpencil_modifier_types.h
+++ b/source/blender/makesdna/DNA_gpencil_modifier_types.h
@@ -54,6 +54,7 @@ typedef enum GpencilModifierType {
   eGpencilModifierType_Multiply = 17,
   eGpencilModifierType_Texture = 18,
   eGpencilModifierType_Lineart = 19,
+  eGpencilModifierType_Length = 20,
   /* Keep last. */
   NUM_GREASEPENCIL_MODIFIER_TYPES,
 } GpencilModifierType;
@@ -484,6 +485,39 @@ typedef enum eLatticeGpencil_Flag {
   GP_LATTICE_INVERT_MATERIAL = (1 << 4),
 } eLatticeGpencil_Flag;
 
+typedef struct LengthGpencilModifierData {
+  GpencilModifierData modifier;
+  /** Material for filtering. */
+  struct Material *material;
+  /** Layer name. */
+  char layername[64];
+  /** Custom index for passes. */
+  int pass_index;
+  /** Flags. */
+  int flag;
+  /** Custom index for passes. */
+  int layer_pass;
+  /** Length. */
+  float start_fac, end_fac;
+  /** Overshoot trajectory factor. */
+  float overshoot_fac;
+  /** Modifier mode. */
+  int mode;
+  char _pad[4];
+} LengthGpencilModifierData;
+
+typedef enum eLengthGpencil_Flag {
+  GP_LENGTH_INVERT_LAYER = (1 << 0),
+  GP_LENGTH_INVERT_PASS = (1 << 1),
+  GP_LENGTH_INVERT_LAYERPASS = (1 << 2),
+  GP_LENGTH_INVERT_MATERIAL = (1 << 3),
+} eLengthGpencil_Flag;
+
+typedef enum eLengthGpencil_Type {
+  GP_LENGTH_RELATIVE = 0,
+  GP_LENGTH_ABSOLUTE = 1,
+} eLengthGpencil_Type;
+
 typedef struct MirrorGpencilModifierData {
   GpencilModifierData modifier;
   struct Object *object;
diff --git a/source/blender/makesdna/intern/dna_defaults.c b/source/blender/makesdna/intern/dna_defaults.c
index 95272fb7804..2d55ea05867 100644
--- a/source/blender/makesdna/intern/dna_defaults.c
+++ b/source/blender/makesdna/intern/dna_defaults.c
@@ -316,6 +316,7 @@ SDNA_DEFAULT_DECL_STRUCT(ThickGpencilModifierData);
 SDNA_DEFAULT_DECL_STRUCT(TimeGpencilModifierData);
 SDNA_DEFAULT_DECL_STRUCT(TintGpencilModifierData);
 SDNA_DEFAULT_DECL_STRUCT(LineartGpencilModifierData);
+SDNA_DEFAULT_DECL_STRUCT(LengthGpencilModifierData);
 
 #undef SDNA_DEFAULT_DECL_STRUCT
 
@@ -541,6 +542,7 @@ const void *DNA_default_table[SDNA_TYPE_MAX] = {
     SDNA_DEFAULT_DECL(TimeGpencilModifierData),
     SDNA_DEFAULT_DECL(TintGpencilModifierData),
     SDNA_DEFAULT_DECL(LineartGpencilModifierData),
+    SDNA_DEFAULT_DECL(LengthGpencilModifierData),
 };
 #undef SDNA_DEFAULT_DECL
 #undef SDNA_DEFAULT_DECL_EX
diff --git a/source/blender/makesrna/RNA_access.h b/source/blender/makesrna/RNA_access.h
index 39a46948321..379e3e77b6e 100644
--- a/source/blender/makesrna/RNA_access.h
+++ b/source/blender/makesrna/RNA_access.h
@@ -336,6 +336,7 @@ extern StructRNA RNA_LatticeModifier;
 extern StructRNA RNA_LatticePoint;
 extern StructRNA RNA_LayerCollection;
 extern StructRNA RNA_LayerObjects;
+extern StructRNA RNA_LengthGpencilModifier;
 extern StructRNA RNA_Library;
 extern StructRNA RNA_Light;
 extern StructRNA RNA_LightProbe;
diff --git a/source/blender/makesrna/intern/rna_gpencil_modifier.c b/source/blender/makesrna/intern/rna_gpencil_modifier.c
index 85a3bd2192c..4aad0741151 100644
--- a/source/blender/makesrna/intern/rna_gpencil_modifier.c
+++ b/source/blender/makesrna/intern/rna_gpencil_modifier.c
@@ -109,6 +109,11 @@ const EnumPropertyItem rna_enum_object_greasepencil_modifier_type_items[] = {
      ICON_MOD_LATTICE,
      "Lattice",
      "Deform strokes using lattice"},
+    {eGpencilModifierType_Length,
+     "GP_LENGTH",
+     ICON_MOD_EDGESPLIT,
+     "Length",
+     "Extend or shrink strokes"},
     {eGpencilModifierType_Noise, "GP_NOISE", ICON_MOD_NOISE, "Noise", "Add noise to strokes"},
     {eGpencilModifierType_Offset,
      "GP_OFFSET",
@@ -188,6 +193,11 @@ static const EnumPropertyItem gpencil_tint_type_items[] = {
     {GP_TINT_GRADIENT, "GRADIENT", 0, "Gradient", ""},
     {0, NULL, 0, NULL, NULL},
 };
+static const EnumPropertyItem gpencil_length_mode_items[] = {
+    {GP_LENGTH_RELATIVE, "RELATIVE", 0, "Relative", "Length in ratio to the stroke's length"},
+    {GP_LENGTH_ABSOLUTE, "ABSOLUTE", 0, "Absolute", "Length in geometry space"},
+    {0, NULL, 0, NULL, NULL},
+};
 #endif
 
 #ifdef RNA_RUNTIME
@@ -233,6 +243,8 @@ static StructRNA *rna_GpencilModifier_refine(struct PointerRNA *ptr)
       return &RNA_OpacityGpencilModifier;
     case eGpencilModifierType_Lattice:
       return &RNA_LatticeGpencilModifier;
+    case eGpencilModifierType_Length:
+      return &RNA_LengthGpencilModifier;
     case eGpencilModifierType_Mirror:
       return &RNA_MirrorGpencilModifier;
     case eGpencilModifierType_Smooth:
@@ -2901,6 +2913,87 @@ static void rna_def_modifier_gpencillineart(BlenderRNA *brna)
   RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
 }
 
+static void rna_def_modifier_gpencillength(BlenderRNA *brna)
+{
+  StructRNA *srna;
+  PropertyRNA *prop;
+
+  srna = RNA_def_struct(brna, "LengthGpencilModifier", "GpencilModifier");
+  RNA_def_struct_ui_text(srna, "Length Modifier", "Stretch or shrink strokes");
+  RNA_def_struct_sdna(srna, "LengthGpencilModifierData");
+  RNA_def_struct_ui_icon(srna, ICON_MOD_EDGESPLIT);
+
+  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_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_text(prop, "End Factor", "Length difference for each segment");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "overshoot_factor", PROP_FLOAT, PROP_FACTOR);
+  RNA_def_property_float_sdna(prop, NULL, "overshoot_fac");
+  RNA_def_property_range(prop, 0.0f, 1.0f);
+  RNA_def_property_ui_text(
+      prop,
+      "Overshoot Factor",
+      "Defines how precise must follow the stroke trajectory for the overshoot extremes");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "mode", PROP_ENUM, PROP_NONE);
+  RNA_def_property_enum_sdna(prop, NULL, "mode");
+  RNA_def_property_enum_items(prop, gpencil_length_mode_items);
+  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, "layer", PROP_STRING, PROP_NONE);
+  RNA_def_property_string_sdna(prop, NULL, "layername");
+  RNA_def_property_ui_text(prop, "Layer", "Layer name");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "material", PROP_POINTER, PROP_NONE);
+  RNA_def_property_flag(prop, PROP_EDITABLE | PROP_ID_SELF_CHECK);
+  RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_LIBRARY);
+  RNA_def_property_ui_text(prop, "Material", "Material used for filtering effect");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "pass_index", PROP_INT, PROP_NONE);
+  RNA_def_property_int_sdna(prop, NULL, "pass_index");
+  RNA_def_property_range(prop, 0, 100);
+  RNA_def_property_ui_text(prop, "Pass", "Pass index");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "invert_layers", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_INVERT_LAYER);
+  RNA_def_property_ui_text(prop, "Inverse Layers", "Inverse filter");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "invert_materials", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_INVERT_MATERIAL);
+  RNA_def_property_ui_text(prop, "Inverse Materials", "Inverse filter");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "invert_material_pass", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_INVERT_PASS);
+  RNA_def_property_ui_text(prop, "Inverse Pass", "Inverse filter");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "layer_pass", PROP_INT, PROP_NONE);
+  RNA_def_property_int_sdna(prop, NULL, "layer_pass");
+  RNA_def_property_range(prop, 0, 100);
+  RNA_def_property_ui_text(prop, "Pass", "Layer pass index");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+
+  prop = RNA_def_property(srna, "invert_layer_pass", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LENGTH_INVERT_LAYERPASS);
+  RNA_def_property_ui_text(prop, "Inverse Pass", "Inverse filter");
+  RNA_def_property_update(prop, 0, "rna_GpencilModifier_update");
+}
+
 void RNA_def_greasepencil_modifier(BlenderRNA *brna)
 {
   StructRNA *srna;
@@ -2976,6 +3069,7 @@ void RNA_def_greasepencil_modifier(BlenderRNA *brna)
   rna_def_modifier_gpencilmultiply(brna);
   rna_def_modifier_gpenciltexture(brna);
   rna_def_modifier_gpencillineart(brna);
+  rna_def_modifier_gpencillength(brna);
 }
 
 #endif