path: root/source/blender/editors/gpencil/gpencil_interpolate.c

/*
 * ***** 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) 2016, Blender Foundation
 * This is a new part of Blender
 *
 * Contributor(s): Antonio Vazquez, Joshua Leung
 *
 * ***** END GPL LICENSE BLOCK *****
 *
 * Operators for interpolating new Grease Pencil frames from existing strokes
 */

/** \file blender/editors/gpencil/gpencil_interpolate.c
 *  \ingroup edgpencil
 */


#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stddef.h>
#include <math.h>

#include "MEM_guardedalloc.h"

#include "BLI_math.h"
#include "BLI_blenlib.h"
#include "BLI_utildefines.h"

#include "BLT_translation.h"

#include "DNA_object_types.h"
#include "DNA_scene_types.h"
#include "DNA_screen_types.h"
#include "DNA_space_types.h"
#include "DNA_view3d_types.h"
#include "DNA_gpencil_types.h"

#include "BKE_context.h"
#include "BKE_global.h"
#include "BKE_gpencil.h"
#include "BKE_library.h"
#include "BKE_report.h"
#include "BKE_screen.h"

#include "UI_interface.h"
#include "UI_resources.h"

#include "WM_api.h"
#include "WM_types.h"

#include "RNA_access.h"
#include "RNA_define.h"
#include "RNA_enum_types.h"

#include "UI_view2d.h"

#include "ED_gpencil.h"
#include "ED_object.h"
#include "ED_screen.h"
#include "ED_view3d.h"
#include "ED_screen.h"
#include "ED_space_api.h"

#include "gpencil_intern.h"

/* ************************************************ */
/* Core/Shared Utilities */

/* Poll callback for interpolation operators */
static int gpencil_interpolate_poll(bContext *C)
{
	bGPdata *gpd = CTX_data_gpencil_data(C);
	bGPDlayer *gpl = CTX_data_active_gpencil_layer(C);
	
	/* only 3D view */
	if (CTX_wm_area(C)->spacetype != SPACE_VIEW3D) {
		return 0;
	}
	
	/* need data to interpolate */
	if (ELEM(NULL, gpd, gpl)) {
		return 0;
	}
	
	return 1;
}

/* Perform interpolation */
static void gp_interpolate_update_points(bGPDstroke *gps_from, bGPDstroke *gps_to, bGPDstroke *new_stroke, float factor)
{
	bGPDspoint *prev, *pt, *next;
	
	/* update points */
	for (int i = 0; i < new_stroke->totpoints; i++) {
		prev = &gps_from->points[i];
		pt = &new_stroke->points[i];
		next = &gps_to->points[i];
		
		/* Interpolate all values */
		interp_v3_v3v3(&pt->x, &prev->x, &next->x, factor);
		pt->pressure = interpf(prev->pressure, next->pressure, factor);
		pt->strength = interpf(prev->strength, next->strength, factor);
		CLAMP(pt->strength, GPENCIL_STRENGTH_MIN, 1.0f);
	}
}

/* ****************** Interpolate Interactive *********************** */

/* Helper: Update all strokes interpolated */
static void gp_interpolate_update_strokes(bContext *C, tGPDinterpolate *tgpi)
{
	tGPDinterpolate_layer *tgpil;
	const float shift = tgpi->shift;
	
	for (tgpil = tgpi->ilayers.first; tgpil; tgpil = tgpil->next) {
		bGPDstroke *new_stroke;
		const float factor = tgpil->factor + shift;
		
		for (new_stroke = tgpil->interFrame->strokes.first; new_stroke; new_stroke = new_stroke->next) {
			bGPDstroke *gps_from, *gps_to;
			int stroke_idx;
			
			if (new_stroke->totpoints == 0) {
				continue;
			}
			
			/* get strokes to interpolate */
			stroke_idx = BLI_findindex(&tgpil->interFrame->strokes, new_stroke);
			
			gps_from = BLI_findlink(&tgpil->prevFrame->strokes, stroke_idx);
			gps_to = BLI_findlink(&tgpil->nextFrame->strokes, stroke_idx);
			
			/* update points position */
			if ((gps_from) && (gps_to)) {
				gp_interpolate_update_points(gps_from, gps_to, new_stroke, factor);
			}
		}
	}
	
	WM_event_add_notifier(C, NC_GPENCIL | NA_EDITED, NULL);
}

/* Helper: Verify valid strokes for interpolation */
static bool gp_interpolate_check_todo(bContext *C, bGPdata *gpd)
{
	ToolSettings *ts = CTX_data_tool_settings(C);
	int flag = ts->gp_sculpt.flag;
	
	bGPDlayer *gpl;
	bGPDlayer *active_gpl = CTX_data_active_gpencil_layer(C);
	bGPDstroke *gps_from, *gps_to;
	int fFrame;
	
	/* get layers */
	for (gpl = gpd->layers.first; gpl; gpl = gpl->next) {
		/* all layers or only active */
		if (((flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ALL_LAYERS) == 0) && (gpl != active_gpl)) {
			continue;
		}
		/* only editable and visible layers are considered */
		if (!gpencil_layer_is_editable(gpl) || (gpl->actframe == NULL)) {
			continue;
		}
		
		/* read strokes */
		for (gps_from = gpl->actframe->strokes.first; gps_from; gps_from = gps_from->next) {
			/* only selected */
			if ((flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ONLY_SELECTED) && ((gps_from->flag & GP_STROKE_SELECT) == 0)) {
				continue;
			}
			/* skip strokes that are invalid for current view */
			if (ED_gpencil_stroke_can_use(C, gps_from) == false) {
				continue;
			}
			/* check if the color is editable */
			if (ED_gpencil_stroke_color_use(gpl, gps_from) == false) {
				continue;
			}
			
			/* get final stroke to interpolate */
			fFrame = BLI_findindex(&gpl->actframe->strokes, gps_from);
			gps_to = BLI_findlink(&gpl->actframe->next->strokes, fFrame);
			if (gps_to == NULL) {
				continue;
			}
			
			return 1;
		}
	}
	return 0;
}

/* Helper: Create internal strokes interpolated */
static void gp_interpolate_set_points(bContext *C, tGPDinterpolate *tgpi)
{
	bGPDlayer *gpl;
	bGPdata *gpd = tgpi->gpd;
	tGPDinterpolate_layer *tgpil;
	bGPDlayer *active_gpl = CTX_data_active_gpencil_layer(C);
	bGPDstroke *gps_from, *gps_to, *new_stroke;
	int fFrame;
	
	/* save initial factor for active layer to define shift limits */
	tgpi->init_factor = (float)(tgpi->cframe - active_gpl->actframe->framenum) / (active_gpl->actframe->next->framenum - active_gpl->actframe->framenum + 1);
	
	/* limits are 100% below 0 and 100% over the 100% */
	tgpi->low_limit = -1.0f - tgpi->init_factor;
	tgpi->high_limit = 2.0f - tgpi->init_factor;
	
	/* set layers */
	for (gpl = gpd->layers.first; gpl; gpl = gpl->next) {
		/* all layers or only active */
		if (((tgpi->flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ALL_LAYERS) == 0) && (gpl != active_gpl)) {
			continue;
		}
		/* only editable and visible layers are considered */
		if (!gpencil_layer_is_editable(gpl) || (gpl->actframe == NULL)) {
			continue;
		}
		
		/* create temp data for each layer */
		tgpil = NULL;
		tgpil = MEM_callocN(sizeof(tGPDinterpolate_layer), "GPencil Interpolate Layer");
		
		tgpil->gpl = gpl;
		tgpil->prevFrame = gpl->actframe;
		tgpil->nextFrame = gpl->actframe->next;
		
		BLI_addtail(&tgpi->ilayers, tgpil);
		
		/* create a new temporary frame */
		tgpil->interFrame = MEM_callocN(sizeof(bGPDframe), "bGPDframe");
		tgpil->interFrame->framenum = tgpi->cframe;
		
		/* get interpolation factor by layer (usually must be equal for all layers, but not sure) */
		tgpil->factor = (float)(tgpi->cframe - tgpil->prevFrame->framenum) / (tgpil->nextFrame->framenum - tgpil->prevFrame->framenum + 1);
		
		/* create new strokes data with interpolated points reading original stroke */
		for (gps_from = tgpil->prevFrame->strokes.first; gps_from; gps_from = gps_from->next) {
			bool valid = true;
			
			/* only selected */
			if ((tgpi->flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ONLY_SELECTED) && ((gps_from->flag & GP_STROKE_SELECT) == 0)) {
				valid = false;
			}
			/* skip strokes that are invalid for current view */
			if (ED_gpencil_stroke_can_use(C, gps_from) == false) {
				valid = false;
			}
			
			/* check if the color is editable */
			if (ED_gpencil_stroke_color_use(tgpil->gpl, gps_from) == false) {
				valid = false;
			}
			
			/* get final stroke to interpolate */
			fFrame = BLI_findindex(&tgpil->prevFrame->strokes, gps_from);
			gps_to = BLI_findlink(&tgpil->nextFrame->strokes, fFrame);
			if (gps_to == NULL) {
				valid = false;
			}
			
			/* create new stroke */
			new_stroke = MEM_dupallocN(gps_from);
			new_stroke->points = MEM_dupallocN(gps_from->points);
			new_stroke->triangles = MEM_dupallocN(gps_from->triangles);
			
			if (valid) {
				/* if destination stroke is smaller, resize new_stroke to size of gps_to stroke */
				if (gps_from->totpoints > gps_to->totpoints) {
					new_stroke->points = MEM_recallocN(new_stroke->points, sizeof(*new_stroke->points) * gps_to->totpoints);
					new_stroke->totpoints = gps_to->totpoints;
					new_stroke->tot_triangles = 0;
					new_stroke->flag |= GP_STROKE_RECALC_CACHES;
				}
				/* update points position */
				gp_interpolate_update_points(gps_from, gps_to, new_stroke, tgpil->factor);
			}
			else {
				/* need an empty stroke to keep index correct for lookup, but resize to smallest size */
				new_stroke->totpoints = 0;
				new_stroke->points = MEM_recallocN(new_stroke->points, sizeof(*new_stroke->points));
				new_stroke->tot_triangles = 0;
				new_stroke->triangles = MEM_recallocN(new_stroke->triangles, sizeof(*new_stroke->triangles));
			}
			
			/* add to strokes */
			BLI_addtail(&tgpil->interFrame->strokes, new_stroke);
		}
	}
}

/* ----------------------- */
/* Drawing Callbacks */

/* Drawing callback for modal operator in screen mode */
static void gpencil_interpolate_draw_screen(const struct bContext *UNUSED(C), ARegion *UNUSED(ar), void *arg)
{
	tGPDinterpolate *tgpi = (tGPDinterpolate *)arg;
	ED_gp_draw_interpolation(tgpi, REGION_DRAW_POST_PIXEL);
}

/* Drawing callback for modal operator in 3d mode */
static void gpencil_interpolate_draw_3d(const bContext *UNUSED(C), ARegion *UNUSED(ar), void *arg)
{
	tGPDinterpolate *tgpi = (tGPDinterpolate *)arg;
	ED_gp_draw_interpolation(tgpi, REGION_DRAW_POST_VIEW);
}

/* ----------------------- */

/* Helper: calculate shift based on position of mouse (we only use x-axis for now.
 * since this is more convenient for users to do), and store new shift value
 */
static void gpencil_mouse_update_shift(tGPDinterpolate *tgpi, wmOperator *op, const wmEvent *event)
{
	float mid = (float)(tgpi->ar->winx - tgpi->ar->winrct.xmin) / 2.0f;
	float mpos = event->x - tgpi->ar->winrct.xmin;
	
	if (mpos >= mid) {
		tgpi->shift = ((mpos - mid) * tgpi->high_limit) / mid;
	}
	else {
		tgpi->shift = tgpi->low_limit - ((mpos * tgpi->low_limit) / mid);
	}
	
	CLAMP(tgpi->shift, tgpi->low_limit, tgpi->high_limit);
	RNA_float_set(op->ptr, "shift", tgpi->shift);
}

/* Helper: Draw status message while the user is running the operator */
static void gpencil_interpolate_status_indicators(tGPDinterpolate *p)
{
	Scene *scene = p->scene;
	char status_str[UI_MAX_DRAW_STR];
	char msg_str[UI_MAX_DRAW_STR];
	
	BLI_strncpy(msg_str, IFACE_("GPencil Interpolation: ESC/RMB to cancel, Enter/LMB to confirm, WHEEL/MOVE to adjust factor"), UI_MAX_DRAW_STR);
	
	if (hasNumInput(&p->num)) {
		char str_offs[NUM_STR_REP_LEN];
		
		outputNumInput(&p->num, str_offs, &scene->unit);
		
		BLI_snprintf(status_str, sizeof(status_str), "%s: %s", msg_str, str_offs);
	}
	else {
		BLI_snprintf(status_str, sizeof(status_str), "%s: %d %%", msg_str, (int)((p->init_factor + p->shift)  * 100.0f));
	}
	
	ED_area_headerprint(p->sa, status_str);
}

/* Update screen and stroke */
static void gpencil_interpolate_update(bContext *C, wmOperator *op, tGPDinterpolate *tgpi)
{
	/* update shift indicator in header */
	gpencil_interpolate_status_indicators(tgpi);
	/* apply... */
	tgpi->shift = RNA_float_get(op->ptr, "shift");
	/* update points position */
	gp_interpolate_update_strokes(C, tgpi);
}

/* ----------------------- */

/* Exit and free memory */
static void gpencil_interpolate_exit(bContext *C, wmOperator *op)
{
	tGPDinterpolate *tgpi = op->customdata;
	tGPDinterpolate_layer *tgpil;
	
	/* don't assume that operator data exists at all */
	if (tgpi) {
		/* remove drawing handler */
		if (tgpi->draw_handle_screen) {
			ED_region_draw_cb_exit(tgpi->ar->type, tgpi->draw_handle_screen);
		}
		if (tgpi->draw_handle_3d) {
			ED_region_draw_cb_exit(tgpi->ar->type, tgpi->draw_handle_3d);
		}
		
		/* clear status message area */
		ED_area_headerprint(tgpi->sa, NULL);
		
		/* finally, free memory used by temp data */
		for (tgpil = tgpi->ilayers.first; tgpil; tgpil = tgpil->next) {
			BKE_gpencil_free_strokes(tgpil->interFrame);
			MEM_freeN(tgpil->interFrame);
		}
		
		BLI_freelistN(&tgpi->ilayers);
		MEM_freeN(tgpi);
	}
	WM_event_add_notifier(C, NC_GPENCIL | NA_EDITED, NULL);
	
	/* clear pointer */
	op->customdata = NULL;
}

/* Init new temporary interpolation data */
static bool gp_interpolate_set_init_values(bContext *C, wmOperator *op, tGPDinterpolate *tgpi)
{
	ToolSettings *ts = CTX_data_tool_settings(C);
	bGPdata *gpd = CTX_data_gpencil_data(C);
	
	/* set current scene and window */
	tgpi->scene = CTX_data_scene(C);
	tgpi->sa = CTX_wm_area(C);
	tgpi->ar = CTX_wm_region(C);
	tgpi->flag = ts->gp_sculpt.flag;
	
	/* set current frame number */
	tgpi->cframe = tgpi->scene->r.cfra;
	
	/* set GP datablock */
	tgpi->gpd = gpd;
	
	/* set interpolation weight */
	tgpi->shift = RNA_float_get(op->ptr, "shift");
	/* set layers */
	gp_interpolate_set_points(C, tgpi);
	
	return 1;
}

/* Allocate memory and initialize values */
static tGPDinterpolate *gp_session_init_interpolation(bContext *C, wmOperator *op)
{
	tGPDinterpolate *tgpi = MEM_callocN(sizeof(tGPDinterpolate), "GPencil Interpolate Data");
	
	/* define initial values */
	gp_interpolate_set_init_values(C, op, tgpi);
	
	/* return context data for running operator */
	return tgpi;
}

/* Init interpolation: Allocate memory and set init values */
static int gpencil_interpolate_init(bContext *C, wmOperator *op)
{
	tGPDinterpolate *tgpi;
	
	/* check context */
	tgpi = op->customdata = gp_session_init_interpolation(C, op);
	if (tgpi == NULL) {
		/* something wasn't set correctly in context */
		gpencil_interpolate_exit(C, op);
		return 0;
	}
	
	/* everything is now setup ok */
	return 1;
}

/* ----------------------- */

/* Invoke handler: Initialize the operator */
static int gpencil_interpolate_invoke(bContext *C, wmOperator *op, const wmEvent *UNUSED(event))
{
	wmWindow *win = CTX_wm_window(C);
	Scene *scene = CTX_data_scene(C);
	bGPdata *gpd = CTX_data_gpencil_data(C);
	bGPDlayer *gpl = CTX_data_active_gpencil_layer(C);
	tGPDinterpolate *tgpi = NULL;

	/* cannot interpolate if not between 2 frames */
	if ((gpl->actframe == NULL) || (gpl->actframe->next == NULL)) {
		BKE_report(op->reports, RPT_ERROR, "Interpolation requires to be between two grease pencil frames in active layer");
		return OPERATOR_CANCELLED;
	}
	
	/* cannot interpolate in extremes */
	if ((gpl->actframe->framenum == scene->r.cfra) || (gpl->actframe->next->framenum == scene->r.cfra)) {
		BKE_report(op->reports, RPT_ERROR, "Interpolation requires to be between two grease pencil frames in active layer");
		return OPERATOR_CANCELLED;
	}
	
	/* need editable strokes */
	if (!gp_interpolate_check_todo(C, gpd)) {
		BKE_report(op->reports, RPT_ERROR, "Interpolation requires some editable stroke");
		return OPERATOR_CANCELLED;
	}
	
	/* try to initialize context data needed */
	if (!gpencil_interpolate_init(C, op)) {
		if (op->customdata)
			MEM_freeN(op->customdata);
		return OPERATOR_CANCELLED;
	}
	else {
		tgpi = op->customdata;
	}
	
	/* Enable custom drawing handlers
	 * It needs 2 handlers because strokes can in 3d space and screen space 
	 * and each handler use different coord system 
	 */
	tgpi->draw_handle_screen = ED_region_draw_cb_activate(tgpi->ar->type, gpencil_interpolate_draw_screen, tgpi, REGION_DRAW_POST_PIXEL);
	tgpi->draw_handle_3d = ED_region_draw_cb_activate(tgpi->ar->type, gpencil_interpolate_draw_3d, tgpi, REGION_DRAW_POST_VIEW);
	
	/* set cursor to indicate modal */
	WM_cursor_modal_set(win, BC_EW_SCROLLCURSOR);
	
	/* update shift indicator in header */
	gpencil_interpolate_status_indicators(tgpi);
	WM_event_add_notifier(C, NC_GPENCIL | NA_EDITED, NULL);
	
	/* add a modal handler for this operator */
	WM_event_add_modal_handler(C, op);
	
	return OPERATOR_RUNNING_MODAL;
}

/* Modal handler: Events handling during interactive part */
static int gpencil_interpolate_modal(bContext *C, wmOperator *op, const wmEvent *event)
{
	tGPDinterpolate *tgpi = op->customdata;
	wmWindow *win = CTX_wm_window(C);
	bGPDframe *gpf_dst;
	bGPDstroke *gps_src, *gps_dst;
	tGPDinterpolate_layer *tgpil;
	const bool has_numinput = hasNumInput(&tgpi->num);
	
	switch (event->type) {
		case LEFTMOUSE: /* confirm */
		case RETKEY:
		{
			/* return to normal cursor and header status */
			ED_area_headerprint(tgpi->sa, NULL);
			WM_cursor_modal_restore(win);
			
			/* insert keyframes as required... */
			for (tgpil = tgpi->ilayers.first; tgpil; tgpil = tgpil->next) {
				gpf_dst = BKE_gpencil_layer_getframe(tgpil->gpl, tgpi->cframe, GP_GETFRAME_ADD_NEW);
				gpf_dst->key_type = BEZT_KEYTYPE_BREAKDOWN;
				
				/* copy strokes */
				BLI_listbase_clear(&gpf_dst->strokes);
				for (gps_src = tgpil->interFrame->strokes.first; gps_src; gps_src = gps_src->next) {
					if (gps_src->totpoints == 0) {
						continue;
					}
					
					/* make copy of source stroke, then adjust pointer to points too */
					gps_dst = MEM_dupallocN(gps_src);
					gps_dst->points = MEM_dupallocN(gps_src->points);
					gps_dst->triangles = MEM_dupallocN(gps_src->triangles);
					gps_dst->flag |= GP_STROKE_RECALC_CACHES;
					BLI_addtail(&gpf_dst->strokes, gps_dst);
				}
			}
			
			/* clean up temp data */
			gpencil_interpolate_exit(C, op);
			
			/* done! */
			return OPERATOR_FINISHED;
		}
		
		case ESCKEY:    /* cancel */
		case RIGHTMOUSE:
		{
			/* return to normal cursor and header status */
			ED_area_headerprint(tgpi->sa, NULL);
			WM_cursor_modal_restore(win);
			
			/* clean up temp data */
			gpencil_interpolate_exit(C, op);
			
			/* canceled! */
			return OPERATOR_CANCELLED;
		}
		
		case WHEELUPMOUSE:
		{
			tgpi->shift = tgpi->shift + 0.01f;
			CLAMP(tgpi->shift, tgpi->low_limit, tgpi->high_limit);
			RNA_float_set(op->ptr, "shift", tgpi->shift);
			
			/* update screen */
			gpencil_interpolate_update(C, op, tgpi);
			break;
		}
		case WHEELDOWNMOUSE:
		{
			tgpi->shift = tgpi->shift - 0.01f;
			CLAMP(tgpi->shift, tgpi->low_limit, tgpi->high_limit);
			RNA_float_set(op->ptr, "shift", tgpi->shift);
			
			/* update screen */
			gpencil_interpolate_update(C, op, tgpi);
			break;
		}
		case MOUSEMOVE: /* calculate new position */
		{
			/* only handle mousemove if not doing numinput */
			if (has_numinput == false) {
				/* update shift based on position of mouse */
				gpencil_mouse_update_shift(tgpi, op, event);
				
				/* update screen */
				gpencil_interpolate_update(C, op, tgpi);
			}
			break;
		}
		default:
			if ((event->val == KM_PRESS) && handleNumInput(C, &tgpi->num, event)) {
				const float factor = tgpi->init_factor;
				float value;
				
				/* Grab shift from numeric input, and store this new value (the user see an int) */
				value = (factor + tgpi->shift) * 100.0f;
				applyNumInput(&tgpi->num, &value);
				tgpi->shift = value / 100.0f;
				
				/* recalculate the shift to get the right value in the frame scale */
				tgpi->shift = tgpi->shift - factor;
				
				CLAMP(tgpi->shift, tgpi->low_limit, tgpi->high_limit);
				RNA_float_set(op->ptr, "shift", tgpi->shift);
				
				/* update screen */
				gpencil_interpolate_update(C, op, tgpi);
				
				break;
			}
			else {
				/* unhandled event - allow to pass through */
				return OPERATOR_RUNNING_MODAL | OPERATOR_PASS_THROUGH;
			}
	}
	
	/* still running... */
	return OPERATOR_RUNNING_MODAL;
}

/* Cancel handler */
static void gpencil_interpolate_cancel(bContext *C, wmOperator *op)
{
	/* this is just a wrapper around exit() */
	gpencil_interpolate_exit(C, op);
}

void GPENCIL_OT_interpolate(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Grease Pencil Interpolation";
	ot->idname = "GPENCIL_OT_interpolate";
	ot->description = "Interpolate grease pencil strokes between frames";
	
	/* callbacks */
	ot->invoke = gpencil_interpolate_invoke;
	ot->modal = gpencil_interpolate_modal;
	ot->cancel = gpencil_interpolate_cancel;
	ot->poll = gpencil_interpolate_poll;
	
	/* flags */
	ot->flag = OPTYPE_UNDO | OPTYPE_BLOCKING;
	
	/* properties */
	RNA_def_float_percentage(ot->srna, "shift", 0.0f, -1.0f, 1.0f, "Shift", "Bias factor for which frame has more influence on the interpolated strokes", -0.9f, 0.9f);
}

/* ****************** Interpolate Sequence *********************** */

static int gpencil_interpolate_seq_exec(bContext *C, wmOperator *op)
{
	Scene *scene = CTX_data_scene(C);
	ToolSettings *ts = CTX_data_tool_settings(C);
	bGPdata   *gpd = CTX_data_gpencil_data(C);
	bGPDlayer *active_gpl = CTX_data_active_gpencil_layer(C);
	bGPDlayer *gpl;
	bGPDframe *prevFrame, *nextFrame, *interFrame;
	bGPDstroke *gps_from, *gps_to, *new_stroke;
	float factor;
	int cframe, fFrame;
	int flag = ts->gp_sculpt.flag;
	
	/* cannot interpolate if not between 2 frames */
	if ((active_gpl->actframe == NULL) || (active_gpl->actframe->next == NULL)) {
		BKE_report(op->reports, RPT_ERROR, "Could not find a pair of grease pencil frames to interpolate between");
		return OPERATOR_CANCELLED;
	}
	/* cannot interpolate in extremes */
	if ((active_gpl->actframe->framenum == scene->r.cfra) || (active_gpl->actframe->next->framenum == scene->r.cfra)) {
		BKE_report(op->reports, RPT_ERROR, "Cannot interpolate as current frame already has strokes");
		return OPERATOR_CANCELLED;
	}
	
	/* loop all layer to check if need interpolation */
	for (gpl = gpd->layers.first; gpl; gpl = gpl->next) {
		/* all layers or only active */
		if (((flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ALL_LAYERS) == 0) && (gpl != active_gpl)) {
			continue;
		}
		/* only editable and visible layers are considered */
		if (!gpencil_layer_is_editable(gpl) || (gpl->actframe == NULL)) {
			continue;
		}
		
		/* store extremes */
		prevFrame = gpl->actframe;
		nextFrame = gpl->actframe->next;
		
		/* Loop over intermediary frames and create the interpolation */
		for (cframe = prevFrame->framenum + 1; cframe < nextFrame->framenum; cframe++) {
			interFrame = NULL;
			
			/* get interpolation factor */
			factor = (float)(cframe - prevFrame->framenum) / (nextFrame->framenum - prevFrame->framenum + 1);
			
			/* create new strokes data with interpolated points reading original stroke */
			for (gps_from = prevFrame->strokes.first; gps_from; gps_from = gps_from->next) {
				/* only selected */
				if ((flag & GP_BRUSHEDIT_FLAG_INTERPOLATE_ONLY_SELECTED) && ((gps_from->flag & GP_STROKE_SELECT) == 0)) {
					continue;
				}
				/* skip strokes that are invalid for current view */
				if (ED_gpencil_stroke_can_use(C, gps_from) == false) {
					continue;
				}
				/* check if the color is editable */
				if (ED_gpencil_stroke_color_use(gpl, gps_from) == false) {
					continue;
				}
				
				/* get final stroke to interpolate */
				fFrame = BLI_findindex(&prevFrame->strokes, gps_from);
				gps_to = BLI_findlink(&nextFrame->strokes, fFrame);
				if (gps_to == NULL) {
					continue;
				}
				
				/* create a new frame if needed */
				if (interFrame == NULL) {
					interFrame = BKE_gpencil_layer_getframe(gpl, cframe, GP_GETFRAME_ADD_NEW);
					interFrame->key_type = BEZT_KEYTYPE_BREAKDOWN;
				}
				
				/* create new stroke */
				new_stroke = MEM_dupallocN(gps_from);
				new_stroke->points = MEM_dupallocN(gps_from->points);
				new_stroke->triangles = MEM_dupallocN(gps_from->triangles);
				
				/* if destination stroke is smaller, resize new_stroke to size of gps_to stroke */
				if (gps_from->totpoints > gps_to->totpoints) {
					new_stroke->points = MEM_recallocN(new_stroke->points, sizeof(*new_stroke->points) * gps_to->totpoints);
					new_stroke->totpoints = gps_to->totpoints;
					new_stroke->tot_triangles = 0;
					new_stroke->flag |= GP_STROKE_RECALC_CACHES;
				}
				
				/* update points position */
				gp_interpolate_update_points(gps_from, gps_to, new_stroke, factor);
				
				/* add to strokes */
				BLI_addtail(&interFrame->strokes, new_stroke);
			}
		}
	}
	
	/* notifiers */
	WM_event_add_notifier(C, NC_GPENCIL | ND_DATA | NA_EDITED, NULL);
	
	return OPERATOR_FINISHED;
}

void GPENCIL_OT_interpolate_sequence(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Interpolate Sequence";
	ot->idname = "GPENCIL_OT_interpolate_sequence";
	ot->description = "Generate 'in-betweens' to smoothly interpolate between Grease Pencil frames";
	
	/* api callbacks */
	ot->exec = gpencil_interpolate_seq_exec;
	ot->poll = gpencil_interpolate_poll;
	
	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
}

/* *************************************************************** */