GP Interpolate Sequence: Tool settings for controlling the shape of interpolation

This commit introduces the ability to use the Robert Penner easing equations
or a Custom Curve to control the way that the "Interpolate Sequence" operator
interpolates between keyframes. Previously, it was only possible to get linear
interpolation between the gp frames.

Workflow:
1) Place current frame between a pair of GP keyframes
2) Open the "Interpolate" panel in the Toolshelf
3) Choose the interpolation type (under "Sequence Options")
4) Adjust settings (e.g. if you're using "Custom Curve", use the curvemap widget
   to define the way that the interpolation proceeds)
5) Click "Sequence" to interpolate
6) Play back/scrub the animation to see if you've got the result you want
7) If you need to make some tweaks, undo, or delete the generated keyframes,
   then repeat the process again from step 4 until you've got the desired result.

1 files changed, 222 insertions, 2 deletions

diff --git a/source/blender/editors/gpencil/gpencil_interpolate.c b/source/blender/editors/gpencil/gpencil_interpolate.c
index ca511493536..287a6f214c0 100644
--- a/source/blender/editors/gpencil/gpencil_interpolate.c
+++ b/source/blender/editors/gpencil/gpencil_interpolate.c
@@ -38,19 +38,22 @@
 
 #include "MEM_guardedalloc.h"
 
-#include "BLI_math.h"
 #include "BLI_blenlib.h"
 #include "BLI_utildefines.h"
+#include "BLI_easing.h"
+#include "BLI_math.h"
 
 #include "BLT_translation.h"
 
+#include "DNA_color_types.h"
+#include "DNA_gpencil_types.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_colortools.h"
 #include "BKE_context.h"
 #include "BKE_global.h"
 #include "BKE_gpencil.h"
@@ -681,6 +684,209 @@ void GPENCIL_OT_interpolate(wmOperatorType *ot)
 
 /* ****************** Interpolate Sequence *********************** */
 
+/* Helper: Perform easing equation calculations for GP interpolation operator */
+static float gp_interpolate_seq_easing_calc(GP_Interpolate_Settings *ipo_settings, float time)
+{
+	const float begin  = 0.0f;
+	const float change = 1.0f;
+	const float duration = 1.0f;
+	
+	const float back = ipo_settings->back;
+	const float amplitude = ipo_settings->amplitude;
+	const float period = ipo_settings->period;
+	
+	eBezTriple_Easing easing = ipo_settings->easing;
+	float result = time;
+	
+	switch (ipo_settings->type) {
+		case GP_IPO_BACK:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_back_ease_in(time, begin, change, duration, back);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_back_ease_out(time, begin, change, duration, back);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_back_ease_in_out(time, begin, change, duration, back);
+					break;
+					
+				default: /* default/auto: same as ease out */
+					result = BLI_easing_back_ease_out(time, begin, change, duration, back);
+					break;
+			}
+			break;
+
+		case GP_IPO_BOUNCE:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_bounce_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_bounce_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_bounce_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease out */
+					result = BLI_easing_bounce_ease_out(time, begin, change, duration);
+					break;
+			}
+			break;
+			
+		case BEZT_IPO_CIRC:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_circ_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_circ_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_circ_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_circ_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+
+		case BEZT_IPO_CUBIC:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_cubic_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_cubic_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_cubic_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_cubic_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		case GP_IPO_ELASTIC:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_elastic_ease_in(time, begin, change, duration, amplitude, period);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_elastic_ease_out(time, begin, change, duration, amplitude, period);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_elastic_ease_in_out(time, begin, change, duration, amplitude, period);
+					break;
+					
+				default: /* default/auto: same as ease out */
+					result = BLI_easing_elastic_ease_out(time, begin, change, duration, amplitude, period);
+					break;
+			}
+			break;
+		
+		case GP_IPO_EXPO:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_expo_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_expo_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_expo_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_expo_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		case GP_IPO_QUAD:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_quad_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_quad_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_quad_ease_in_out(time, begin, change, duration);
+					break;
+				
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_quad_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		case GP_IPO_QUART:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_quart_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_quart_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_quart_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_quart_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		case GP_IPO_QUINT:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_quint_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_quint_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_quint_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_quint_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		case GP_IPO_SINE:
+			switch (easing) {
+				case BEZT_IPO_EASE_IN:
+					result = BLI_easing_sine_ease_in(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_OUT:
+					result = BLI_easing_sine_ease_out(time, begin, change, duration);
+					break;
+				case BEZT_IPO_EASE_IN_OUT:
+					result = BLI_easing_sine_ease_in_out(time, begin, change, duration);
+					break;
+					
+				default: /* default/auto: same as ease in */
+					result = BLI_easing_sine_ease_in(time, begin, change, duration);
+					break;
+			}
+			break;
+		
+		default:
+			printf("%s: Unknown interpolation type - %d\n", __func__, ipo_settings->type);
+			break;
+	}
+	
+	return result;
+}
+
 static int gpencil_interpolate_seq_exec(bContext *C, wmOperator *op)
 {
 	bGPdata   *gpd = CTX_data_gpencil_data(C);
@@ -730,6 +936,20 @@ static int gpencil_interpolate_seq_exec(bContext *C, wmOperator *op)
 			/* get interpolation factor */
 			factor = (float)(cframe - prevFrame->framenum) / (nextFrame->framenum - prevFrame->framenum + 1);
 			
+			if (ipo_settings->type == GP_IPO_CURVEMAP) {
+				/* custom curvemap */
+				if (ipo_settings->custom_ipo) {
+					factor = curvemapping_evaluateF(ipo_settings->custom_ipo, 0, factor);
+				}
+				else {
+					BKE_report(op->reports, RPT_ERROR, "Custom interpolation curve does not exist");
+				}
+			}
+			else if (ipo_settings->type >= GP_IPO_BACK) {
+				/* easing equation... */
+				factor = gp_interpolate_seq_easing_calc(ipo_settings, factor);
+			}
+			
 			/* create new strokes data with interpolated points reading original stroke */
 			for (gps_from = prevFrame->strokes.first; gps_from; gps_from = gps_from->next) {
 				bGPDstroke *new_stroke;