GPencil: Per-layer option to always show onion skinning

Sometimes it can be useful to be able to keep onion skins visible in the
OpenGL renders and/or when doing animation playback. In particular, there
are two use cases where this is quite useful:
 1) For creating a cheap motion-blur effect, especially when the before/after
    values are also animated.
 2) If you've animated a shot with onion skinning enabled, the poses may end
    up looking odd if the ghosts are not shown (as you may have been accounting
    for the ghosts when making the compositions).

This option can be found as the small "camera" toggle between the "Use Onion Skinning"
and "Use Custom Colors" options.

3 files changed, 21 insertions, 3 deletions

diff --git a/source/blender/editors/gpencil/drawgpencil.c b/source/blender/editors/gpencil/drawgpencil.c
index 9da3c3ee088..2fd574d6523 100644
--- a/source/blender/editors/gpencil/drawgpencil.c
+++ b/source/blender/editors/gpencil/drawgpencil.c
@@ -1419,8 +1419,16 @@ static void gp_draw_data_layers(
 
 #undef GP_DRAWFLAG_APPLY
 		
-		/* draw 'onionskins' (frame left + right) */
-		if ((gpl->flag & GP_LAYER_ONIONSKIN) && !(dflag & GP_DRAWDATA_NO_ONIONS)) {
+		/* Draw 'onionskins' (frame left + right)
+		 *   - It is only possible to show these if the option is enabled
+		 *   - The "no onions" flag prevents ghosts from appearing during animation playback/scrubbing
+		 *     and in renders
+		 *   - The per-layer "always show" flag however overrides the playback/render restriction,
+		 *     allowing artists to selectively turn onionskins on/off during playback
+		 */
+		if ((gpl->flag & GP_LAYER_ONIONSKIN) && 
+		    ((dflag & GP_DRAWDATA_NO_ONIONS) == 0 || (gpl->flag & GP_LAYER_GHOST_ALWAYS))) 
+		{
 			/* Drawing method - only immediately surrounding (gstep = 0),
 			 * or within a frame range on either side (gstep > 0)
 			 */
diff --git a/source/blender/makesdna/DNA_gpencil_types.h b/source/blender/makesdna/DNA_gpencil_types.h
index 23b73424da5..0364d855f69 100644
--- a/source/blender/makesdna/DNA_gpencil_types.h
+++ b/source/blender/makesdna/DNA_gpencil_types.h
@@ -244,6 +244,7 @@ typedef struct bGPDlayer {
 	float inverse[4][4];    /* inverse matrix (only used if parented) */
 	char parsubstr[64];     /* String describing subobject info, MAX_ID_NAME-2 */
 	short partype, pad;
+	
 	float tintcolor[4];     /* Color used to tint layer, alpha value is used as factor */
 	float opacity;          /* Opacity of the layer */
 } bGPDlayer;
@@ -275,7 +276,9 @@ typedef enum eGPDlayer_Flag {
 	/* Use high quality fill (instead of buggy legacy OpenGL Fill) */
 	GP_LAYER_HQ_FILL        = (1 << 11),
 	/* Unlock color */
-	GP_LAYER_UNLOCK_COLOR = (1 << 12)
+	GP_LAYER_UNLOCK_COLOR 	= (1 << 12),
+	/* always show onion skins (i.e. even during renders/animation playback) */
+	GP_LAYER_GHOST_ALWAYS	= (1 << 13),
 } eGPDlayer_Flag;
 
 /* Grease-Pencil Annotations - 'DataBlock' */
diff --git a/source/blender/makesrna/intern/rna_gpencil.c b/source/blender/makesrna/intern/rna_gpencil.c
index 41a73cfc528..14da990278c 100644
--- a/source/blender/makesrna/intern/rna_gpencil.c
+++ b/source/blender/makesrna/intern/rna_gpencil.c
@@ -1242,6 +1242,13 @@ static void rna_def_gpencil_layer(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "After Color", "Base color for ghosts after the active frame");
 	RNA_def_property_update(prop, NC_GPENCIL | ND_DATA, "rna_GPencil_update");
 	
+	prop = RNA_def_property(srna, "use_ghosts_always", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LAYER_GHOST_ALWAYS);
+	RNA_def_property_ui_text(prop, "Always Show Ghosts",
+	                         "Ghosts are shown in renders and animation playback. Useful for special effects (e.g. motion blur)");
+	RNA_def_property_update(prop, NC_GPENCIL | ND_DATA, "rna_GPencil_update");
+	
+	
 	/* Flags */
 	prop = RNA_def_property(srna, "hide", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LAYER_HIDE);