GPencil: Add option to draw new strokes on back of layer

For artist point of view is very useful to have an option to draw by
default the new strokes on back of all strokes in the layer.

3 files changed, 19 insertions, 2 deletions

diff --git a/source/blender/editors/gpencil/gpencil_paint.c b/source/blender/editors/gpencil/gpencil_paint.c
index d637affe79d..cebcbfe9317 100644
--- a/source/blender/editors/gpencil/gpencil_paint.c
+++ b/source/blender/editors/gpencil/gpencil_paint.c
@@ -692,6 +692,7 @@ static void gp_stroke_newfrombuffer(tGPsdata *p)
 	bGPDspoint *pt;
 	tGPspoint *ptc;
 	bGPDbrush *brush = p->brush;
+	ToolSettings *ts = p->scene->toolsettings;
 	
 	int i, totelem;
 	/* since strokes are so fine, when using their depth we need a margin otherwise they might get missed */
@@ -925,8 +926,16 @@ static void gp_stroke_newfrombuffer(tGPsdata *p)
 	gps->palcolor = palcolor;
 	strcpy(gps->colorname, palcolor->info);
 
-	/* add stroke to frame */
-	BLI_addtail(&p->gpf->strokes, gps);
+	/* add stroke to frame, usually on tail of the listbase, but if on back is enabled the stroke is added on listbase head 
+	* because the drawing order is inverse and the head stroke is the first to draw. This is very useful for artist
+	* when drawing the background
+	*/
+	if (ts->gpencil_flags & GP_TOOL_FLAG_PAINT_ONBACK) {
+		BLI_addhead(&p->gpf->strokes, gps);
+	}
+	else {
+		BLI_addtail(&p->gpf->strokes, gps);
+	}
 	gp_stroke_added_enable(p);
 }
 
diff --git a/source/blender/makesdna/DNA_scene_types.h b/source/blender/makesdna/DNA_scene_types.h
index 90e8d8b8270..a4934cc1f24 100644
--- a/source/blender/makesdna/DNA_scene_types.h
+++ b/source/blender/makesdna/DNA_scene_types.h
@@ -2088,6 +2088,8 @@ typedef enum eGPencil_Flags {
 	GP_TOOL_FLAG_PAINTSESSIONS_ON       = (1 << 0),
 	/* When creating new frames, the last frame gets used as the basis for the new one */
 	GP_TOOL_FLAG_RETAIN_LAST            = (1 << 1),
+	/* Add the strokes below all strokes in the layer */
+	GP_TOOL_FLAG_PAINT_ONBACK = (1 << 2)
 } eGPencil_Flags;
 
 /* toolsettings->gpencil_src */
diff --git a/source/blender/makesrna/intern/rna_scene.c b/source/blender/makesrna/intern/rna_scene.c
index abac6b98199..156c327f97c 100644
--- a/source/blender/makesrna/intern/rna_scene.c
+++ b/source/blender/makesrna/intern/rna_scene.c
@@ -2612,6 +2612,12 @@ static void rna_def_tool_settings(BlenderRNA  *brna)
 	                         "are included as the basis for the new one");
 	RNA_def_property_update(prop, NC_SCENE | ND_TOOLSETTINGS, NULL);
 	
+	prop = RNA_def_property(srna, "use_gpencil_draw_onback", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "gpencil_flags", GP_TOOL_FLAG_PAINT_ONBACK);
+	RNA_def_property_ui_text(prop, "Draw Strokes on Back",
+		"When draw new strokes, the new stroke is drawn below of all strokes in the layer");
+	RNA_def_property_update(prop, NC_SCENE | ND_TOOLSETTINGS, NULL);
+
 	prop = RNA_def_property(srna, "grease_pencil_source", PROP_ENUM, PROP_NONE);
 	RNA_def_property_enum_bitflag_sdna(prop, NULL, "gpencil_src");
 	RNA_def_property_enum_items(prop, gpencil_source_3d_items);