GPencil: Layers with alpha = 0 should not be editable

* Added a new API function to test if a GPencil layer is visible or not
* Replaced all editability checks with this new "super check"
* Replaced all magic number thresholds for opacity visiblity with a single define

2 files changed, 32 insertions, 2 deletions

diff --git a/source/blender/blenkernel/BKE_gpencil.h b/source/blender/blenkernel/BKE_gpencil.h
index dc436288eff..24e330d927f 100644
--- a/source/blender/blenkernel/BKE_gpencil.h
+++ b/source/blender/blenkernel/BKE_gpencil.h
@@ -58,6 +58,11 @@ struct bGPdata *gpencil_data_duplicate(struct bGPdata *gpd, bool internal_copy);
 void gpencil_frame_delete_laststroke(struct bGPDlayer *gpl, struct bGPDframe *gpf);
 
 
+/* Stroke and Fill - Alpha Visibility Threshold */
+#define GPENCIL_ALPHA_OPACITY_THRESH 0.001f
+
+bool gpencil_layer_is_editable(const struct bGPDlayer *gpl);
+
 /* How gpencil_layer_getframe() should behave when there
  * is no existing GP-Frame on the frame requested.
  */
diff --git a/source/blender/blenkernel/intern/gpencil.c b/source/blender/blenkernel/intern/gpencil.c
index afdac83f68a..485c4f5b29f 100644
--- a/source/blender/blenkernel/intern/gpencil.c
+++ b/source/blender/blenkernel/intern/gpencil.c
@@ -435,16 +435,41 @@ void gpencil_frame_delete_laststroke(bGPDlayer *gpl, bGPDframe *gpf)
 
 /* -------- GP-Layer API ---------- */
 
+/* Check if the given layer is able to be edited or not */
+bool gpencil_layer_is_editable(const bGPDlayer *gpl)
+{
+	/* Sanity check */
+	if (gpl == NULL)
+		return false;
+	
+	/* Layer must be: Visible + Editable */
+	if ((gpl->flag & (GP_LAYER_HIDE | GP_LAYER_LOCKED)) == 0) {
+		/* Opacity must be sufficiently high that it is still "visible"
+		 * Otherwise, it's not really "visible" to the user, so no point editing...
+		 */
+		if ((gpl->color[3] > GPENCIL_ALPHA_OPACITY_THRESH) || (gpl->fill[3] > GPENCIL_ALPHA_OPACITY_THRESH)) {
+			return true;
+		}
+	}
+	
+	/* Something failed */
+	return false;
+}
+
+/* Look up the gp-frame on the requested frame number, but don't add a new one */
 bGPDframe *BKE_gpencil_layer_find_frame(bGPDlayer *gpl, int cframe)
 {
 	bGPDframe *gpf;
-
+	
+	/* Search in reverse order, since this is often used for playback/adding,
+	 * where it's less likely that we're interested in the earlier frames
+	 */
 	for (gpf = gpl->frames.last; gpf; gpf = gpf->prev) {
 		if (gpf->framenum == cframe) {
 			return gpf;
 		}
 	}
-
+	
 	return NULL;
 }