Index: trunk/src/layer.c
===================================================================
--- trunk/src/layer.c	(revision 32974)
+++ trunk/src/layer.c	(revision 32975)
@@ -1115,6 +1115,19 @@
 	return NULL;
 }
 
+rnd_layer_id_t pcb_layer2id(pcb_data_t *data, pcb_layer_t *ly)
+{
+	long lid;
+
+	if (ly == NULL) return -1;
+
+	lid = ly - data->Layer;
+	if ((lid < 0) || (lid >= data->LayerN))
+		return -1;
+
+	return lid;
+}
+
 void pcb_layer_link_trees(pcb_layer_t *dst, pcb_layer_t *src)
 {
 	/* we can't link non-existing trees - make sure src does have the trees initialized */
Index: trunk/src/layer.h
===================================================================
--- trunk/src/layer.h	(revision 32974)
+++ trunk/src/layer.h	(revision 32975)
@@ -206,6 +206,9 @@
 /* Return the layer pointer (or NULL on invalid or virtual layers) for an id */
 pcb_layer_t *pcb_get_layer(pcb_data_t *data, rnd_layer_id_t id);
 
+/* Return the layer ID within data, or -1 if ly is not in data */
+rnd_layer_id_t pcb_layer2id(pcb_data_t *data, pcb_layer_t *ly);
+
 /* Return the name of a layer (real or virtual) or NULL on error
    NOTE: layer names may not be unique; returns the first case sensitive hit;
    slow linear search */