diff --git a/sch2fig/test/comp b/sch2fig/test/comp
index 55e1052..b406607 100755
--- a/sch2fig/test/comp
+++ b/sch2fig/test/comp
@@ -1,9 +1,23 @@
 #!/bin/sh
+
+usage()
+{
+	echo "usage: $0 [dir]" 1>&2
+	exit 1
+}
+
+
+[ "$2" ] && usage
+[ "${1#-}" != "$1" ] && usage
+
+dir=${1:-.}
+
 diffs=0
-rm -f _diff*png
-for n in out*.png; do
-	ref=ref${n#out}
-	diff=_diff${n#out}
+rm -f $dir/_diff*png
+for n in $dir/out*.png; do
+	out=`basename "$n"`
+	ref=$dir/ref${out#out}
+	diff=$dir/_diff${out#out}
 	if ! compare -metric AE $ref $n - >/dev/null; then
 		diffs=`expr $diffs + 1`
 		compare -metric AE $ref $n $diff