diff --git a/gentextures.sh b/gentextures.sh
index 09d4067..57d70a7 100755
--- a/gentextures.sh
+++ b/gentextures.sh
@@ -1,56 +1,116 @@
 #!/bin/bash
 
-# This line defines the base of the filename, e.g. cotton_xxx_xxx.png
-TEXTURE="cotton"
+TEXTURE=$1
+COMPOSITE=$2
 
-# This defines the name of the bright red source texture that will
-# be used to generate all of the others.
 base_texture="red_base_"$TEXTURE".png"
 
-base_colors="red orange yellow lime green aqua cyan skyblue blue violet magenta redviolet"
+if [ -z $TEXTURE ] || [ $TEXTURE == "--help" ] || [ $TEXTURE == "-h" ] ; then {
 
-echo -e "\nCurrently configured to generate filenames based on "$TEXTURE"."
-
-if [ ! -e "$base_texture" ] ; then {
-	echo -e "\nPlease supply a proper base texture from which to generate all"
-	echo "of the colors.  It must be named "$base_texture" and must"
-	echo -e "be placed in the directory you ran this script from.\n"
+	echo -e "\nUsage:"
+	echo -e "\ngentextures.sh basename [overlay filename]"
+	echo -e "\nThis script requires one or two parameters which supply the"
+	echo -e "base filename of the textures, and an optional overlay.  The"
+	echo -e "<basename> is the first part of the filename that your textures"
+	echo -e "will use when your mod is done.  For example, if you supply the"
+	echo -e "word 'cotton', this script will produce filenames like cotton_red.png"
+	echo -e "or 'cotton_dark_blue_s50.png'.  The texture that this script will"
+	echo -e "read and recolor is derived from this parameter, and will be of"
+	echo -e "the form 'red_base_xxxxx.png', where 'xxxx' is the basename."
+	echo -e "\nYou can also supply an optional overlay image filename."
+	echo -e "This image will be composited onto the output files after they"
+	echo -e "have been colorized, but without being modified.  This is useful"
+	echo -e "when you have some part of your base image that will either get"
+	echo -e "changed unpredictably or undesirably.  Simply draw two images -"
+	echo -e "one containing the whole image to be colored, and one containing"
+	echo -e "the parts that should not be changed, with transparency where the"
+	echo -e "base image should show through.\n"
 	exit 1
 } fi
 
-pushd . >/dev/null
+
+if [[ ! -z $TEXTURE && ! -e $base_texture ]]; then {
+	echo -e "\nThe basename 'red_base_"$TEXTURE".png' was not found."
+        echo -e "\nAborting.\n"
+	exit 1
+} fi
+
+if [[ ! -z $COMPOSITE && ! -e $COMPOSITE ]]; then {
+	echo -e "\nThe requested composite file '"$COMPOSITE"' was not found."
+        echo -e "\nAborting.\n"
+	exit 1
+} fi
+
+
+convert $base_texture -modulate 1,2,3 tempfile.png 1>/dev/null 2>/dev/null
+
+if (( $? )) ; then {
+	echo -e "\nImagemagick failed while testing the base texture file."
+	echo -e "\nEither the base file 'red_base_"$TEXTURE".png isn't an image,"
+	echo "or it is broken, or Imagemagick itself just didn't work."
+	echo -e "\nPlease check and correct your base image and try again."
+	echo -e "\nAborting.\n"
+	exit 1
+} fi
+
+composite_file=""
+
+if [ ! -z $COMPOSITE ] ; then {
+	convert $base_texture -modulate 1,2,3 $COMPOSITE -composite tempfile.png 1>/dev/null 2>/dev/null
+
+	if (( $? )) ; then {
+		echo -e "\nImagemagick failed while testing the composite file."
+		echo -e "\nEither the composite file '"$COMPOSITE"' isn't an image"
+		echo "or it is broken, or Imagemagick itself just didn't work."
+		echo -e "\nPlease check and correct your composite image and try again."
+		echo -e "\nAborting.\n"
+		exit 1
+	} fi
+
+	composite_file=$COMPOSITE" -composite"
+} fi
+
+rm tempfile.png
+
+base_colors="red orange yellow lime green aqua cyan skyblue blue violet magenta redviolet"
+
+echo -e -n "\nGenerating filenames based on "$base_texture
+if [ ! -z $COMPOSITE ] ; then {
+	echo ","
+	echo -n "using "$COMPOSITE" as an overlay"
+} fi
+echo -e "...\n"
+
+rm -rf generated-textures
 mkdir generated-textures
-cd generated-textures
 
 hue=0
 for name in $base_colors ; do
 	hue2=`echo "scale=10; ("$hue"*200/360)+100" |bc`
 	echo $name "("$hue" degrees)"
 	echo "     dark"
-	convert ../$base_texture -modulate  33,100,$hue2 $TEXTURE"_dark_"$name".png"
+	convert $base_texture -modulate  33,100,$hue2 $composite_file "generated-textures/"$TEXTURE"_dark_"$name".png"
 	echo "     medium"
-	convert ../$base_texture -modulate  66,100,$hue2 $TEXTURE"_medium_"$name".png"
+	convert $base_texture -modulate  66,100,$hue2 $composite_file "generated-textures/"$TEXTURE"_medium_"$name".png"
 	echo "     bright"
-	convert ../$base_texture -modulate 100,100,$hue2 $TEXTURE"_"$name".png"
+	convert $base_texture -modulate 100,100,$hue2 $composite_file "generated-textures/"$TEXTURE"_"$name".png"
 	echo "     dark, 50% saturation"
-	convert ../$base_texture -modulate  33,50,$hue2  $TEXTURE"_dark_"$name"_s50.png"
+	convert $base_texture -modulate  33,50,$hue2  $composite_file "generated-textures/"$TEXTURE"_dark_"$name"_s50.png"
 	echo "     medium, 50% saturation"
-	convert ../$base_texture -modulate  66,50,$hue2  $TEXTURE"_medium_"$name"_s50.png"
+	convert $base_texture -modulate  66,50,$hue2  $composite_file "generated-textures/"$TEXTURE"_medium_"$name"_s50.png"
 	echo "     bright, 50% saturation"
-	convert ../$base_texture -modulate 100,50,$hue2  $TEXTURE"_"$name"_s50.png"
+	convert $base_texture -modulate 100,50,$hue2  $composite_file "generated-textures/"$TEXTURE"_"$name"_s50.png"
 	hue=$((hue+30))
 done
 
 echo "greyscales"
 echo "     black"
-convert ../$base_texture -modulate  15,0,0  $TEXTURE"_black.png"
+convert $base_texture -modulate  15,0,0  $composite_file "generated-textures/"$TEXTURE"_black.png"
 echo "     dark grey"
-convert ../$base_texture -modulate  50,0,0  $TEXTURE"_darkgrey.png"
+convert $base_texture -modulate  50,0,0  $composite_file "generated-textures/"$TEXTURE"_darkgrey.png"
 echo "     medium grey"
-convert ../$base_texture -modulate 100,0,0  $TEXTURE"_mediumgrey.png"
+convert $base_texture -modulate 100,0,0  $composite_file "generated-textures/"$TEXTURE"_mediumgrey.png"
 echo "     light grey"
-convert ../$base_texture -modulate 150,0,0  $TEXTURE"_lightgrey.png"
+convert $base_texture -modulate 150,0,0  $composite_file "generated-textures/"$TEXTURE"_lightgrey.png"
 echo "     white"
-convert ../$base_texture -modulate 190,0,0  $TEXTURE"_white.png"
-
-popd >/dev/null
+convert $base_texture -modulate 190,0,0  $composite_file "generated-textures/"$TEXTURE"_white.png"