Generate a blank copy of the grid in a separate image

README.md

@@ -12,11 +12,14 @@ This is the number of **columns** and **rows**, not the number of separators.
 For example, specifying "2" columns will result in **one** line splitting the image vertically.
 Border lines around the edge of the image are always generated.
 
+`griddle` also generates an image named "blank_grid.png", which is an exact copy of the grid generated on top of the input image, but with a blank white background.
+This can then be printed out and used to draw on, instead of manually trying to recreate the grid on paper, if you both have a printer and are using a kind of paper that can be used in said printer.
+(I don't have a printer, but I figured someone might find this feature useful.)
+
 It supports any input and output file formats that ImageMagick supports.
 The input and output files don't need to be the same format; you can input a PNG and output a JPEG, for example.
 
 ## Planned Features
-- Generate an additional output image (or PDF?) containing a blank copy of just the generated grid which can be printed out, to save the effort of manually recreating the grid on your drawing paper.
 - Improve the CLI interface with `--options` to remove the arbitrary ordering of `<number_of_columns>` and `<number_of_rows>`.
 - Make border lines optional.
 - Allow the user to adjust the color and transparency of grid lines.

griddle

@@ -43,7 +43,7 @@ generate_lines() {
 	done
 }
 
-create_image() {
+draw_image_with_grid() {
 	inputfile="$1"
 	lines="$2"
 	outputfile="$3"
@@ -51,6 +51,14 @@ create_image() {
 	convert -draw "$lines" "$inputfile" "$outputfile"
 }
 
+draw_blank_grid() {
+	lines="$1"
+	width="$2"
+	height="$3"
+
+	convert -size "${width}x${height}" canvas:white -draw "$lines" blank_grid.png
+}
+
 main() {
 	if test "$#" -ne 4; then
 		printf "Usage: griddle <inputfile> <columns> <rows> <outputfile>\n"
@@ -83,7 +91,8 @@ main() {
 
 	lines="$(generate_lines $columns $rows $width $height)"
 
-	create_image "$inputfile" "$lines" "$outputfile"
+	draw_image_with_grid "$inputfile" "$lines" "$outputfile"
+	draw_blank_grid "$lines" "$width" "$height"
 }
 
 main "$@"