Add README.md

.gitignore

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-testcanvas.png
+*.png

README.md

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+# griddle
+
+`griddle` is a program for practicing drawing using the [Grid Technique](https://www.art-is-fun.com/grid-method).
+It takes an input image and uses [ImageMagick](https://imagemagick.org) to automatically draw a grid (of specified dimensions) over it.
+
+## Installation
+`griddle` is a shell script; you can copy it to somewhere in your `$PATH` or just clone the repository and execute it.
+
+## Usage
+The CLI interface for `griddle` is `griddle <inputfile> <number_of_columns> <number_of_rows> <outputfile>`.
+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.
+
+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.
+- Add a default output filename (something like `<inputfilename>_grid.<inputextension>`) if no output filename is explicitly given.

griddle

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 #!/bin/sh
 
+set -e
+
+WIDTH=1
+HEIGHT=2
+
+get_image_dimension() {
+	inputfile="$1"
+	dimension="$2"
+
+	height="$(identify $inputfile | cut -d ' ' -f 3 | cut -d 'x' -f $dimension)"
+	printf "%s" "$height"
+}
+
+generate_lines() {
+	columns="$1"
+	rows="$2"
+
+	width="$3"
+	height="$4"
+
+	# Generate border lines
+	border_width=$((width - 1))
+	border_height=$((height - 1))
+	printf "line 0, 0 0, %d " $border_height
+	printf "line 0, 0 %d, 0 " $border_width
+	printf "line 0, %d %d, %d " $border_height $border_width $border_height
+	printf "line %d, 0 %d, %d " $border_width $border_width $border_height
+
+	# Generate column separators
+	colwidth=$((width / columns))
+	for i in $(seq 1 $columns); do
+		current_colwidth=$((colwidth * i))
+		printf "line %d, 0 %d, %d " $current_colwidth $current_colwidth $height 
+	done
+
+	# Generate row separators
+	rowheight=$((height / rows))
+	for i in $(seq 1 $rows); do
+		current_rowheight=$((rowheight * i))
+		printf "line 0, %d %d, %d " $current_rowheight $width $current_rowheight
+	done
+}
+
+create_image() {
+	inputfile="$1"
+	lines="$2"
+	outputfile="$3"
+
+	convert -draw "$lines" "$inputfile" "$outputfile"
+}
+
 main() {
-	if test "$#" -ne 2; then
-		echo "Usage: griddle <inputfile> <outputfile>"
+	if test "$#" -ne 4; then
+		printf "Usage: griddle <inputfile> <columns> <rows> <outputfile>\n"
 		return 1
 	fi
 
 	inputfile="$1"
-	outputfile="$2"
+	if ! test -e "$inputfile"; then
+		printf "Input file %s does not exist.\n" "$inputfile"
+		return 2
+	fi
 
-	echo "$inputfile" "$outputfile"
+	columns="$2"
+	rows="$3"
+
+	outputfile="$4"
+	if test -e "$outputfile"; then
+		printf "Output file $outputfile already exists. Overwrite? (y/N) "
+		read response
+		if ! test "$response" = "y" -o "$response" = "Y"; then
+			printf "Not overwriting $outputfile.\n"
+			return 0
+		else
+			printf "Overwriting $outputfile.\n"
+		fi
+	fi
+
+	width="$(get_image_dimension $inputfile $WIDTH)"
+	height="$(get_image_dimension $inputfile $HEIGHT)"
+
+	lines="$(generate_lines $columns $rows $width $height)"
+
+	create_image "$inputfile" "$lines" "$outputfile"
 }
 
 main "$@"