diff --git a/README.md b/README.md
index 552d474..a423bf8 100644
--- a/README.md
+++ b/README.md
@@ -1,62 +1,24 @@
 # tile-bleed-margin-generator
-Automatically stretch out the 1px edge of each tile in a sprite sheet to compensate for tile bleed in 2D game development.
 
-## bleed.sh
+Automatically stretch out the 1px edge of each tile in a tile sheet to compensate for tile bleed in 2D game development.
 
-Change the script to be specific to your case.
+This should solve rounding errors which can cause tiles to slightly bleed into their neighbors, such as the very common "gap between tiles" problem in Unity. The script started as a fork but I added so many features that it ended up quite different from [the original](https://github.com/cjonasw/tile-bleed-margin-generator).
 
-### You'll need to figure out a few things:
+Alternatively, you might want to look at chonkypixel's solution which is integrated in the Unity editor. I haven't tested it myself, but it has a UI which does exactly the same thing. https://assetstore.unity.com/packages/2d/textures-materials/tiles/2d-tile-gap-fixing-tool-157060
 
-1. First you need to know how big your tiles are; mine are *16x16* (`TILE_SIZE = 16`)
-2. then how big your spritesheet is; mine is *640x576* (`SPRITESHEET_WIDTH = 640`, `SPRITESHEET_HEIGHT = 576`)
-3. How many columns there are:
-```
-COLUMN_COUNT = (SPRITESHEET_WIDTH / TILE_SIZE)
-```
+## Dependencies
 
-so mine is: `(640/16)`
+This script was written for bash and depends on ImageMagick (tested using `ImageMagick 6.9.7-4`). The `convert`, `montage` and `identify` commands must be in your PATH.
 
-4. How many rows there are:
+Until someone makes a Windows version (`.cmd` or `.ps`) you should be able to run it on Windows 10 using bash in the Windows Subsystem for Linux, provided you've installed ImageMagick within your virtual linux.
 
-```
-ROW_COUNT = (SPRITESHEET_HEIGHT / TILE_SIZE)
-```
+## Sample usage
 
-so mine is: `(576/16)`
+    ./bleed.sh input.png output.png --tile-size 16 16 --bleed 3 --input-gap 4 4 --output-gap 1 1 --input-offset 20 20 --output-offset 3 3
 
-5. How many tiles are in the spritesheet:
+The script determines the number of tiles based on the size of the file. It ignores content above and the the left of the specified `--input-offset`, and any leftover space that is smaller than a tile on the bottom right. See the sample files in the repository.
 
-```
-NUMBER_OF_TILES = COLUMN_COUNT * ROW_COUNT
-```
+## Known issues
 
-so in my case I had to do: `(640/16)*(576/16)` which is 1440.
-
-```
-NUMBER_OF_TILES = 1440
-```
-
-6. You need the file name of the spritesheet too.
-
-That's all you need. so now:
-
-### Modify the script to your case
-
-1. Change all instances of `16` to your `TILE_SIZE`
-2. Change `spritesheet.png` to your file
-3. Change all instances of `1439` to your `NUMBER_OF_TILES - 1` so in my case it was *`1439`* instead of `1440`. (This is due to indexing starting at 0)
-4. Change all instances of `48` to your `TILESIZE` multiplied by 3 (`TILESIZE * 3` so mine was `16*3`)
-5. Change all instances of `40` to your `COLUMN_COUNT`
-6. Change all instances of `36` to your `ROW_COUNT`
-
-Make sure your script is in the same directory as the file, open Terminal and `cd` to this folder and run:
-
-`sh bleed.sh`
-
-you should now have loads of files, but the only one you are interested in is:
-
-`spritesheet-bleed.png`
-
-## TODO: Improvements
-
-Make all of these calculations automatic and based on arguments.
+-   Unknown options or options with the wrong number of parameters produce unclear error messages.
+-   The `--help` flag doesn't work properly because the command fails on missing arguments first. Mitigated by showing the usage on missing arguments.
diff --git a/bleed.sh b/bleed.sh
index 4f3a6ce..93c1c98 100644
--- a/bleed.sh
+++ b/bleed.sh
@@ -1,3 +1,226 @@
-convert -crop 16x16 spritesheet.png sprite.png
-convert sprite-{0..1439}.png -set option:distort:viewport 48x48-16-16 -virtual-pixel Edge -filter point -distort SRT 0 +repage sprite-bleed.png
-montage sprite-bleed-{0..1439}.png -tile 40x36 -geometry 48x48+0+0 -background none spritesheet-bleed.png
+#!/bin/bash
+
+usage() {
+	cat <<-EOM
+		Usage: bleed INPUT_FILE OUTPUT_FILE [OPTIONS...]
+		
+		Extends the edge of of every tile in a tileset by 1px to avoid issues with
+		neighboring tile bleeding and gaps between rendered tiles.
+		
+		Tested with ImageMagick 6.9.7-4
+		
+		Options must appear after the positional arguments:
+		  -t, --tile-size XX YY         the size of the tiles
+		                                (defaults to 16x16)
+		  -b, --bleed XX                how much bleeding should be produced around
+		                                each generated tile
+		                                (defaults to 1)
+		  -g, --input-gap XX YY         the horizontal and vertical gap between
+		                                tiles in the input image
+		                                (defaults to 0x0)
+		  -o, --input-offset XX YY      the offset of the first tile from the
+		                                top left corner of the input image
+		                                (defaults to 0x0)
+		  -G, --output-gap XX YY        the horizontal and vertical gap to leave
+		                                between tiles in the output image as
+		                                transparent pixels
+		                                (defaults to 0x0)
+		  -O, --output-offset XX YY     the margins added to the output image
+		                                (defaults to 0x0)
+		  -f, --force                   overwrite the output file if it exists
+		  -v, --verbose                 display computations during execution
+		  -h, --help                    display this help text and exit
+	EOM
+}
+
+
+#### INITIALIZE VARIABLES
+
+positional_args=()
+
+input_path=
+input_tile_width=16
+input_tile_height=16
+input_offset_x=0
+input_offset_y=0
+input_gap_x=0
+input_gap_y=0
+
+output_path=
+output_offset_x=0
+output_offset_y=0
+output_gap_x=0
+output_gap_y=0
+output_bleed=1
+
+
+#### PARSE ARGUMENTS
+
+if [[ $# -lt 2 ]] ; then
+	echo "Missing arguments."
+	usage
+	exit 1
+fi
+
+input_path=$1
+shift
+output_path=$1
+shift
+
+while [[ $# > 0 ]] ; do
+	case $1 in
+		-t | --tile-size )
+			shift
+			input_tile_width=$1
+			shift
+			input_tile_height=$1
+			;;
+		-b | --bleed )
+			shift
+			output_bleed=$1
+			;;
+		-g | --input-gap )
+			shift
+			input_gap_x=$1
+			shift
+			input_gap_y=$1
+			;;
+		-o | --input-offset )
+			shift
+			input_offset_x=$1
+			shift
+			input_offset_y=$1
+			;;
+		-G | --output-gap )
+			shift
+			output_gap_x=$1
+			shift
+			output_gap_y=$1
+			;;
+		-O | --output-offset )
+			shift
+			output_offset_x=$1
+			shift
+			output_offset_y=$1
+			;;
+		-f | --force )
+			force=true
+			;;
+		-v | --verbose )
+			verbose=true
+			;;
+		-fv | -vf )
+			force=true
+			verbose=true
+			;;
+		-h | --help )
+			usage
+			exit
+			;;
+		* )
+			# todo: make sure --help flag is always checked
+			echo "Incorrect option sequence." >&2
+			usage >&2
+			exit 1
+	esac
+	shift
+done
+
+
+#### VALIDATE ARGUMENTS
+
+if [[ ! -f $input_path ]] ; then
+	echo "Input file not found." >&2; exit 1
+fi
+
+if [[ -e $output_path && $force != true ]] ; then
+	echo "A file with the specified output path already exists." >&2; exit 1
+fi
+
+int_regex='^[0-9]+$'
+assert_int() {
+	if ! [[ $2 =~ $int_regex ]] ; then
+		echo "$1 must be an integer" >&2; exit 1
+	fi
+}
+assert_coords() {
+	if ! [[ $2 =~ $int_regex && $3 =~ $int_regex ]] ; then
+		echo "$1 must be two integers separated by a space" >&2; exit 1
+	fi
+}
+
+assert_coords "--tile-size" $input_tile_width $input_tile_height
+assert_int "--bleed" $output_bleed
+assert_coords "--input-gap" $input_gap_x $input_gap_y
+assert_coords "--input-offset" $input_offset_x $input_offset_y
+assert_coords "--output-gap" $output_gap_x $output_gap_y
+assert_coords "--output-offset" $output_offset_x $output_offset_y
+
+
+#### BEGIN COMPUTATION
+
+# ENABLE VERBOSE MODE IF DESIRED
+if [[ $verbose == true ]] ; then
+	set -x
+fi
+
+# GET IMAGE SIZE
+input_file_width=`identify -format %w $input_path`
+input_file_height=`identify -format %h $input_path`
+
+# CROP MARGINS
+input_right_margin=$(( ($input_file_width + $input_gap_x - $input_offset_x) % ($input_tile_width + $input_gap_x) ))
+input_bottom_margin=$(( ($input_file_height + $input_gap_y - $input_offset_y) % ($input_tile_height + $input_gap_y) ))
+input_usefull_width=$(( $input_file_width - $input_offset_x - $input_right_margin ))
+input_usefull_height=$(( $input_file_height - $input_offset_y - $input_bottom_margin ))
+crop_margins_geometry="${input_usefull_width}x${input_usefull_height}+${input_offset_x}+${input_offset_y}"
+
+# SLICE INTO TILES
+slice_width=$(($input_tile_width + $input_gap_x))
+slice_height=$(($input_tile_height + $input_gap_y))
+slice_to_tiles_geometry="${slice_width}x${slice_height}"
+
+# REMOVE TILE GAP
+crop_tile_gap_geometry="${input_tile_width}x${input_tile_width}+0+0"
+
+# ADD BLEEDING
+bleeding_width=$(($input_tile_width + (2 * $output_bleed) ))
+bleeding_height=$(($input_tile_height + (2 * $output_bleed) ))
+distort_bleed_geometry="${bleeding_width}x${bleeding_height}-${output_bleed}-${output_bleed}"
+
+# ADD TILE GAP
+output_tile_width=$(($bleeding_width + $output_gap_x ))
+output_tile_height=$(($bleeding_height + $output_gap_y ))
+output_tile_geometry="${output_tile_width}x${output_tile_height}+0+0"
+
+# MERGE TILES
+columns=$(( ($input_file_width + $input_gap_x - $input_offset_x) / ($input_tile_width + $input_gap_x) ))
+rows=$(( ($input_file_height + $input_gap_y - $input_offset_y) / ($input_tile_height + $input_gap_y) ))
+
+# REMOVE LAST GAP
+output_usefull_width=$(( ($output_tile_width * $columns) - $output_gap_x ))
+output_usefull_height=$(( ($output_tile_height * $rows) - $output_gap_y ))
+crop_extra_gap_geometry="${output_usefull_width}x${output_usefull_height}+0+0"
+
+# ADD MARGIN
+output_total_width=$(($output_usefull_width + (2 * $output_offset_x) ))
+output_total_height=$(($output_usefull_height + (2 * $output_offset_y) ))
+output_size_geometry="${output_total_width}x${output_total_height}"
+
+# DISABLE VERBOSE OUTPUT
+{ set +x; } 2>/dev/null
+
+
+#### SUBMIT TASK TO IMAGEMAGICK
+
+convert $input_path \
+	-crop $crop_margins_geometry +repage +gravity \
+	-crop $slice_to_tiles_geometry +repage \
+	-crop $crop_tile_gap_geometry +repage \
+	-define distort:viewport=$distort_bleed_geometry -filter point -distort SRT 0 +repage \
+	-background none -extent $output_tile_geometry miff:- |
+montage - -tile ${columns}x${rows} -geometry +0+0 -background none +repage miff:- |
+convert - \
+	-crop $crop_extra_gap_geometry +repage \
+	-gravity center -background none -extent $output_size_geometry \
+	$output_path
\ No newline at end of file
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