A script which uses ImageMagick to develop scans of C-41 film negatives into color-corrected positive images

c41lab

#!/bin/bash
# ====================================================================================================================
#
#   Converted for bash by @stollcri (stollcri at gmail dot com), 2020-07-28
#
#   Originally downloaded from: https://sites.google.com/site/c41digitallab/the-complexity
#
# ====================================================================================================================
#   C41LAB - Version 1.2
# 
#   Copyright (C) 2013 S.Terland - email: <c41.script@gmail.com>
# 
#   This program is free software: you can redistribute it and/or modify it under the terms of the GNU General 
#   Public License as published by the Free Software Foundation, either version 3 of the License.
# 
#   This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the 
#   implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License 
#   for more details.
# 
#   You should have received a copy of the GNU General Public License along with this program.  If not, 
#   see <http://www.gnu.org/licenses/>.
# 
# ====================================================================================================================
#   
#   Description:
#          C41Lab is a Windows Command Script for developing positive images from C41 negatives. 
# 
#          The script was developed with batch processing in mind, transforming negatives to positive images that 
#          do not require more post processing than can be expected from images taken by a digital camera. It utilizes 
#          several techniques to transform the negative and achieves a good white balance most of the time. 
#          The only thing that seems to disrupt a perfect white balance is when a picture is very biased 
#          towards a particular color. But this can usually be easily corrected in post processing.
#            
#   Usage:
#          C41Lab <Input negative (TIF)> <Input non-exposed negative (TIF)> <Output image (TIF)>
# 
#          Make sure you use 16-bit TIF as the input, using 8-bit will give poor results. The non-exposed negative
#          reference can be any size, but must be free of all dust particles and exposure. A small crop of 
#          the non-exposed portion between the actual images on the film strip is usually the best reference. 
#          Note that you can use the same reference for a whole film, i.e. every 36 or 24 pictures as long as
#          the entire film was scanned with the same exposure settings.
# 
#          Important! Input images must be linear scanned negatives. C41 will not work with logarithmic scans.
#   
#   Prerequisite:
#          Latest version of the free ImageMagick utility installed - see <http://www.imagemagick.org> for details
# 
#   Remember:
#          Old film based cameras where of varying quality resulting in larger exposure variances than what can be
#          expected from today's simplest digital cameras. This script cannot improve the underlying quality of 
#          your negatives. This being said, Adobe PhotoShop, LightRoom and similar products may do wonders in post  
#          processing for difficult images.        
# 
# ====================================================================================================================

echo
echo "====================== C41Lab v1.2 ======================"

# Verify that input files do exist
if [ ! -f "$1" ]; then
	echo "Source image not found: $1"
	exit
fi
SRC_IMAGE=$1

if [ ! -f "$2" ]; then
	echo "Reference image not found: $2"
	exit
fi
REF_IMAGE=$2

if [ -x "$3" ]; then
	echo "Output image not defined: $3"
	exit
fi
OUT_IMAGE=$3

# ======================================== DEFINE VARIABLES ==========================================================

# If you want to change the brightness of your images you can change Gamma (this overrides the calculated value)
GAMMA=1.0
# Using negated gamma approach to enhance contrast and improve color saturation 
CONTRAST=0.75
# Colorspace is set to sRGB and don't show warnings - Ensures 16-bit color depth in each color channel (RGB)
CSPACE="-set colorspace sRGB -quiet"
# Push the image a bit away from minimum and maximum to minimize clipping.
BUFFER_LOW=100
BUFFER_HIGH=0.99

# Set several values for helper files and directories
TEMP_DIR=$HOME/.temp_C41Lab
TEMP1="$TEMP_DIR/temp1_color_shift.tif"
TEMP2="$TEMP_DIR/temp2_gamma_corrected.tif"
TEMP3="$TEMP_DIR/temp3_white_balance.tif"
BLACK_REF="$TEMP_DIR/black_ref.tif"

# Create the temporary directory
if [ ! -d "$TEMP_DIR" ]; then
	mkdir "$TEMP_DIR"
fi

# ====== Analyze the reference image =====
echo "--- Normalize Scanning Exposure of Negative"
# The exposure of the scanned image may vary slightly, calculate necessary corrections to ensure an even result
# across different films. Note! if the exposure is very under- or overexposed the colors of the final image will
# be affected. Note! This operation will not affect the image exposure, it only compensates for the fact that the 
# actual film differs in transparency and/or scanning exposure thus affecting the exposure across different films.

convert $REF_IMAGE $CSPACE -compress none $BLACK_REF

L=$(convert $CSPACE $BLACK_REF -colorspace gray -format "mid_exp=%[fx: mean/2]" info:)
mid_exp=$(echo "$L" | head -n1 | awk -F= '{print $2}')

L=$(convert $CSPACE $BLACK_REF -colorspace gray -format "exp_corr_gamma=%[fx: (log(maxima - $mid_exp)/log($mid_exp))]\nmax_exp=%[fx: maxima]" info:)
exp_corr_gamma=$(echo "$L" | head -n1 | awk -F= '{print $2}')
max_exp=$(echo "$L" | tail -n1 | awk -F= '{print $2}')

if [ "$exp_corr_gamma" == "nan" ]; then
	exp_corr_gamma=1.0
fi

# Check if any of the color channels of the reference image reaches QuantumRange. If so, clipping is highly likely.
if [ "$max_exp" == "1" ]; then
	echo    - Warning! - Negative seems to be overexposed.
	echo                 See histogram of negative "${OUT_IMAGE}_ref_hist.jpg" for more details
	convert $CSPACE $SRC_IMAGE -define histogram:unique-colors=false histogram:${OUT_IMAGE}_neg_hist.jpg
fi

# ====== Start Image Transformation =====
echo "--- Create Positive and Perform Color Shift"
# Enhance exposure of negative. Based on the provided black-reference (the unexposed negative) move all colors correctly to  
# black (Low value => zero) and add a small buffer on low and high end to minimize clipping.
# Must normalize each color channel before applying gamma to maintain color balance. Applying "negated" gamma to enhance colors and contrast.

L=$(convert $BLACK_REF $CSPACE -gamma $exp_corr_gamma -negate -format "red_shift=%[fx: minima.r*QuantumRange]\ngreen_shift=%[fx: minima.g*QuantumRange]\nblue_shift=%[fx: minima.b*QuantumRange] " info:)
red_shift=$(echo "$L" | head -n1 | awk -F= '{print $2}')
green_shift=$(echo "$L" | head -n2 | tail -n1 | awk -F= '{print $2}')
blue_shift=$(echo "$L" | tail -n1 | awk -F= '{print $2}')

convert $SRC_IMAGE $CSPACE -compress none -gamma $exp_corr_gamma -negate \
-channel R -evaluate subtract $red_shift -normalize \
-channel G -evaluate subtract $green_shift -normalize \
-channel B -evaluate subtract $blue_shift -normalize \
-channel RGB,sync -evaluate multiply $BUFFER_HIGH -evaluate add $BUFFER_LOW $TEMP1

echo "--- Perform Color Correction"
# Calculate the gamma correction factors to correct color balance.
L=$(convert $TEMP1 $CSPACE -resize 1x1 -format "Green_GAMMA=%[fx: (log(1/maxima.g)/log(1/maxima.r))]\nBlue_GAMMA=%[fx: (log(1/maxima.b)/log(1/maxima.r))] " info:)
Green_GAMMA=$(echo "$L" | head -n1 | awk -F= '{print $2}')
Blue_GAMMA=$(echo "$L" | tail -n1 | awk -F= '{print $2}')

# Performing gamma correction of colors based on the physical absorption principle of
# "log(actual luminosity) /log(maximum luminosity)" of each color dimension (RGB)
convert $TEMP1 $CSPACE \
-channel G -gamma $Green_GAMMA \
-channel B -gamma $Blue_GAMMA \
-channel RGB,sync \
-gamma $CONTRAST -negate \
-gamma $CONTRAST -negate $TEMP2

echo "--- Enhance White Balance "
# Based on the "Grey World Principle" perform final white balance enhancement of image
L=$(convert $TEMP2 $CSPACE -resize 1x1 -format "green_fact=%[fx: (log(maxima.g)/log(maxima.r))]\n blue_fact=%[fx: (log(maxima.b)/log(maxima.r))] " info:)
green_fact=$(echo "$L" | head -n1 | awk -F= '{print $2}' | xargs)
blue_fact=$(echo "$L" | tail -n1 | awk -F= '{print $2}' | xargs)

if [ "$green_fact" == "nan" ]; then
	green_fact=1.0
fi

if [ "$blue_fact" == "nan" ]; then
	blue_fact=1.0
fi

convert $TEMP2 $CSPACE -channel G -gamma $green_fact -channel B -gamma $blue_fact -channel RGB,sync $TEMP3

echo --- Enhance Brightness
# The earlier transformations can make the image stray away from its original brightness. Perform necessary gamma correction.
# Find average gray of the original image and calculate the compensation factor for the brightness mean
L=$(convert $TEMP1 $CSPACE -colorspace gray -format "image_mean=%[fx: mean] " info:)
image_mean=$(echo "$L" | awk '{print $1}' | head -n1 | awk -F= '{print $2}')

# Apply compensation factor to the final image
L=$(convert $TEMP3 $CSPACE -colorspace gray -resize 1x1 -format "final_gamma=%[fx: (log(maxima)/log($image_mean))] " info:)
final_gamma=$(echo "$L" | awk '{print $1}' | head -n1 | awk -F= '{print $2}')

if [ "$final_gamma" == "nan" ]; then
	final_gamma=1.0
fi

convert $CSPACE $TEMP3 -gamma $final_gamma -gamma $GAMMA -compress Zip $OUT_IMAGE

echo --- Remove Temporary Files
# Delete temporary files
# rm $TEMP1
# rm $TEMP2
# rm $TEMP3
# rm $BLACK_REF

# Make it look nice when finished
echo "--- Done!"
echo "=========================================================="
echo