Imagemagick Batch operation conditional on Filesize - How? - bash

Question

I'm running Imagemagick on a command line Ubuntu terminal in Windows 10 - using the built in facility in Windows 10 - the Ubuntu App.
I am a complete linux novice but have installed imagemagick in the above environment.
My task - Auto remove the black(ish) border and deskew the images of thousands of scanned 35mm slides.
I can successfully run commands such as
mogrify -fuzz 35% -deskew 80% -trim +repage *.tif
The problem is:-
The border is not crisply defined nor is completely black, hence the -fuzz. Some images are over-trimmed at a certain fuzz, while others are not trimmed enough.
So what I want to do is to have two passes at this, with different fuzz %, for these reasons:-
1st pass with a low Fuzz%. Many images will not be trimmed at all but I have found that the ones that are susceptible to over-trimming will trim Ok with low %
Since all the images start with an identical filesize, the ones that have trimmed Ok will have a lower filesize (note these are tifs not jpgs)
So what I need to do is set a file size condition for the second pass at higher fuzz% THAT IGNORES file sizes below a certain value and does not perform any operation.
In this way, with few errors, all the images will be trimmed correctly.
So the question
- How can I adjust the command line to have 2 passes and to ignore a lower file size on the second pass?
I have a horrible feeling the the answer will be a script. I have no idea how to construct or set up Ubuntu to run this so if so, please can you point me to help for that also!!

Answer 1

In ImageMagick, you could do something like the following:
Get the input filesize
Use convert to deskew and trim.
Then find the new file
Then compare the new to the old to compute the percentdifference to some percent threshold
If the percent difference is less than some threshold, then the processing did not trim enough
So reprocess with a higher fuzz value and write over the input; otherwise keep the first one only and do not write over the old one.
Unix syntax.
Choose two fuzz values
Choose a percent change threshold
Create a new empty directory to hold the output (results)
cd
cd desktop/Originals
fuzz1=20
fuzz2=40
threshpct=10
list=`ls`
for img in $list; do
filesize=`convert -ping $img -precision 16 -format "%b" info: | sed 's/[B]*$//'`
echo "filesize=$filesize"
convert $img -background black -deskew 40% -fuzz $fuzz1% ../results/$img
newfilesize=`convert -ping ../results/$img -precision 16 -format "%b" info: | sed 's/[B]*$//'`
test=`convert xc: -format "%[fx:100*($filesize-$newfilesize)/$filesize<$threshpct?1:0]" info:`
echo "newfilesize=$newfilesize; test=$test;"
[ $test -eq 1 ] && convert $img -background black -deskew 40% -fuzz $fuzz2% ../results/$img
done
The issue is that you need to be sure you set your TIFF compression for the output the same as for the input so that the file sizes are equivalent and presumably the new size is not larger than the old one as happens with JPG.
Note that the sed is used to remove the letter B (bytes) from the file size, so they can be compared as numerals and not strings. The -precision 16 forces "%b" to report as B and not KB or MB.