I'm trying to merge every pair of images from a folder and combine that result into a PDF file with imagemagick and parallelize the process with GNU Parallel.
parallel -N2 convert \( {1} -rotate 30 \) {2} +append miff:- ::: *jpeg | convert - out.pdf
The problem is that I need to rotate the first argument and an error occurs.
Error: /bin/sh: -c: line 0: syntax error near unexpected token `01.jpeg'
/bin/sh: -c: line 0: `convert ( 01.jpeg -rotate 30 ) 02.jpeg +append miff:-'
...
How can I process one of the arguments GNU parallel is receiving?
First, you better use parallel -k else the output will be in the wrong order.
Second, you don't need parentheses to ensure your -rotate only applies to the first image because you haven't loaded a second at that point.
So, you are looking at:
parallel -k -N2 convert {1} -rotate 30 {2} +append miff:- ::: *jpeg | convert - out.pdf
or maybe:
parallel -k -N2 'convert {1} -rotate 30 {2} +append miff:-' ::: *jpeg | convert - out.pdf
In answer to your question, ImageMagick will apply operators (like -rotate) to all images in the loaded stack. So:
convert im1.png im2.png -rotate ...
will rotate both images. Whereas:
convert im1.png -rotate 90 im2.png ...
will only rotate the first image. If you want to only rotate the second, you have 2 choices, either put the second in parentheses so the -rotate only applies to it:
convert im1.png \( im2.png -rotate 90 \) ...
or, load the second image first and rotate it, then load the first and swap the order:
convert im2.png -rotate 90 im1.png +swap ...