Imagemagick Batch Processing

Imagemagick provides powerful tools for image manipulation. These can be easily automated in small shell scripts or command lines.

Shellscript batch processing

Some of the following examples use basename $file to not overwrite the original file. Replace it by $file to overwrite the original.

Scale a bunch of photos

This scales all .JPG files in the directory ../mypics to 2048x1536 pixels. Note that the file extension (.JPG) is case sensitive and will not match .jpg files. The aspect ratio is preserved.

for file in ../mypics/*.JPG; do echo $file; convert "$file" -scale 2048x1536 -quality 85 `basename "$file"`; done

Crop out a section of PNG files

This cuts out the upper left 6x17 pixels. +0+0 adjusts the offset. Use -depth 8 to get 8bit PNGs instead of 16bit.

for file in ../my_pngs/*.png; do echo $file; convert $file -crop 6x17+0+0! -depth 8 `basename $file`; done

... can of course be combined with -scale, here with different parameters. ! ignores the aspect ratio and ensures that the output images have the exact resolution given:

for file in ../my_pngs/*.png; do echo $file; convert $file -crop 90x45+0+13 -scale 72x36! -depth 8 `basename $file`; done

Fade in/out

A fade in and out effect for PNG sequences, easily done with -modulate:

# Fade in, 60 frames:
i=0; for file in ../my_pngs/*.png; do echo $file; convert $file -modulate $(( ( $i * 100 ) / 60 )) -depth 8 $file; i=$(($i + 1)); done

#Fade out, 60 frames:
i=60; for file in ../my_pngs/*.png; do echo $file; convert $file -modulate $(( ( $i * 100 ) / 60 )) -depth 8 $file; i=$(($i - 1)); done

Attention: These two examples overwrite the original files.

Process multiple directories

Applying the same procedure to images in all subdirectories of the current path:

for dir in `find -type d`; do for file in $dir/*.png; do echo $file; convert $file -crop 24x48+12+0! -depth 8 $file; done; done

Attention: Overwrites the original files.

What else?

• convert -rotate 90 Rotates the image 90 degrees (or any multiple of 90 degrees)
• composite -compose Add multiple images, also with transparency

Imagemagick convert can do much more image conversions. For example blurring, various resize methods and colour filters or even blending images. See the Command-line options for details.