ImageMagick reminder

mogrify vs convert

Both commands can be used almost equally, but mogrify is better fitted to bulk modifications whereas convert is better adapted to single file modifications. It is nonetheless possible to do everything with both commands, as shown below.

Resize images

It is rather easy to resize a bunch of images to harmonize their size (and converting them, if needed, in an other format) with -resize option. To get images with a constant height, use -resize x[height]; for a constant width, use -resize [width]x, with dimensions given in pixels. It is also possible to keep the name of the file using -set filename:base %[base].

Here is an example to convert images with a constant height of 100 pixels:

convert ../{image-pattern} -resize x100 -set filename:base "%[base]" "%[filename:base]_100.png"

File conversion

mogrify and convert can be used to convert files easily. To modify image quality, it is possible to play with resolution in dpi with density option (set to 72 by default).

Here is an example to convert EPS files to PNG:

mogrify -format png -density 150  *eps

Animations

ImageMagick is useful to create animations — as animated GIF — from a bunch of images. You can set a delay between each frame (and therefore define something like a framerate) with -delay option.

Here is a simple example:

convert -delay 20 $(ls *png) animation.gif

And if you wish to realise an animation with a selection on files:

convert -delay 20 $(ls *png | head -n40) animation.gif

Remove images from an animation

First step is to retrieve images from the animation:

convert animation.gif +adjoin temp_%02d.gif

Then, you can select every n frames with a for loop on all the images. For example, you can check that the iterator is a multiple of 2, and if it is true, to copy it in a new file:

j=0; for i in $(ls temp_*gif); do if [ $(( $j%2 )) -eq 0 ]; then cp $i sel_`printf %02d $j`.gif; fi; j=$(echo "$j+1" | bc); done

If, on the contrary, you wih to keep all the files that are not divisible by 2, you can replace -eq by -ne.

Lastly, you can create the new animation:

convert -delay 20 $( ls sel_*) new_animation.gif