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Cropping Images and Video

Cropping images is something that is commonly done, however cropping many images at a time can become tedious. Furthermore cropping a video can be seen to be almost impossible without a non-linear video editing app at hand. This guide will show you the tools that imagemagick and ffmpeg provide to make thes tasks almost effortless.

Cropping an Image with ImageMagick

Removing Pixels from Edge of Image

Say we want to remove a certain number of pixel from each border of our image. Let's start by removing 200 pixels from the left and right of the image

The command is the following:

convert original.jpg -shave 200x0 output.jpg

To remove 200 pixels from the top and bottom, the command is: 

convert original.jpg -shave 0x200 output.jpg

But what if we want different values for each side? Then we can combine two crop flags: 

                      #left,top      right,bottom
convert test.png -crop +180+140 -crop -60-140 cropped.png

Removing White Regions of an Image

If you just want to remove the white regions of an image, ImageMagick has a dedicated command for that: 

convert -trim input.jpg output.jpg

You can also use: 

mogrify -trim file.jpg
if you don't want to produce a copy of the original file.

Remove Transparent Border of Image

If you ever stumble upon a png file with a large transparent border, you add in the +repage flag to remove the transparent border aswell: 

convert input.png -trim +repage output.png

Cropping a Video with FFMPEG

Cropping videos in FFMPEG requires a little more thinking, let's start with the command: 

ffmpeg -i input.mp4 -filter:v "crop=w:h:x:y" output.mp4

What this all means:

  • -i input.mp4 specifies the input video (input.mp4 being the input / original video in this case)
  • -filter:v (can be abbreviated to -vf) specifies we're using a video filter

  • "crop=W:H:X:Y" means we're using the "crop" video filter, with 4 values:

    • w the width of the output video (so the width of the cropped region), which defaults to the input video width (input video width = iw, which is the same as in_w); out_w may also be used instead of w
    • h the height of the output video (the height of the cropped region), which defaults to the input video height (input video height = ih, with in_h being another notation for the same thing); out_h may also be used instead of h
    • x the horizontal position from where to begin cropping, starting from the left (with the absolute left margin being 0)
    • y the vertical position from where to begin cropping, starting from the top of the video (the absolute top being 0)

  • output.mp4 is the new, cropped video file.

A few notes:

  • The filter will automatically center the crop if x and y are omitted, so x defaults to (iw-w)/2, and y to (ih-h)/2
  • There is also an optional keep_aspect option that you can set to 1 to force the output display aspect ratio to be the same of the input (example usage: "crop=100:100:0:0:keep_aspect=1"). This won't work with images, that's why you don't see a separate example with screenshot here.
  • FFmpeg gets the original input video width (iw) and height (ih) values automatically, so you can perform mathematical operations using those values (e.g. iw/2 for half the input video width, or ih-100 to subtract 100 pixels from the input video height).