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How to remove every second frame from an animated gif?


Question

I have a folder full of videos that I want to convert to an animated gifs. ffmpeg/avconv does a bad job of doing it directly, so I instead convert the video to a gif by first outputting each frame as a png and then converting back to gif using imagemagick. The problem is that this results in a large gif in terms of file size. To solve this I want to "drop" every second or nth frame from the gif, either by skipping every image file when converting to a gif or by removing frames from a gif. How can I do this on Ubuntu (13.04) using imagemagick or some other command-line utility?

2013/09/02
1
23
9/2/2013 9:08:00 PM

Accepted Answer

Using a bash script

To do this from the command line, you could use a utility called Gifsicle. There is no built in method to delete every other frame, so you'll need to get your hands dirty with some scripting.

Here is a quick script I made to do just a single GIF:

#!/bin/bash
# This script will take an animated GIF and delete every other frame
# Accepts two parameters: input file and output file
# Usage: ./<scriptfilename> input.gif output.gif

# Make a copy of the file
cp $1 $2

# Get the number of frames
numframes=`gifsicle $1 -I | grep -P "\d+ images" --only-matching | grep -P "\d+" --only-matching`

# Deletion
let i=0
while [[ $i -lt $numframes  ]]; do
    rem=$(( $i % 2 ))

    if [ $rem -eq 0 ]
    then
        gifsicle $2 --delete "#"$(($i/2)) -o $2 
    fi

    let i=i+1 
done

I tested it out with a simple countdown GIF:

enter image description here

And here is the result after running it through the script:

enter image description here

This script is of course not bulletproof, but it should lead you in the right direction.

2016/09/27
14
9/27/2016 7:39:00 PM

Here's a simpler solution using gifsicle than JohnB's script:

gifsicle -U input.gif `seq -f "#%g" 0 2 99` -O2 -o output.gif

This command should work in most Unix shells; I've tested it in bash. Replace input.gif and output.gif with the input and output file names, and 99 with the number of frames in your animation. (It's OK to use a larger number, but gifsicle will complain about it.)

Some notes:

  • The -U switch will merge frames in the input animation with the preceding ones, so that each frames stands alone and doesn't depend in any others. You really want to do this before doing pretty much anything with animations, otherwise you're likely to get messy results. (If your input animation is already unoptimized, gifsicle may print a warning about it, but this is also completely harmless.)

  • Conversely, the -O2 switch re-optimizes the output animation to minimize the file size. With JohnB's sample animation, it shrinks the output size down by 27%.

  • The seq command just outputs a sequence of numbers from 0 to 99, counting up in steps of 2. The -f "#%g" makes it print a # before each number, which makes gifsicle understand it as a frame selection instead of a file name. The backticks (`) around the seq command cause its output to be included as parameters in the gifsicle command line.

If you don't want the gif to speed up, you can use gifsicle -I input.gif to get the current frame delay, multiply it by 2 and use gifsicle -d ${delay} ....

2016/04/03