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?

Answer 1

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} ....

Answer 2

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:

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

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

Answer 3

NOTE: This answer was posted before there was a command line/open source requirement, but I'm leaving it up as it may help someone else in the future

---

Using Photoshop

This is not an open source or command line solution, but you can do this with Photoshop:

File ▸ Import ▸ Video Frames to Layers...

"Limit To Every __ Frames" will do the trick for you

Answer 4

I think I need new more sophisticated explanation (frame delay is also important thing that should be considered too)... For this new process you'll need to download JREPL.BAT from this link : http://www.dostips.com/forum/viewtopic.php?t=6044

Firstly copy captured gif into input.gif file then start command interpreter and type :

gifsicle input.gif -I "#-1" >input.txt

look into input.txt and see what is the length of animation - how many frames it contains... also look into delay of frame and if its 0.07s that means that you should put option -d14 (7msec*2) into this line in process.cmd : gifsicle -b -U -d14 input.gif , after edit save process.cmd

then edit test.bat and change value of variable 166 from this line to match to number of frames of animation : for /L %%i IN (1,2,166) DO echo "#%%i">> input.bat , after edit save test.bat...

then start process.cmd and croped animation with every odd frame will be processed into file input.gif

P.S. The advantage of this method is also that you have full control of what do you want to delete from gif (every second frame (1,2,166), or every third (1,3,166)), and so on just change middle number in batch line, but be aware that if you specify (1,3,166), you should accordingly change -d14 (delay option) from process.cmd to reflect change (7msec*3/2 = 10), so instead of (7msec*2/1 = 14), option for delay should be -d10...

Other files (for example of removing every second frame) : test.bat :

@echo off
echo gifsicle -b input.gif --delete>> input.bat
for /L %%i IN (1,2,166) DO echo  "#%%i">> input.bat
echo  --done>> input.bat
call jrepl.bat "\n" "" /x /m /f input.bat /o -

process.cmd :

gifsicle -b -U -d14 input.gif
call "test.bat"
call "input.bat"
gifsicle -b -O3 input.gif
erase "input.bat"

And helper readme file input.txt :

* input.gif 166 images
  logical screen 1366x768
  global color table [256]
  background 15
  loop forever
  + image #165 1x1 at 1365,767 transparent 15
    disposal asis delay 0.07s

Answer 5

Here is my solution using batch script processing...

Firstly copy captured-original animated gif into input.gif file then start command interpreter and type :

gifsicle input.gif -I "#-1" >input.txt

look into input.txt and see what is the length of animation - how many frames it contains...

input.txt

* input.gif 166 images
  logical screen 1366x768
  global color table [256]
  background 15
  loop forever
  + image #165 1x1 at 1365,767 transparent 15
    disposal asis delay 0.07s

then edit-create test.bat and change value of variable len=specify_length_number_from_input.txt and save test.bat...

test.bat

@echo off
set /A len=166
set /A i=1
set /A ii=0
:loop
if %i%==%len% goto :eof
if %ii%==0 (set /A ii=1) else (set /A ii=0)
set /A iii=%ii%*%i%
if %i%==%iii% echo gifsicle -b input.gif --delete "#1" --done
set /A i=%i%+1
goto :loop

then finally start process1.bat and process2.bat and croped animation with every odd frame will be processed into file input.gif

process1.bat

gifsicle -b -U input.gif
test.bat > input.bat

process2.bat

call "input.bat"
gifsicle -b -O2 input.gif
erase "input.bat"