The goal#

Given a series of images, present them in a video with a blend effect between the images. The frames between the input frames should gradually transition between the previous and next image. This, and other transition effects, could likely be implemented using a slideshow generator, but it can also be done quite easily using a shell script.

The script#

The final script I wrote is blend.sh. The following commands fetch and run it:

$ wget https://gist.githubusercontent.com/dperelman/2b4b86233aa13d13c0ab/raw/91c7988e27288af8aeb25ade11d0cab90355702f/blend.sh
$ chmod +x blend.sh
$ ./blend.sh slideshow.mkv first.png second.jpg third.gif
$ mplayer slideshow.mkv

The input images may be in any format. The extension of slideshow.mkv will be used by FFmpeg to guess the desired video format (H.264 in a Matroska Multimedia Container for .mkv).

The details#

FFmpeg#

To encode a video from the command line, the relevant tools are FFmpeg and MEncoder. I choose FFmpeg because I generally find it easier to use, but I tend to switch between the two when one isn't doing what I want.

We reduce the problem of generating a video to one of generating the frames for that video using the FFmpeg input mode image2, which takes in a sequence of images as the frames for a video.

ImageMagick#

To generate the frames, we will use the ImageMagick family of tools, which are a set of command-line tools that will do pretty much any image manipulation task you can imagine. The primary downside of ImageMagick is that it does so much, it sometimes takes a bit of reading through the documentation to figure out the right settings for a simple task.

The basic ImageMagick command is convert. In the script, it is used to convert the input image from any format ImageMagick supports to a PNG (the frames are stored as PNG because it uses lossless compression):

convert "$image" "$dir/image${num}.png"

Blending#

ImageMagick's composite command with the -blend option is used to make the intermediate frames. See the examples section of the ImageMagick documentation for how to use -blend.

The code for performing the blend is¹

# Use bc for math to get decimals.
percent="$(echo "$fade_amount/$frames_per_image*100" \
    | bc -lq)"
composite -blend "$percent" "$image" "$prev" \
    "$dir/image$num"".png"

The computation of $percent is done with bc in order to get decimals:

$ fade_amount=7
$ frames_per_image=30
$ echo "$fade_amount/$frames_per_image*100"
7/30*100
$ echo $((fade_amount/frames_per_image*100))
0
$ echo $((100*fade_amount/frames_per_image))
23
$ echo "100*$fade_amount/$frames_per_image" | bc
23
$ echo "$fade_amount/$frames_per_image*100" | bc -l
23.33333333333333333300

Cleaning up#

The script make a temporary directory using mktemp -d and deletes it using rm -rf when it is done. This means the script writes the frames it generates to a directory in /tmp, so if using this to create a video for many images, make sure you have enough space in /tmp for the frames or use the -p option of mktemp to place the temporary directory somewhere there is enough space. The frames will take approximately $frames_per_image (default=30) times as much space as the input images.