How do I sum images?

Question

I have a number of photos. I want to add these images togther pixel-wise (not mean or median). I have Imagemagick installed, and I tried:

composite -compose plus IMG* ~/Desktop/sum.png

But the output image

wasn't noticeably brighter than the input images:

Since I have 30 input images, I expected the output to be much brighter.

Why isn't this working, and how do I sum images, using either Imagemagick, or other free tool?

Answer 1

Imagemagick cannot be used to compose 30 images in one step. The documentation states:

Image Composition is the process of merging two (and only two) images in a variety of ways.

The following simple bash script creates a black 100x100 image (temp.png) and compose all the test_*.jpg images (assuming all of them are 100x100) one by one to this image.

convert -size 100x100 canvas:black temp.png
for f in test_*.jpg; do
  composite temp.png "${f}" -compose Plus temp.png
done

Answer 2

You can use GIMP.

1. Open the 1st image.

2. Open the 2nd image as a layer (Ctrl + Alt + O).

3. Change layer mode to "Addition" (Dialogue - Layers - Mode).

4. Open next image as a layer.

5. Change layer mode to "Addition".

6. Proceed until all images are added.

You can add all images at once (selecting them with Ctrl while opening as a layer) but you'll still have to change layer mode individually for every layer.

This procedure is also possible with any graphic editor which is capable to work with layers (Paint.net, Photoshop (not free!) etc).

Answer 3

After attempting to apply the repeated composite technique with HDR images and ending up with clamped output, I discovered the -evaluate-sequence operator. Here's an example:

convert *.exr -evaluate-sequence Add output.exr

This takes all the .exr files (or whatever format you want) in the current directory and sums them into a new image, output.exr.

If that doesn't give proper HDR output you may need to ensure you have a version of image magick built with HDR enabled.

Answer 4

Thanks to asalamon74 for a solution using a bash script, which I modified to use PNG rather than JPEG, as szulat suggested.

convert -size 100x100 canvas:black temp.png

for f in IMG_*.jpg; do
  composite temp.png ${f} -compose Plus temp.png
done

I tested, and this works.