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

[![enter image description here](https://i.stack.imgur.com/wnlnM.jpg)](https://i.stack.imgur.com/wnlnM.jpg)

wasn't noticeably brighter than the input images:

[![enter image description here](https://i.stack.imgur.com/GH7tW.jpg)](https://i.stack.imgur.com/GH7tW.jpg)

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: 
Imagemagick cannot be used to compose 30 images in one step. The [documentation](http://www.imagemagick.org/Usage/compose/) 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

```
You can use [GIMP](https://www.gimp.org/).

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).