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

```
After attempting to apply the repeated `composite` technique with HDR images and ending up with clamped output, I discovered the [`-evaluate-sequence`](https://www.imagemagick.org/script/command-line-options.php#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](https://www.imagemagick.org/script/high-dynamic-range.php).